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Fungal keratitis is commonly caused by Fusarium species and less commonly by Candida species. Recent outbreaks of Fusarium keratitis were associated with contact lens wear and with ReNu with MoistureLoc contact lens care solution, and biofilm formation on contact lens/lens cases was proposed to play a role in this outbreak. However, no in vitro model for contact lens-associated fungal biofilm has been developed. In this study, we developed and characterized in vitro models of biofilm formation on various soft contact lenses using three species of Fusarium and Candida albicans. The contact lenses tested were etafilcon A, galyfilcon A, lotrafilcon A, balafilcon A, alphafilcon A, and polymacon. Our results showed that clinical isolates of Fusarium and C. albicans formed biofilms on all types of lenses tested and that the biofilm architecture varied with the lens type. Moreover, differences in hyphal content and architecture were found between the biofilms formed by these fungi. We also found that two recently isolated keratitis-associated fusaria formed robust biofilms, while the reference ATCC 36031 strain (recommended by the International Organization for Standardization guidelines for testing of disinfectants) failed to form biofilm. Furthermore, using the developed in vitro biofilm model, we showed that phylogenetically diverse planktonic fusaria and Candida were susceptible to MoistureLoc and MultiPlus. However, Fusarium biofilms exhibited reduced susceptibility against these solutions in a species- and time-dependent manner. This in vitro model should provide a better understanding of the biology and pathogenesis of lens-related fungal keratitis.
Fusarium keratitis is a devastating ocular disease and an important cause of morbidity and blindness (34, 47). This disease has been reported in different parts of the world, particularly in tropical areas (1, 2), where it may account for more than 50% of all ocular mycoses (3). A major increase in the incidence of fungal keratitis during the past 2 years has made it a relatively new public health concern, drawing broad international attention. Fungal keratitis is caused by filamentous fungi (especially Fusarium and Aspergillus) and by yeast-like fungi (particularly Candida albicans) (5).
Recently several investigations, including those by the Singapore Health Ministry and the U.S. Centers for Disease Control and Prevention (CDC) and in the French West Indies, reported outbreaks of Fusarium keratitis associated with contact lens wear (16, 18, 29). Although fungal keratitis is generally associated with trauma and prior application of corticosteroids (6), patients in these outbreaks had no history of recent ocular trauma (16). The majority of the confirmed Fusarium keratitis cases reported by the CDC indicated the wearing of soft contact lenses and use of the alexidine-containing contact lens cleaning solution ReNu with MoistureLoc (“MoistureLoc”) (16). Also, as noted by the CDC, although the market share of ReNu Multipurpose solution (“MultiPlus,” containing polyhexamethylene biguanide) was five times higher than that of MoistureLoc, the majority of reported Fusarium keratitis cases involved the use of the latter lens cleaning solution, thus implicating it in the outbreak (16).
C. albicans also causes fungal keratitis and lens-associated infections, although they are less common than Fusarium-associated keratitis. However, ocular infections caused by the former are usually associated with an underlying condition (25, 47). In a retrospective review of fungal keratitis cases at the University of Florida from 1999 to 2006, Iyer et al. (25) showed that the most commonly isolated fungi in these cases were Fusarium (41%) and Candida (14%), followed by Curvularia (12%) and Aspergillus (12%). Although contact lens-related infections associated with fungi, especially Fusarium, have gained prominence recently, Iyer et al. (25) showed that the incidence of this disease was increasing even before the latest outbreak. In this regard, before 2004, trauma was the most common risk factor for fungal keratitis (51%), compared to contact lens use (40%), while after 2005, this disease was associated more with contact lens use (risk factor 52%) than with trauma (29%). These studies underscore the need for detailed investigations into the biology and pathogenesis of Fusarium and non-Fusarium keratitis.
One mechanism by which keratitis-associated Fusarium exhibits a drug-resistant phenotype may be formation of biofilms (microbial communities that are embedded in an endogenous extracellular matrix). In this regard, bacterial biofilms have been demonstrated on contact lenses obtained from patients with culture-proven keratitis (20, 45, 50). However, investigations of fungal biofilms associated with contact lenses and lens cases are virtually nonexistent, and the lack of robust in vitro models has resulted in scant information about the pathogenesis of Fusarium keratitis.
In the current study, we established an in vitro model of contact lens-associated Fusarium and Candida biofilms and used this model to characterize biofilms formed by these organisms on soft contact lenses belonging to the four FDA-designated groups (groups I, II, III, and IV). In addition, silicone hydrogel lenses, which represent a lens type with different monomer polymer backbones and surface treatments and greatly enhanced oxygen permeability compared to traditional hydrogels (10), were also examined. Finally, we used our in vitro model to determine whether biofilms formed by Fusarium and Candida isolates were susceptible to MoistureLoc and MultiPlus contact lens care solutions. Our results showed that both Fusarium and C. albicans can form biofilms on the different lens types tested, but there were pronounced differences in biofilm architecture, hyphal morphology, biofilm thickness, and metabolic activity between Fusarium and Candida. Furthermore, while the two contact lens solutions tested were active against the planktonic (free-floating) forms of Fusarium and Candida, these solutions were less effective against Fusarium biofilms in a strain- and time-dependent manner. Moreover, these solutions were ineffective against Candida biofilms. Our studies suggest that the suboptimal activity of MoistureLoc against Fusarium infections in recent outbreaks may be due to the ability of these pathogens to form resistant biofilms.
The three keratitis-associated fusaria included in this study were characterized using DNA sequence data from three loci as previously described (16). Results of multilocus DNA sequence typing indicate that the fusaria represent three phylogenetically distinct species. Two of these are members of the species-rich Fusarium solani species complex (FSSC)] (51), while the third is a member of the Fusarium oxysporum species complex (FOSC) 3-a widespread clonal lineage (40). FSSC multilocus haplotype 1-b strain MRL8609 (= NRRL 47513) was isolated from a patient with fungal keratitis not associated with contact lens use at University Hospitals Case Medical Center, Cleveland, OH, while the FOSC 3-a strain MRL8996 (= NRRL 47514) was isolated from a patient with contact lens-associated fungal keratitis at Cleveland Clinic Foundation. ATCC 36031 (= NRRL 47512) FSSC multilocus haplotype 2-c, which was originally isolated from a human corneal ulcer in Nigeria in the mid-1970s, was acquired from the American Type Culture Collection (Manassas, VA) and used as a reference isolate in our Fusarium-related experiments. The ATCC 36031 strain is the recommended reference isolate in the International Organization for Standardization (ISO 14729) guidelines for testing of the antimicrobial activity of lens care solutions in vitro. C. albicans strain SC5314, a clinical isolate obtained from a candidiasis patient, was a generous gift from William Fonzi (Georgetown University, Washington, DC). All of the fusaria are available upon request from the Agricultural Research Service Culture Collection (NRRL), National Center for Agricultural Utilization Research, Peoria, IL.
Fusarium isolates were grown at 37°C for 40 h in Sabouraud dextrose broth (SDB) (Difco Laboratories, Detroit, MI). To collect conidia following incubation, conidia of the Fusarium species were harvested and hyphae were removed by filtration through sterile gauze. Conidia were washed with phosphate-buffered saline (PBS) and standardized to 1 × 103 conidia/ml for growth rate studies and 1 × 106 conidia/ml for biofilm formation experiments. C. albicans was grown overnight at 37°C in yeast nitrogen base medium (YNB) (Difco Laboratories) supplemented with 50 mM glucose. Candida cells were harvested, washed with PBS, and standardized to 1 × 107 blastospores/ml for biofilm formation experiments. Growth of Fusarium in SDB, YNB, yeast potato dextrose (YPD) broth (Difco Laboratories), and RPMI 1640 (Mediatech, Inc., Herndon, VA) was monitored to identify the optimal medium to be used for biofilm formation by fusaria.
Soft contact lenses used in the present study included the following: etafilcon A (Vistakon; Johnson & Johnson, Jacksonville, FL), galyfilcon A (Vistakon), lotrafilcon A (CIBA Vision, Duluth, GA), balafilcon A (Bausch & Lomb, Rochester, NY), alphafilcon A (Bausch & Lomb), and polymacon (Bausch & Lomb). Properties (water content and surface charge) of these contact lenses are shown in Table Table1.1. All the lenses used had a power of +1.50 diopters. The lens care solutions tested were MoistureLoc (lot number GG5033; Bausch & Lomb) and MultiPlus (lot number GF6002; Bausch & Lomb). Lens care solutions were stored at room temperature as per the manufacturer's suggestions.
To evaluate biofilm formation by Fusarium and Candida isolates, soft contact lenses were washed with PBS, placed in 12-well tissue culture plates with 4 ml standardized cell suspension (1 × 107 cells/ml for C. albicans and 1 × 106 conidia/ml for the fusaria), and incubated for 90 min at 37°C (adherence phase). Nonadherent cells were removed from soft contact lenses by gentle washing with 4 ml PBS. Next, soft contact lenses were immersed in medium (SDB for the fusaria, and YNB for C. albicans) and incubated for 48 h at 37°C on a rocker. Biofilms were quantified using a tetrazolium XTT [2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide] assay as described previously (13). Planktonic cells were grown in 12-well plates in the absence of a contact lens.
To characterize the gross morphology of biofilms formed on contact lenses, lenses were seeded with fungi and allowed to adhere and then form biofilms for 48 h. Next, a digital camera was used to capture images of the biofilms to compare their gross morphologies and appearances.
The architecture of biofilms formed on soft contact lenses was analyzed using confocal scanning laser microscopy (CSLM), following our previously described method (13). Briefly, soft contact lenses containing biofilms were transferred to 12-well plates and incubated for 45 min at 37°C in 4 ml of PBS containing the fluorescent stains FUN-1 (10 mM) and concanavalin A-Alexa Fluor 488 conjugate (ConA) (25 mg/ml). FUN-1 (long-pass filter; excitation wavelength, 543 nm; emission, 560 nm) is converted to orange-red cylindrical intravacuolar structures by metabolically active cells, while ConA (long-pass filter; excitation wavelength, 488 nm; emission, 505 nm) binds to glucose and mannose residues of fungal cell wall polysaccharides and emits green fluorescence. After incubation with the dyes, the lenses were flipped and placed on a 35-mm-diameter glass-bottom petri dish (MatTek Corp., Ashland, MA). Stained biofilms were observed by using a Zeiss LSM510 confocal scanning laser microscope equipped with argon and HeNe lasers and mounted on a Zeiss Axiovert100 M microscope (Carl Zeiss, Inc.). All observations were conducted with a water immersion C-apochromat objective (403; numerical aperture, 1.2).
The antifungal activities of MoistureLoc and MultiPlus lens care solutions against planktonic forms of Fusarium and Candida were evaluated using the ISO 14729 standalone contact lens disinfectant test (24) and the 2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-5-[(phenylamino)carbonyl]-2H-tetrazolium hydroxide (XTT)-based metabolic activity assay. Activities of the lens care solutions against fungal biofilms were determined by measuring their metabolic activities using the XTT-based assay.
ISO 14729 guidelines are the standard used by the industry to demonstrate activities of contact lens care solutions against planktonically grown microorganisms (24). These guidelines specify that an active contact lens disinfectant must reduce the viability of fungal species by 1 log (90%) within the time recommended by the product's manufacturer. For evaluation of the antifungal activities of MoistureLoc and MultiPlus solutions against Fusarium strains ATCC 36031 (FSSC 2-c), MRL8609 (FSSC 1-b), and MRL8996 (FOSC 3-a), isolates were grown on potato dextrose agar plates for 10 days at 25°C. Conidia were harvested using a procedure based on the ISO standard and washed three times with Dulbecco's PBS plus 0.05% Tween 80. Conidial suspensions were adjusted to 5.0 × 107 conidia/ml using a hemacytometer. Subsequently, a 0.1-ml suspension of each Fusarium strain was mixed with 10 ml of each lens care solution and incubated at 25°C for 1, 2, 3, 4, and 20 h. At each time point, 1 ml of the resultant mixture was taken, diluted with Dey-Engley neutralizing broth (DEB) (Difco Laboratories), and spread on Sabouraud dextrose agar plates. After the plates were incubated for 2 days at 25°C, viable CFU were counted. Each strain was tested three independent times. A similar ISO 14729-based approach was used to assess the inhibitory activities of these lens care solutions against C. albicans SC5314.
The XTT-based assay has been used previously to monitor fungal damage (38), to quantify Candida and Cryptococcus biofilms, and also to determine antibiofilm activities of antifungal agents (11, 13, 14, 23, 27, 28, 31, 35, 42). In this study, we used this method to evaluate the abilities of contact lens care solutions to inhibit planktonic and biofilm forms of Fusarium and Candida. Briefly, planktonic cells grown in the absence or presence of lens care solutions were incubated with XTT (1 mg/ml) and menadione (1 mM; Sigma Chemical Co., St. Louis, MO) at 37°C for 5 h. Mitochondrial dehydrogenase-mediated conversion of XTT tetrazolium salt to a formazan product in live cells resulted in a colorimetric change, which was measured using a microtiter plate reader (Bio-Rad Laboratories, Hercules, CA) at 492 nm.
To evaluate the ability of MoistureLoc or MultiPlus to inhibit contact lens-associated fungal biofilms, Fusarium or Candida biofilms formed on contact lenses were incubated with 4 ml of either solution in a 12-well plate for 4 h or 20 h, followed by incubation with 4 ml of DEB for 10 min to stop the action of these solutions (as per ISO recommendations). Cells treated with only DEB served as controls. Next, lens-associated biofilms were washed with PBS and their metabolic activity was determined as described above.
All experiments were performed in triplicate. Statistical analysis was performed using analysis of variance followed by the Bonferroni/Dunn post hoc test for comparisons of biofilms on different contact lenses and an unpaired t test for evaluating the activity of lens care solutions. A P value of <0.05 was considered statistically significant.
The DNA sequence data generated in this study have been deposited in GenBank under accession numbers EU251484 to EU251492.
To understand the pathogenesis and mechanisms involved in Fusarium biofilm formation on contact lenses, it is necessary to establish a robust in vitro model of such biofilms. As the first step in developing such a model, we monitored growth of MRL8609 FSSC 1-b in four different media (SDB, YNB, YPD, and RPMI) by determining the number of microconidia and percent hyphal elements formed in each medium. No significant difference was observed in the numbers of conidia/growth rates of MRL8609 FSSC 1-b grown in SDB, YPD, or RPMI (growth rate = 6.61 ± 0.095, 7.11 ± 0.17, or 6.15 ± 0.37 conidia/h, respectively; P > 0.05) (Fig. 1A and B). However, the growth rate of MRL8609 was significantly lower in YNB medium than in the other three media tested (growth rate = 4.77 ± 0.018 conidia/h; P < 0.05 for each comparison). Although no difference in the growth rates was observed between MRL8609 grown in SDB, YPD, or RPMI, the percentage of hyphae harvested from MRL8609 grown in SDB or YNB was higher than that when cultured in YPD or RPMI (Fig. (Fig.1C).1C). It is important to note that MRL8609 growth was not influenced by enriched medium, since SDB (an enriched medium) induced abundant hyphal formation while YPD (also an enriched medium) induced only minimal hyphal formation. Moreover, YNB (a minimal medium) induced significantly more hyphal formation by MRL8609 than YPD. Since clinical presentation of Fusarium keratitis is commonly associated with the presence of a large number of hyphal elements in affected tissues and hyphal elements are known to assist fungal invasion of tissue (12), and since SDB supported an optimal growth rate and maximum hyphal formation, we selected it as the optimal growth medium and incubation for 48 h as the optimal time needed to form a fully mature Fusarium biofilm.
Analysis of the gross morphologies of biofilms formed on contact lenses showed that although MRL8609 FSSC 1-b was able to form biofilms on all lenses examined (Fig. 1D to H), there was a difference in the abilities of the biofilms to adhere to the contact lenses tested. In this regard, biofilms formed by strain MRL8609 on etafilcon A and polymacon lenses were loosely attached to the surfaces of soft contact lenses and very easily dislodged when manipulated. In contrast, biofilms formed by strain MRL8609 on lotrafilcon A, balafilcon A, and alphafilcon A lenses adhered firmly to the surfaces of soft contact lenses. In general, C. albicans biofilms adhered firmly to the lenses, were compact, and were not easily detached, and no differences were observed in gross morphology and adhesion of biofilms formed by this pathogen on different lens types (data not shown).
Since the lenses tested in this study differ in their surface properties (surface charge and water content; Table Table1),1), we hypothesized that the ability of Fusarium to form biofilm will vary with different contact lenses. To test this hypothesis, we quantified biofilms formed on six commonly used lenses using the XTT-based method. Our analyses revealed that as measured by metabolic activity, strain MRL8609 FSSC 1-b formed significantly more biofilms on lotrafilcon A than on balafilcon A (P < 0.0001), galyfilcon A (P = 0.0029), etafilcon A (P = 0.0030), or alphafilcon A (P < 0.0001) lenses (Fig. (Fig.2).2). Additionally, significantly more biofilms were formed on polymacon lenses than on balafilcon A (P = 0.0003) or alphafilcon A (P = 0.0021) lenses. We also found that strain MRL8609 formed significantly more biofilms on both etafilcon A and galyfilcon A lenses than were formed on balafilcon A lenses (P = 0.001 for both comparisons). By way of contrast, the ISO 14729-recommended reference isolate ATCC 36031 FSSC 2-c failed to form biofilm on the lenses tested (data not shown). These studies revealed that among silicone hydrogel lenses, maximum biofilms were formed by strain MRL8609 FSSC 1-b on lotrafilcon A lenses while minimum biofilms were formed on balafilcon A lenses.
Since Candida is the second most common fungus causing keratitis, we evaluated its ability to form biofilms on different lenses. Metabolic activity assays revealed that in similarity to Fusarium isolates, the Candida isolate tested also formed significantly more biofilms on lotrafilcon A than on galyfilcon A (P < 0.0001), balafilcon A (P < 0.0001), etafilcon A (P = 0.0012), or alphafilcon A (P < 0.0001) lenses (Fig. (Fig.2).2). No difference was observed in Candida biofilms formed on lotrafilcon A and polymacon A lenses (P > 0.05).
Taken together, these results demonstrate that Fusarium and Candida can form biofilms on contact lenses and that this ability is influenced by the lens type.
Since the architecture of C. albicans biofilm is influenced by the substrate used (e.g., dentures or catheters) (15) and since contact lenses differ in their surface properties (Table (Table1),1), we hypothesized that the architecture of Fusarium biofilms formed on different contact lenses may also be influenced by the type of lenses used in this study. As shown in Fig. Fig.3,3, CSLM analyses showed that biofilms formed by strain MRL8609 FSSC 1-b on soft contact lenses were composed of profuse hyphae. Biofilms formed on lotrafilcon A and balafilcon A were characterized by numerous filaments with yellow staining within the hyphal elements, which resulted from dual staining with carbohydrate (green; ConA) and metabolically active (red; FUN-1) dyes (Fig. 3C and D). Biofilms formed on etafilcon A, galyfilcon A, and alphafilcon A lenses contained abundant extracellular matrix (Fig. 3A, B, and E, arrows). Moreover, biofilms formed on the polymacon lens had a diffuse, granular appearance, with ConA staining visible throughout the biofilm (Fig. (Fig.3F).3F). Additionally, biofilms formed by strain MRL8609 exhibited similar architecture and thicknesses at the center and periphery of the soft contact lenses (data not shown).
Analyses of biofilm thickness revealed that biofilms formed by strain MRL8609 FSSC 1-b on lenses with low water content were thinner than those formed on lenses with high water content. This pattern was observed for both nonionic and ionic lenses. Thus, for nonionic lenses, biofilms formed on the low-water-content lotrafilcon A and polymacon lenses (water content = 24% and 28%, respectively) were thinner than those formed on galyfilcon A and alphafilcon A, which have high water content (47% and 64%, respectively) (Fig. (Fig.4).4). Similarly, among ionic lens-associated biofilms, those formed on balafilcon A (with 36% water content) were thinner than biofilms formed on the etafilcon A lens (with 58% water content). These studies suggested a water content-dependent effect of the lens type on the ability of Fusarium to form biofilms on lenses. However, more-detailed studies are needed to determine the relevance of such an effect.
Taken together, our results clearly demonstrate that contact lens-associated Fusarium biofilms are hypha rich and have homogenous architecture, with some surface-dependent differences in biofilm architecture.
Next, we used CSLM to determine whether the architecture of lens-associated biofilms formed by C. albicans is also dependent on lens surface properties and whether this architecture is similar to that of Fusarium biofilms. As shown in Fig. 3G to L, C. albicans biofilm architecture was distinctive for each lens type. For example, C. albicans biofilm formed on etafilcon A lenses was composed primarily of yeast cells with an abundant extracellular matrix (Fig. (Fig.3G),3G), whereas biofilms formed on lotrafilcon A lenses consisted of sparse yeast cells interspersed within a diffuse matrix (Fig. (Fig.3I).3I). Moreover, C. albicans biofilms formed on balafilcon A were rich in hyphae with few yeast cells and a distinct granular extracellular matrix (Fig. (Fig.3J).3J). In contrast, C. albicans biofilms formed on galyfilcon A, alphafilcon A, and polymacon lenses were a mix of yeast and hyphae, with minimal detectable extracellular matrix (Fig. 3H, K, and L). Our analyses showed that the architecture of C. albicans biofilms formed at the periphery of lotrafilcon A and polymacon contact lenses differed from those formed at the center. Specifically, among biofilms formed on lotrafilcon A, the central area contained matrix-rich, dense, “mushroom-like” structures (data not shown), similar to those seen in bacterial biofilms (9, 17, 43, 44). In Candida biofilms formed on a polymacon lens, the peripheral region contained sparse yeast cells, while biofilms formed in the centers of the lenses contained abundant yeast and hyphal elements. Analyses of biofilm thickness formed on lenses revealed that C. albicans biofilms formed on balafilcon A lenses were significantly thicker than those formed on the other lenses (P < 0.0001 for all comparisons) (Fig. (Fig.4).4). There was no significant difference in thickness between biofilms formed by C. albicans on the remaining five lenses (P > 0.05), suggesting a lack of correlation between lens water content and biofilm thickness. These results suggested that lens surface properties modulate the morphology and architecture of Candida biofilms but not their thickness.
Having established the in vitro biofilm model, we used it to examine the abilities of three human keratitis-associated Fusarium isolates to form biofilms on the lotrafilcon A lens using the XTT assay. This contact lens was selected because it allowed abundant biofilm formation. As per ISO 14729 recommendations for testing activities of lens care solutions, the reaction of these disinfectants was stopped after the manufacturer-recommended contact time by adding DEB. Therefore, in our assays, we used DEB-treated cells as controls. To determine whether DEB by itself interfered with the ability of fungi to form biofilms on contact lenses, we compared the abilities of the three Fusarium species FSSC 1-b (MRL8609), FSSC 2-c (ATCC 36031), and FOSC 3-a (MRL8996) and C. albicans SC5314 to form a biofilm on lotrafilcon A lens in the absence or presence of DEB. The XTT assay revealed that exposure of cells to DEB reduced the XTT activity of the biofilm by almost 50% compared to that of the untreated control (Fig. (Fig.5).5). Although DEB reduced the level of biofilms formed by the isolates tested, the biofilms formed were significant and did not interfere with our ability to study the activity of contact lens care solutions against fungal biofilms. In contrast to the clinical isolates MRL8609 FSSC 1-b and MRL8996 FOSC 3-a, the ATCC 36031 reference isolate FSSC 2-c lacked the ability to form a biofilm in the presence or absence of DEB (Fig. (Fig.55).
Determination of CFU after 4 h of incubation (ISO 14729 guidelines) is the standard method for evaluating activities of lens care solutions against planktonically grown microbes. Moreover, since biofilm growth is assessed by using the XTT assay and not by CFU determination and to allow comparison of the activities of lens care solutions against planktonic and biofilm-associated cells, in the current study we evaluated the effects of MoistureLoc and MultiPlus against planktonically grown fusaria and C. albicans using both the CFU and XTT assays. As shown in Fig. Fig.6,6, planktonically grown fusaria (MRL8609 [FSSC 1-b], ATCC 36031 [FSSC 2-c], and MRL8996 [FOSC 3-a]) and C. albicans isolate SC5314 were susceptible to both MoistureLoc and MultiPlus solutions. Incubation of planktonically grown cells in the lens care solutions for 4 h, as suggested by the manufacturer and the ISO document, resulted in a >1 log reduction of CFU for the three fusaria isolates (Fig. 6A to C) and the C. albicans isolate (Fig. (Fig.6G)6G) tested. Results from the XTT assay also revealed similar patterns (Fig. 6B, D, F, and H), with complete inhibition of metabolic activity observed after 4 h of incubation. These results demonstrated that the ISO- and XTT-based methods generate similar results regarding the activities of lens case solutions, showing that both MoistureLoc and MultiPlus are active against planktonically grown Fusarium and C. albicans.
Next, we used the in vitro model we developed to test whether MoistureLoc and MultiPlus could inhibit biofilm formation by MRL8609 (FSSC 1-b), MRL8996 (FOSC 3-a), or C. albicans SC5314 on the lotrafilcon A lens. In contrast to the activities of lens care solutions against planktonically grown fungi (percent reductions of 53 ± 1% and 51 ± 4% compared to that for the untreated control [mean ± standard deviation [SD]) (Table (Table2),2), these solutions only partially inhibited biofilm formation following a 4-h exposure. Extending the treatment period for both solutions to 20 h resulted in slightly more inhibition of biofilms formed by MRL8906. In contrast, biofilms formed by MRL8996 were not significantly reduced by MultiPlus or MoistureLoc solutions (Table (Table2).2). However, in both cases, the lens care solutions failed to completely inhibit biofilm formation by the two phylogenetically divergent species of Fusarium tested. In contrast to the partial inhibitory effect of MoistureLoc and MultiPlus solutions against Fusarium biofilms, these agents had no inhibitory effect on biofilms formed on contact lenses at 4 h or 20 h by C. albicans isolate SC5314 (Table (Table22).
Taken together, our studies showed that biofilms formed by Fusarium isolates exhibited reduced susceptibility to MoistureLoc and MultiPlus lens care solutions, while C. albicans biofilm was completely resistant to these solutions.
In this study, we developed a reproducible in vitro model of fungal biofilm formation on contact lenses and demonstrated that Fusarium and Candida—major etiological agents of fungal keratitis—can form biofilms on contact lenses. Using the in vitro model, we showed that the amount of lens-associated biofilms formed and their composition and architecture were dependent on the species used and the type of lens tested. We also demonstrated that the XTT method can be used to evaluate fungal biofilm formation on contact lenses and to determine the inhibitory effects of lens care solutions, e.g., MoistureLoc and MultiPlus, on these biofilms. Our studies revealed that while MoistureLoc and MultiPlus lens care solutions exhibited potent activity against planktonically grown species of Fusarium within the FSSC and FOSC and C. albicans, these solutions were less effective against Fusarium biofilms formed on contact lenses and had no effect on Candida biofilms formed on contact lenses under the same conditions.
Gross morphological data showed that Fusarium and Candida differ in their ability to form biofilms on contact lenses. While the attachment of Fusarium biofilms varied with the lens type, Candida biofilms were more tightly bound to all of the contact lenses tested. The loose association of Fusarium mats was recently observed in studies investigating the attachment and penetration of contact lenses (4). Ahearn et al. (4) showed that Fusarium mats tended to be loosely associated with the lenses and could be released from the lenses by vigorous shaking or rinsing of the lens. Similar to our findings, attachment to the lens surface ranged from a loose association of conidia and hyphae to firmly attached hyphae that were difficult or impossible to remove (4). It is noteworthy that Ahearn et al. (4) reported that the two isolates of “F. solani” tested differed in their ability to attach to hydrogel contact lenses. Given that multilocus DNA sequence typing has shown that these isolates represent two phylogenetically distinct species within the FSSC (39) (GSU AFR4 [FSSC 1-b] and GSU 81036 [FSSC 4-a]), it is possible that some of the observed differences may be species or strain specific. Because at least 18 medically relevant phylogenetically distinct species are nested within the FSSC (51) and because human pathogenic fusaria within other species complexes are also phylgenetically diverse (39), future studies on biofilm formation employing fusaria should adopt the multilocus haplotype nomenclatural system initially presented in the 2005-2006 multistate keratitis outbreak investigation so that the findings from different studies can be compared directly. The difference in the abilities of the fusaria and C. albicans to tightly adhere to soft contact lenses could be due to the high propensity of C. albicans to adhere to surfaces (22). Another possible reason could be the ability of Candida to grow as yeast and hyphal forms in the biofilm while only hyphae were formed in Fusarium biofilms. In this regard, earlier studies showed that C. albicans forms a biphasic biofilm, with yeast cells forming an anchor that is strongly attached to the substrate (15). Therefore, the basal layer may account for the strong attachment of Candida biofilms on contact lenses.
Metabolic activity-based quantification and thickness measurements demonstrated that Fusarium and Candida can form biofilms on commonly used soft contact lenses and that the amount of biofilms formed by Fusarium was influenced by the surface properties of the lens used. In an earlier study, Chandra et al. (15) showed that modification of a polyurethane surface by adding 6% polyethylene oxide (thus modifying the surface hydrophobicity and charge) led to minimal biofilm formation by C. albicans on the resulting surface. A similar effect of the substrate on bacterial biofilms was shown by Okajima et al. (41). Since the lens types tested in the current study differed with respect to water content, ionic state, monomer backbone, and surface treatment, it is possible that these variables may influence the ability of fungi to form biofilms on contact lenses. Moreover, the lenses used in the current study were procured from commercial sources, and their surface properties varied in an unmatched manner. Therefore, it was not possible to unequivocally show a direct relationship between the lens ionic charge and water content. As such, additional comparative studies employing matched lens types differing in only one variable are necessary to determine the influence of surface properties on the ability of microbes to form biofilms. It is notable that the lenses used in the current study were fresh; surface properties may change in lenses that have been worn on an extended basis, and this may contribute to different levels of biofilm formation. This aspect needs to be investigated further.
Our CSLM analyses showed that while Fusarium biofilms were composed of a homogenous layered mesh of hyphal elements, Candida biofilms had a heterogeneous architecture consisting of yeast and hyphal elements. Moreover, Fusarium tended to form a uniform biofilm at the center and the periphery of the contact lenses, while the architecture of Candida biofilms formed in the center and at the periphery of the lens differed. Such differences in biofilm characteristics between organisms are not surprising and have been demonstrated earlier for different bacteria and for different candida species. For example, Kuhn et al. (31) showed that C. albicans isolates produced more biofilm on silicone elastomer than Candida parapsilosis, Candida glabrata, and Candida tropicalis isolates and that biofilms formed by C. parapsilosis had a patchy, “mushroom-like” structure while those formed by C. albicans were more continuous in nature. Furthermore, Chandra et al. (13) showed that Candida biofilms formed on a denture surface were composed only of yeast cells while those formed on catheter discs consisted of a basal yeast layer overlaid by one that is hyphal.
An alternative explanation for the observed differences in biofilm formation between central and peripheral regions of the lenses for Candida biofilms may be associated with the design of the contact lens, since all lenses used in our study had powers of +1.50 diopters and were slightly thicker in the center than the periphery. It is possible that the lenses showing differences in peripheral-to-central thickness may have an irregular surface texture which can influence biofilm formation. In this regard, Chandra et al. (13, 14) showed that C. albicans biofilms were thicker at raised areas present on the surface of dentures. However, this does not explain why no differences were observed in biofilms formed by Fusarium at the center and peripheral areas of the lenses.
Our data did not demonstrate a direct correlation between biofilm metabolic activity, determined by the XTT assay, and its thickness, measured by CSLM. Such a lack of correlation is not surprising, since biofilms may contain fewer fungal cells embedded in a thick matrix, as reported earlier for Candida biofilms formed on modified surfaces (15).
Comparison of the antifungal activities of MoistureLoc against Fusarium and Candida cells grown planktonically or as a biofilm showed that planktonically grown cells were susceptible to both solutions using the manufacturer-recommended incubation time (4 h), as well as during an extended incubation (20 h). In contrast, Fusarium biofilms formed on contact lens were less susceptible than planktonically grown cells to MoistureLoc and MultiPlus solutions. It was not possible to completely eradicate Fusarium biofilms even with an extended treatment period (20 h). Interestingly, the FOSC 3-a strain MRL8996, which was obtained from a contact lens wearer with extensive corneal damage, exhibited resistance to the lens care solutions even after the extended incubation of 20 h, suggesting that some fusaria may be more resistant to these solutions than others. In addition, Candida biofilms were completely resistant to MoistureLoc and MultiPlus solutions. Our data are in agreement with those of May et al. (36), who examined the antimicrobial activities of a number of disinfectant solutions, recommended for use with rigid gas-permeable or hard contact lenses, against planktonic and adhered cells of bacteria and C. albicans. Their findings showed that while most solutions gave marked inhibition (99.99% reduction within 4 h) of planktonically growing cells, cells of all microorganisms adhering to wells of polyethylene contact lens cases showed various degrees of survival after 4, 6, and 12 h of exposure to most contact lens solutions. Similarly, Wilson et al. (49) showed that biofilms of Pseudomonas aeruginosa, Serratia marcescens, Staphylococcus epidermidis, Streptococcus pyogenes, and C. albicans formed on wells of polyethylene contact lens cases retained viability with certain soft contact lens disinfectant solutions after exposure for the manufacturer's minimum recommended disinfection times.
Lens care solutions in contact lens cases can become concentrated and often form dried films due to evaporation and because these cases are often topped off by users instead of being emptied and then refilled regularly. The effect of MoistureLoc drying on the growth of F. solani-F. oxysporum complex was investigated by Zhang et al. (51), who showed that MoistureLoc films on plastic surfaces of lens cases can support the growth of selective isolates of this fungus. The in vitro model developed in the current study was not suited to addressing this question. However, we found that overnight (20 h) incubation of MoistureLoc and MultiPlus solutions at 37°C did not lead to any loss of activity. Since an overnight incubation is not expected to result in major evaporation, the final concentration of active ingredients in the lens care solutions is not expected to change noticeably. Detailed investigations are needed to elucidate the role of temperature and/or compliance-related fluctuations in the concentration of lens care solutions on their antifungal activities.
Evans and Dart (21) showed that bacteria grown as biofilm show reduced susceptibility to contact lens disinfecting solutions compared to that of planktonic cells. Similarly, Kuhn et al. (32) showed that catheter-associated biofilms formed by several Candida species were resistant to commonly used antifungal agents. Other investigators have also shown that Candida biofilms were resistant to antifungal agents while the planktonic form of the same isolate was susceptible (7, 8, 13, 14, 30). It is clear that the mode of microbial growth (planktonic versus biofilm) influences the susceptibility of microbial cells to lens care solutions, where biofilms express a resistance phenotype. In contrast to our findings, Dyavaiah et al. (19) recently reported that biofilms formed by six Fusarium keratitis isolates on contact lenses were susceptible to MoistureLoc. The reason for the disagreement between our results and those reported by Dyavaiah et al. (19) may be related to differences in methods used to form biofilm. In our study, we allowed Fusarium to form biofilms in the presence of growth medium and demonstrated the presence of a carbohydrate-rich extracellular matrix (an important characteristic of biofilms) in which fungal elements were embedded. In an earlier study, Simmons et al. (46) monitored lens colonization using a method in which fungal isolates were incubated in the presence of a balanced salt solution with lens disinfectants. Dyavaiah et al. (19) followed this method to evaluate fungal biofilm formation on lenses. However, the conditions used by these investigators promote initial attachment of the fungi but do not allow biofilm formation. In agreement with the results reported by these investigators, we also found, using both the industry standard ISO 14729 and XTT-based methods, that planktonic Fusarium and Candida cells were susceptible to lens disinfectants. The recent Fusarium keratitis outbreaks have been associated with loss of antimicrobial activity during contact lens storage, selective growth of Fusarium in globules of partially dried deposits of MoistureLoc, use of a “no-rub” procedure to care for contact lenses, a general decrease in effectiveness of MoistureLoc solution compared with other lens solutions, and binding of Fusarium to different contact lens materials (4, 26). Our current findings suggest that the ability of Fusarium spp. to form biofilms on contact lenses may also contribute to their reduced susceptibility to MoistureLoc and MultiPlus.
Our results revealed that two recently isolated keratitis-causing fusaria formed robust biofilms, while the ATCC 36031 reference isolate (recommended by the ISO guidelines for evaluating the antimicrobial effects of lens disinfectants) failed to form a biofilm on soft contact lenses. The finding that the ATCC isolate did not form a biofilm is not surprising, since growth and virulence phenotypes of laboratory strains tend to change with multiple laboratory passage. Thus, it is possible that the ATCC 36031 isolate tested in this study may also have lost its virulence and hence failed to form a biofilm on soft contact lenses. Another compelling reason why the ATCC isolate recommended by the ISO should be changed is that in contrast to the two fusaria included in our experiments (i.e., FSSC 1-b [MRL8609] and FOSC 3-a [MRL8996]), which were well represented in the 2005-2006 keratitis outbreaks within the United States (16), the FSSC 2-c multilocus haplotype represented by ATCC 36031 appears to be a rare genotype in that it is represented only by a single strain from Nigeria isolated from a corneal ulcer in the mid-1970s (51). Our study, like those of others (33), revealed other inadequacies in the testing procedures recommended by the FDA Premarket Notification [510(k)] Guidance Document for Contact Lens Care Products (48), where only one Fusarium isolate (24) is used to challenge the disinfectants. Currently, the disinfecting effects of contact lens care solutions for licensing purposes continue to be tested against planktonically grown microbial cells. As stated by McLaughlin-Borlace et al. (37), microbial contamination of lens storage cases is widespread, for both asymptomatic wearers and those with corneal infections, despite good compliance. Since biofilms are intimately associated with contact lenses and their carrier cases, it will be prudent to incorporate testing for activity against biofilms as part of the licensing procedures. Therefore, we propose that testing of new lens care solutions/disinfectants should include testing for activity against planktonic and biofilm-associated microbes. Moreover, a representative of the most common genotypes involved in the recent Fusarium keratitis outbreaks, such as FOSC 3-a, FSSC 1-a, and FSSC 2-d (16, 39), should be considered for inclusion among reference test isolates.
In conclusion, we established an in vitro model of Candida and Fusarium biofilm formation on contact lenses. The metabolic activity, thickness, and architecture of these fungal biofilms were dependent on lens type. Importantly, Fusarium biofilms were less susceptible than planktonically grown cells to MoistureLoc and MultiPlus solutions, which exhibited strain- and time-dependent activity against contact lens-associated Fusarium biofilms but were inactive against Candida biofilms. It is possible that the recently reported increase in the incidence of Fusarium keratitis among contact lens wearers may be partly due to formation of biofilms by fusaria on lens, lens cases, corneal tissue, or a combination of these surfaces. The role of biofilm formation in fungal keratitis needs to be investigated further using an expanded set of keratitis-related Fusarium isolates. This work is currently under way.
The in vitro model we developed for lens-associated fungal biofilms will be helpful for better understanding the biology, pathogenesis, and antifungal resistance of Fusarium and Candida biofilms and their role in contact lens-related fungal keratitis. In addition, this model has utility in evaluation of lens care solutions for their antibiofilm activity.
This work was supported by funds from the NIH (grants R01 DE017486-01A1 and R01DE 13932-4), the Bristol Myers Squibb Freedom to Discover Award to M.A.G., an American Heart Association (Scientist Development Grant 0335313N) award to P.K.M., and NIH grant P30 EY11373 (to E.P.) and Research to Prevent Blindness (RPB) Foundation and Ohio Lions Eye Research Foundation grants to J.H.L. E.P. is a recipient of an RPB Senior Investigator Award. The assistance of the Confocal Scanning Laser Microscopy core facility (NCI grant P30CA43703-12) at Case Western Reserve University is gratefully acknowledged.
Published ahead of print on 12 November 2007.