Does inflation of the catheter balloon with triclosan stop crystalline biofilm formation by the mutant strains?
Two of the mutant strains, M48 and M55, and their parental strain, P. mirabilis B2, were inoculated into bladder models that were fitted with all-silicone catheters. Control models in which catheter balloons were inflated with sterile water and test models with catheters primed with triclosan (10 mg/ml in 5% polyethylene glycol) in the retention balloons were set up for each mutant and the wild type. The urine was supplied to the models until the catheters became blocked or for a maximum of 48 h. The times to blockage of the catheters recorded from triplicate experiments are shown in Table . It is clear that triclosan prevented catheter blockage by the wild type and strain M48. In the case of strain M55, however, triclosan failed to inhibit encrustation and catheter blockage. The mean times to blockage of 36 h for control catheters and 44 h for test catheters were not significantly different (P > 0.05).
Effect of triclosan on times to catheter blockage, rates of catheter encrustation, pHs, and viable cells in bladder models inoculated with the wild-type P. mirabilis B2 strain and its mutantsa
The pHs and the viable cell populations of the residual urine samples in the models at the time of blockage or at 48 h are presented in Table . The mean pHs in the test models inoculated with strains B2 and M48 were significantly lower (P < 0.05) than those in all the other test and control models. The mean pHs of and viable cell populations in the urine in the test models inoculated with M55, however, were not significantly different from those in the control models (P > 0.05).
Calcium and magnesium analyses were performed on the catheters removed from models at the time of blockage or at 48 h. The rates of encrustation on the control and test catheters were calculated and are summarized in Table . Triclosan significantly reduced the encrustation rates for B2 and M48 (P < 0.05) but not for M55. The MIC of triclosan for M55 was 40 μg/ml, compared to 2 μg/ml for M48. At first sight, it might seem strange that urinary concentrations of triclosan around 0.1 mg/liter inhibit catheter encrustation by a strain with an MIC of 2.0 mg/liter. A possible explanation, however, is that as the catheterized bladder is a continuous culture system, a concentration of an antibacterial agent that merely decreases the growth rate could be sufficient to ensure a significant reduction in the bacterial population and pH of the residual urine.
Triclosan is a widely used antibacterial agent that has been incorporated into an extensive range of health care and consumer products. Concern has been expressed that the scale of its exploitation might result in the selection of resistant organisms (23
). These concerns have been compounded by suggestions that resistance to triclosan might be linked to cross-resistance to antibiotics (15
There is certainly evidence that, under laboratory conditions, exposure to triclosan has resulted in the reduced susceptibility of some bacterial species to this biocide. The exposure of an Escherichia coli
strain with an MIC of 0.8 mg/liter to triclosan has resulted in the selection of mutants with MICs ranging from 2 to 80 mg/liter (18
). The repeated subculture in triclosan (0.01 mg/liter) of a Staphylococcus aureus
strain with an MIC of 0.025 mg/liter selected for stable mutants with MICs of up to 1 mg/liter (28
). The decreased susceptibility to triclosan was not associated, however, with an increase in resistance to any of six antibiotics. Ledder et al. (14
) exposed a wide range of enteric, skin, and oral species to a total of 10 subcultures on agar containing gradients of triclosan. The results suggested that the selection of strains with increased tolerance to triclosan was not widespread, with most species showing no alteration in their MICs to triclosan. The exceptions were an E. coli
strain for which the MIC increased from 0.00024 to 16 mg/liter and a Klebsiella oxytoca
strain (from 0.00012 to 0.49 mg/liter). These strains exhibited no increase in MICs to antibiotics. In contrast, other studies have reported that E. coli
variants that had acquired reduced susceptibility to triclosan in the laboratory had also gained cross-resistance to antibiotics (1
). A recent study by Karatzas et al. (10
), for example, reported that prolonged exposure of Salmonella enterica
serovar Typhimurium to triclosan in the laboratory selected for strains having a 2,000-fold increase in their MICs (up to 64 mg/liter). These variants were also less susceptible to chloramphenicol, tetracycline, ampicillin, and acriflavine. Evidence was presented that resistance was due to the overexpression of an efflux pump. The results reported in Table indicate that P. mirabilis
can be added to the list of species that have developed elevated MICs to triclosan after exposure to the biocide under laboratory conditions. In this case, however, the decreased susceptibility to triclosan was not associated with changes in susceptibility to antibiotics.
Several groups have investigated whether the extensive use of triclosan in health care, dental, and domestic situations has resulted in the selection of resistant organisms. While strains of S. aureus
with MICs of 2 to 4 mg/liter were isolated from patients who had daily baths with triclosan (4
), the general picture is that acquired resistance to triclosan has rarely been found in organisms isolated from clinical sources (29
). The extended use of triclosan in dental products has not led to decreased susceptibility to triclosan and other antibacterial agents (5
). A large-scale double-blind randomized intervention trial showed that the use of triclosan-containing (0.2%) liquid hand-washing soap in households for periods of 12 months had no effect on the susceptibility of either gram-negative bacteria or staphylococci from the hands of occupants. Neither did the use of this antibacterial product lead to any significant increases in antibacterial drug resistance (2
). A study of the effect of the exposure of domestic-drain biofilm microcosms to low levels of triclosan showed no changes in antimicrobial susceptibility (17
). Those authors concluded that the emergence of antibiotic resistance through the domestic use of triclosan is improbable (17
It has been argued that there is no convincing evidence in the literature that the use of triclosan has resulted in the development of clinically significant levels of resistance or associated antibiotic resistance (22
). It is clear that the MICs recorded for “resistant” strains are orders of magnitude below the concentrations of triclosan used in practice (2 to 20 g/liter). Strains with elevated MICs have not been able to survive “in-use” concentrations of the biocide (4
). In the case of the triclosan strategy to prevent catheter encrustation by P. mirabilis
, however, this argument does not hold. Although triclosan at 10 mg/ml is used to inflate catheter balloons, the concentrations of the biocide diffusing into the residual bladder urine are around 0.1 mg/liter (8
), very close to the MICs for this species. The strain with an MIC of 40 mg/liter that had been selected by prior exposure to triclosan did not respond to the strategy (Table ). The possibility of the selection of strains resistant to triclosan in this context is thus of more concern.
We have not advocated the use of triclosan to prevent or control catheter-associated urinary tract infections. Indeed, pathogens such as Pseudomonas aeruginosa
, Serratia marcescens
, and Morganella morganii
, which infect the catheterized urinary tract, are not sensitive to triclosan and can form extensive biofilms on catheters that have been primed with triclosan (8
). We have suggested that the strategy should be deployed only when encrustation of the catheters is being induced by P. mirabilis
. All 118 clinical and environmental isolates of P. mirabilis
that we have tested are uniformly susceptible to triclosan, with MICs ranging from 0.1 to 0.3 mg/liter (9
). Concerns about the development of resistance to triclosan should not, therefore, currently preclude its use in the prevention of the complication of catheter encrustation. In view of the results presented here, however, in any clinical trial or subsequent clinical use of the strategy, it will be important to monitor the urinary flora of the catheterized patients for signs of the emergence of less susceptible strains or the selection of intrinsically resistant species that might coincidently also be resistant to antibiotics.