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Background

Methicillin-resistant S. aureus (MRSA) with low susceptibility to glycopeptides is uncommon.

Case presentation

The case of a 50-year-old non-drug addict patient presenting with tricuspid valve infective endocarditis (IE) by MRSA resistant to vancomycin and linezolid is presented. There was response only to quinupristin/dalfopristin. He had a motorcycling accident four years before undergoing right above-the-knee amputation and orthopaedic fixation of the left limb. There were multiple episodes of left MRSA-osteomyelitis controlled after surgery and vancomycin therapy. MRSA isolated from the blood at the time of IE presented with the same profile than the isolated four years earlier. Sequential treatment with teicoplanin-cotrimoxazole and Linezolid associated to vancomycin – rifampicin – cotrimoxazole had no improvement. Infection was controlled after 28 days of therapy with quinupristin/dalfopristin.

Conclusion

The literature presents only a few cases of MRSA IE not susceptible to glycopeptides in not drug addicted patients. This case shows the comparison of a highly-resistant MRSA after previous S. aureus osteomyelitis treated with glycopeptides. This is the first description of successful treatment of resistant-MRSA IE of the tricuspid valve complicated by multiple pulmonary septic infarction with quinupristin/dalfopristin

An increase in the distribution of vancomycin MIC values among methicillin (meticillin)-resistant Staphylococcus aureus (MRSA) isolates has been noted. It is postulated that the shift in vancomycin MIC values may be associated with a concurrent rise in the MIC values of other anti-MRSA agents. Scant data are available on the correlation between vancomycin MIC values and the MIC values of other anti-MRSA agents. This study examined the correlation between vancomycin MIC values and the MIC values of daptomycin, linezolid, tigecycline, and teicoplanin among 120 patients with bloodstream infections caused by MRSA at a tertiary care hospital between January 2005 and May 2007. For each included patient, the MIC values of the antibiotics under study were determined by the Etest method and were separated into the following two categories: day 1 (index) and post-day 1 (subsequent). For subsequent isolates, the MIC values for each antibiotic from the post-day 1 terminal isolate were used. Among the index isolates, there was a significant correlation (P value, <0.01) between the MIC values for vancomycin and daptomycin and between the MIC values for vancomycin and teicoplanin. The MIC values for daptomycin were significantly correlated with linezolid, tigecycline, and teicoplanin MIC values. Among the 48 patients with subsequent isolates, vancomycin MIC values were significantly correlated with MIC values for daptomycin, linezolid, and teicoplanin (ρ value of ≥0.38 for all comparisons). This study documented an association between vancomycin MIC values and the MIC values of other anti-MRSA antibiotics among patients with bloodstream infections caused by MRSA primarily treated with vancomycin.

TD-1792 is a glycopeptide-cephalosporin heterodimer antibiotic with activity against a broad spectrum of Gram-positive pathogens that includes methicillin-susceptible and -resistant Staphylococcus aureus. The objective of the present study was to evaluate the in vitro activity of TD-1792 against a collection of clinical isolates of vancomycin-intermediate Staphylococcus spp. (VISS), heteroresistant VISS (hVISS), and vancomycin-resistant S. aureus (VRSA). The TD-1792, vancomycin, daptomycin, linezolid, and quinupristin-dalfopristin MICs and minimum bactericidal concentrations (MBCs) were determined for 50 VISS/hVISS isolates and 3 VRSA isolates. Time-kill experiments (TKs) were then performed over 24 h with two vancomycin-intermediate S. aureus strains and two VRSA strains, using each agent at multiples of the MIC. TD-1792 and daptomycin were also evaluated in the presence and absence of 50% human serum to determine the effects of the proteins on their activities. Most of the VISS/hVISS isolates were susceptible to all agents except vancomycin. TD-1792 exhibited the lowest MIC values (MIC90 = 0.125 μg/ml), followed by quinupristin-dalfopristin and daptomycin (MIC90 = 1 μg/ml) and then linezolid (MIC90 = 2 μg/ml). The presence of serum resulted in a 2- to 8-fold increase in the TD-1792 and daptomycin MIC values. In TKs, QD demonstrated bactericidal activity at multiples of the MIC that simulated therapeutic levels, whereas linezolid was only bacteriostatic. Both TD-1792 and daptomycin demonstrated rapid bactericidal activities against all isolates tested. The presence of proteins had only a minimal impact on the activity of TD-1792 in TKs. TD-1792 exhibited significant in vitro activity against multidrug-resistant Staphylococcus isolates and represents a promising candidate for the treatment of infections caused by Gram-positive organisms.

The in vitro activity of daptomycin was compared with those of vancomycin, linezolid, and quinupristin-dalfopristin against a variety (n = 203) of gram-positive bacteria, including methicillin-resistant Staphylococcus aureus and S. epidermidis (MRSA and MRSE, respectively), vancomycin-resistant enterococci (VRE), and vancomycin-intermediate S. aureus (VISA). Overall, daptomycin was more active against all organisms tested, except Enterococcus faecium and VISA, against which its activity was similar to that of quinupristin-dalfopristin. In time-kill studies with MRSA, MRSE, VRE, and VISA, daptomycin demonstrated greater bactericidal activity than all other drugs tested, killing ≥3 log CFU/ml by 8 h. Daptomycin may be a potential alternative drug therapy for multidrug-resistant gram-positive organisms and warrants further investigation.

Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) infections have increased dramatically over the last two decades. The types of infections can range from complicated skin and skin structure infections (cSSSI) to pneumonia and endocarditis. Oral antimicrobial therapy, such as trimethoprim-sulfamethoxazole, clindamycin, long-acting tetracyclines, or linezolid may provide enhanced benefit to those with uncomplicated cutaneous lesions when used in conjunction with incision and drainage in an outpatient setting. However, resistance, susceptibilities, patient-specific circumstances, and adverse effects can impact a healthcare professional’s choice of antibiotics. In patients with complicated infections requiring hospitalization or parenteral treatment, vancomycin remains the drug of choice, even though increased resistance and decreased efficacy have crept into clinical practice. Linezolid, quinupristin/dalfopristin, daptomycin, and tigecycline are alternative intravenous agents for the treatment of CA-MRSA. Investigational agents such as dalbavancin, telavancin, oritivancin, iclaprim, ceftobiprole, ceftaroline, and others may expand our therapeutic armamentarium for the treatment of infections caused by CA-MRSA in the future.

Vancomycin and daptomycin MIC results for 1,800 randomly selected oxacillin (methicillin [meticillin])-resistant Staphylococcus aureus (MRSA) bloodstream isolates from nine U.S. hospitals (collected from 2002 to 2006) were determined by a reference broth microdilution (BMD) method using frozen-form panels with precise incremental dilutions and by the Etest technique. The Etest provided vancomycin and daptomycin MIC results that were consistently higher (0.5 to 1.5 log2 dilution steps) than those provided by the reference BMD method. The dominant MRSA population (91.2% of MRSA isolates) would be categorized as vancomycin nonsusceptible by the MIC results from the Etest method if the susceptibility breakpoint was adjusted downward to ≤1 μg/ml, as suggested by clinical outcome studies.

This study used CLSI broth microdilution to test the activity of telavancin and comparator antimicrobial agents against 67 methicillin (meticillin)-resistant Staphylococcus aureus (MRSA) isolates. Twenty-six vancomycin-intermediate S. aureus (VISA) strains were among the isolates tested; all strains were susceptible to telavancin at ≤1 μg/ml, whereas 12/26 (46%) of these isolates were nonsusceptible to daptomycin at the same concentration. All strains were susceptible to quinupristin-dalfopristin, while resistance was found to all other drugs tested. Telavancin demonstrated potent activity against all vancomycin-susceptible isolates as well as against heterogeneously VISA and VISA resistance phenotypes. In multistep resistance selection studies, telavancin yielded one stable mutant after 43 days in one MRSA strain out of the 10 MRSA strains tested with the MIC rising eightfold from 0.25 μg/ml (parent) to 2 μg/ml. MICs for this clone did not increase further when passages were continued for the maximum 50 days. In contrast, daptomycin selected stable resistant clones (MIC increase of >4×) after 14 to 35 days in 4 of 10 MRSA strains with MICs increasing from 1 to 2 μg/ml (parents) to 4 to 8 μg/ml (resistant clones). Sequencing analysis of daptomycin resistance determinants revealed point mutations in the mprF genes of all four stable daptomycin-resistant clones. Teicoplanin gave rise to resistant clones after 14 to 21 days in 2 of 10 MRSA strains with MICs rising from 1 to 2 μg/ml (parents) to 4 to 16 μg/ml (stable resistant clones). Linezolid selected stable resistant clones after 22 to 48 days in 2 of 10 MRSA strains with MICs rising from 2 to 4 μg/ml (parents) to 32 μg/ml (resistant clones). Vancomycin yielded no resistant clones in 10 MRSA strains tested; however, MICs increased two- to fourfold from 1 to 8 μg/ml to 2 to 16 μg/ml after 50 days. No cross-resistance was found with any clone/antimicrobial combination. The two enterococci developed resistance to daptomycin, and one developed resistance to linezolid. Single-step mutation frequencies for telavancin (<4.0 × 10−11 to <2.9 × 10−10 at 2× MIC) were lower than the spontaneous mutation frequencies obtained with the comparators.

We describe here a clinical daptomycin treatment failure in a patient with recurrent methicillin-resistant Staphylococcus aureus (MRSA) bacteremia in whom daptomycin was administered after a failed empirical treatment course with vancomycin and piperacillin-tazobactam. We had the opportunity to compare the genome sequences of an isogenic pair of daptomycin-susceptible and -resistant MRSA isolates obtained before and after initiation of daptomycin therapy, respectively. The genotype of both isolates was USA800, ST5, SCCmec type IV, agr type II. There was no increase in cell wall thickness in the daptomycin-resistant strain despite having decreased susceptibility to both vancomycin and daptomycin. By comparing the genome sequences by pyrosequencing, we identified a polymorphism (S337L) in the tenth transmembrane segment of the multiple peptide resistance factor, MprF, encoding lysyl phosphatidylglycerol transferase. This enzyme has been shown previously to promote repulsion of daptomycin at the cell surface by addition of positively charged lysine to phosphatidylglycerol. Also, the hlb open reading frame (ORF) encoding the β-toxin was interrupted by a prophage in the daptomycin-susceptible strain; this phage was missing in the daptomycin-resistant isolate and the hlb ORF was restored. Loss of the phage in the resistant isolate also resulted in loss of the virulence factor genes clpP, scn, and sak. This is the first study to use pyrosequencing to compare the genomes of a daptomycin-susceptible/resistant MRSA isolate pair obtained during failed daptomycin therapy in humans.

The state of Hawai‘i has the highest prevalence of methicillin-resistant Staphylococcus aureus (MRSA) infection in the United States. Since vancomycin is the most frequently-prescribed antibiotic for healthcare-associated MRSA infection, there is concern for development of vancomycin resistance. We report on a 61 year-old woman with history of previous successful treatments of MRSA bacteremia with vancomycin. She was later hospitalized for catheter-related MRSA bacteremia that persisted despite vancomycin treatment. The vancomycin minimal inhibitory concentration (MIC) was initially 1–2 µg/ml, suggesting susceptibility, but changed to 4µg/ml. At this level, the organism was classified as a vancomycin-intermediate Staphylococcus aureus (VISA). Therapy was changed from vancomycin to daptomycin, and the patient's blood cultures were sterilized. High suspicion of VISA should be raised in MRSA-infected patients who fail or have a history of vancomycin therapy so that additional susceptibility testing and appropriate antibiotic therapy can be promptly commenced to reduce the morbidity associated with VISA infection.

This study investigated “creep” in vancomycin and daptomycin MICs among methicillin-resistant Staphylococcus aureus (MRSA) isolates from blood cultures over a 5-year period in a hospital in the United Kingdom, using different susceptibility testing methods. Trends in vancomycin and daptomycin susceptibility were evaluated by using Etest performed prospectively on isolates in routine clinical practice from December 2007 to December 2010 (n = 102). Comparison was made to results from prospective testing of subcultures at the Scottish MRSA Reference Laboratory, using an automated system (Vitek 2) and retrospective testing (Etest and CLSI reference broth microdilution [BMD] method) of stored isolates from 2006 to 2010 (n = 208). Spearman's rank correlations revealed a significant increase in vancomycin MIC (P = 0.012) and a significant decrease in daptomycin MIC (P = 0.03) by year of study for Etest results from the time of isolation. However, neither trend was replicated in MICs from automated or retrospective testing. The Friedman test revealed a significant difference between vancomycin MICs obtained from the same samples by different testing methods (χ2 [3 degrees of freedom] = 97; P < 0.001). MICs from automated testing and Etest analysis of stored isolates were significantly lower than those from Etest analysis at the time of isolation for both antibiotics (P < 0.001). Effects of storage on the MIC appeared within the first 6 months of storage. Inconsistent evidence on vancomycin MIC creep and the relevance of the MIC to clinical outcome may arise from differences in susceptibility testing methods, including storage of isolates. There is a need to standardize and streamline susceptibility testing of vancomycin against MRSA.

The efficacies of daptomycin, teicoplanin, and vancomycin were compared in the therapy of experimental Staphylococcus aureus endocarditis. Rabbits infected with either of two methicillin-susceptible strains (SA-12871 or its moderately teicoplanin-resistant derivative SA-12873) or a methicillin-resistant S. aureus strain (MRSA-494) were treated with daptomycin, 8 mg/kg of body weight, every 8 h; teicoplanin, 12.5 mg/kg (low-dose teicoplanin [teicoplanin-LD], excluding MRSA-494) or 40 mg/kg (high-dose teicoplanin [teicoplanin-HD]) every 12 h; or vancomycin, 17.5 mg/kg every 6 h, for 4 days. Compared with no treatment daptomycin, teicoplamin-HD, and vancomycin significantly reduced bacterial counts of all test strains in vegetations and renal and splenic tissues (P less than 0.001). Teicoplanin-LD was equally effective against SA-12871 but failed against SA-12873, with three of six animals still being bacteremic at the end of therapy. For SA-12871, daptomycin was as effective as teicoplanin-HD and was superior to teicoplanin-LD and vancomycin (P = 0.02) in lowering vegetation bacterial counts. There were no differences between daptomycin, teicoplanin-HD, or vancomycin in the reduction of bacterial counts in tissues for any of the test strains. In rabbits infected with SA-12871, vegetations from 33% of teicoplanin-LD-treated, 6% of teicoplanin-HD-treated, and 13% of daptomycin-treated animals yielded organisms for which there were up to eightfold increases in the MICs. Resistance may have contributed to early death in one daptomycin-treated animal. No increases in the MICs for the test strain were detected in animals infected with SA-12873 or MRSA-494. We conclude that in this model and against these strains of S. aureus, daptomycin and teicoplanin-HD are as efficacious as vancomycin, but diminished susceptibility to both can develop during therapy.

Objectives

In a prospective, randomized trial, daptomycin was non-inferior to standard therapy for Staphylococcus aureus bacteraemia and right-sided endocarditis. Since rates of infection due to methicillin-resistant S. aureus (MRSA) infection are increasing and treatment outcomes for bacteraemia caused by MRSA are generally worse than those observed with methicillin-susceptible S. aureus bacteraemia, clinical characteristics and treatment results in the trial’s pre-specified subset of patients with MRSA were analysed.

Methods

Clinical characteristics and outcomes of patients receiving daptomycin were compared with those receiving vancomycin plus low-dose gentamicin. Success was defined as clinical improvement with clearance of bacteraemia among patients who completed adequate therapy, received no potentially effective non-study antibiotics and had negative blood cultures 6 weeks after end of therapy.

Results

Twenty of the 45 (44.4%) daptomycin patients and 14 of the 43 (32.6%) vancomycin/gentamicin patients were successfully treated (difference 11.9%; confidence interval −8.3 to 32.1). Success rates for daptomycin versus vancomycin/gentamicin were 45% versus 27% in complicated bacteraemia, 60% versus 45% in uncomplicated bacteraemia and 50% versus 50% in right-sided MRSA endocarditis. Cure rates in patients with septic emboli and in patients who received pre-enrolment vancomycin were similar between treatment groups. However, in both treatment groups, success rates were lower in the elderly (≥75 years). Persisting or relapsing bacteraemia occurred in 27% of daptomycin and 21% of vancomycin/gentamicin patients; among these patients, MICs of ≥2 mg/L occurred in five daptomycin and four vancomycin/gentamicin patients. The clinical course of several patients may have been influenced by lack of surgical intervention.

Conclusions

Daptomycin was an effective alternative to vancomycin/gentamicin for MRSA bacteraemia or right-sided endocarditis.

We examined the activity of CB-181963, a novel cephalosporin, against methicillin-resistant Staphylococcus aureus (MRSA) (n = 200), methicillin-susceptible S. aureus (MSSA) (n = 50), glycopeptide-intermediate Staphylococcus species (GISS) (n = 47), and VRSA (n = 2) isolates. CB-181963 exhibited MIC profiles similar to those of linezolid against MRSA and GISS; however, activity against MSSA was similar to that of vancomycin. Time-kill study results of investigations of activity against MRSA, MSSA, and GISS at 24 h were as follows: CB-181963 activity = vancomycin activity > linezolid activity (P < 0.001); CB-181963 = quinupristin-dalfopristin = vancomycin > linezolid (P < 0.05); CB-181963 > linezolid (P = 0.003); and CB-181963 = quinupristin-dalfopristin = vancomycin. CB-181963 may provide an alternative treatment for multidrug-resistant staphylococci.

A total of 207 Staphylococcus aureus strains, including 105 well-characterized strains with decreased susceptibility to vancomycin (17 vancomycin-intermediate S. aureus [VISA] and 88 heteroresistant VISA [hVISA] strains) and 102 wild-type methicillin-resistant S. aureus (MRSA-WT) strains were tested by reference/standardized broth microdilution and disk diffusion methods, as well as by Etest (AB BIODISK, Solna, Sweden), against daptomycin and vancomycin. The lowest concentration of antimicrobial agent that killed ≥99.9% of the initial inoculum was defined as the minimum bactericidal concentration (MBC) endpoint, and time-kill curves were performed in selected strains to further evaluate bactericidal activity. All MRSA-WT and hVISA strains were inhibited by ≤1 μg/ml of daptomycin, while the VISA strains showed slightly higher daptomycin MICs (range, 0.5 to 4 μg/ml). All daptomycin MBC results were at the MIC or twofold higher. In contrast, 14.7% of MRSA-WT, 69.3% of hVISA, and all VISA strains showed a vancomycin MBC/MIC ratio of ≥32 or an MBC of ≥16 μg/ml (tolerant). The correlation coefficients between broth microdilution and disk diffusion method results were low for daptomycin (0.07) and vancomycin (0.11). Eight (3.8%) strains (all hVISA or VISA) were “nonsusceptible” to daptomycin by broth microdilution methods but susceptible by the disk diffusion method. For vancomycin, 35 (16.9%) strains were nonsusceptible by broth microdilution methods but susceptible by disk diffusion methods. In conclusion, daptomycin was highly bactericidal against S. aureus strains, and its bactericidal activity was not affected by decreased susceptibility to vancomycin. In contrast, many (one in seven) contemporary MRSA-WT, the majority of hVISA, and all VISA strains showed vancomycin MBC/MIC ratios consistent with tolerance, a predictor of poor clinical response. Disk diffusion tests generally failed to detect strains categorized as nonsusceptible to daptomycin or vancomycin by the reference broth microdilution method or Etest, and reassessment of breakpoints should be immediately attempted for MIC methods suggested as the test of choice.

A vancomycin-resistant Staphylococcus aureus (VRSA) isolate was obtained from a patient in Pennsylvania in September 2002. Species identification was confirmed by standard biochemical tests and analysis of 16S ribosomal DNA, gyrA, and gyrB sequences; all of the results were consistent with the S. aureus identification. The MICs of a variety of antimicrobial agents were determined by broth microdilution and macrodilution methods following National Committee for Clinical Laboratory Standards (NCCLS) guidelines. The isolate was resistant to vancomycin (MIC = 32 μg/ml), aminoglycosides, β-lactams, fluoroquinolones, macrolides, and tetracycline, but it was susceptible to linezolid, minocycline, quinupristin-dalfopristin, rifampin, teicoplanin, and trimethoprim-sulfamethoxazole. The isolate, which was originally detected by using disk diffusion and a vancomycin agar screen plate, was vancomycin susceptible by automated susceptibility testing methods. Pulsed-field gel electrophoresis (PFGE) of SmaI-digested genomic DNA indicated that the isolate belonged to the USA100 lineage (also known as the New York/Japan clone), the most common staphylococcal PFGE type found in hospitals in the United States. The VRSA isolate contained two plasmids of 120 and 4 kb and was positive for mecA and vanA by PCR amplification. The vanA sequence was identical to the vanA sequence present in Tn1546. A DNA probe for vanA hybridized to the 120-kb plasmid. This is the second VRSA isolate reported in the United States.

The activity of telavancin and comparators was assessed against a contemporary (2007 and 2008) global collection of 10,000 isolates of Staphylococcus aureus. Telavancin was very active against methicillin-susceptible and -resistant S. aureus (MSSA and MRSA, respectively; MIC50/90 for both, 0.12/0.25 μg/ml; 100.0% susceptible). This agent was 2-, 4-, and 8-fold more potent than daptomycin (MIC90, 0.5 μg/ml), vancomycin or quinupristin-dalfopristin (MIC90, 1 μg/ml), and linezolid (MIC90, 2 μg/ml) against MRSA, respectively. These data show a potent activity of telavancin tested against a current global collection of S. aureus.

Staphylococcus aureus bacteremia is a common disease with a high risk of mortality and complications. An increasing proportion of cases are methicillin-resistant S.aureus (MRSA), and methicillin-resistance is being observed from both community-acquired bacteremias and in healthcare-associated infections. The duration of bacteremia and transesophageal echocardiographic findings are useful in predicting the likelihood of complications including endocarditis. Therapy with vancomycin has been the mainstay in the treatment of MRSA bacteremias, but is associated with a long duration of bacteremia on therapy and relapses. Loss of susceptibility to vancomycin, due to thickened cell walls and through the acquisition of the vanA gene, has been described. Daptomycin is newly approved lipopeptide that is highly bactericidal against most strains of MRSA. In a randomized trial, daptomycin was demonstrated to be effective in the treatment of S. aureus bacteremia and right-sided endocarditis. However treatment failures associated with isolates with daptomycin non-susceptibility are reported, and there is a correlation between isolates with reduced vancomycin susceptibility and reduced daptomycin susceptibility. Daptomycin is a useful alternative to vancomycin in the therapy of MRSA bacteremia and endocarditis. However the appropriate role of daptomycin in optimizing therapy with MRSA bacteremia and endocarditis remains to be elucidated.

Staphylococcus aureus clinical isolates with vancomycin MICs of 2 μg/ml have been associated with vancomycin therapeutic failure and the heteroresistant vancomycin-intermediate S. aureus (hVISA) phenotype. A population analysis profile (PAP) with an area under the curve (AUC) ratio of ≥0.9 for the AUC of the clinical isolate versus the AUC for hVISA strain Mu3 is most often used for determining hVISA, but it is time-consuming and labor-intensive. A collection of 140 MRSA blood isolates with vancomycin MICs of 2 μg/ml by reference broth microdilution and screened for hVISA using PAP-AUC (21/140 [15%] hVISA) were tested by additional methods to detect hVISA. The methods included (i) Etest macromethod using vancomycin and teicoplanin test strips, brain heart infusion (BHI) agar, and a 2.0 McFarland inoculum; (ii) Etest glycopeptide resistance detection (GRD) using vancomycin-teicoplanin double-sided gradient test strips on Mueller-Hinton agar (MHA) with 5% sheep blood and a 0.5 McFarland inoculum; and (iii) BHI screen agar plates containing 4 μg/ml vancomycin and 16 g/liter casein using 0.5 and 2.0 McFarland inocula. Each method was evaluated using PAP-AUC as the reference method. The sensitivity of each method for detecting hVISA was higher when the results were read at 48 h. The Etest macromethod was 57% sensitive and 96% specific, Etest GRD was 57% sensitive and 97% specific, and BHI screen agar was 90% sensitive and 95% specific with a 0.5 McFarland inoculum and 100% sensitive and 68% specific with a 2.0 McFarland inoculum. BHI screen agar with 4 μg/ml vancomycin and casein and a 0.5 McFarland inoculum had the best sensitivity and specificity combination, was easy to perform, and may be useful for clinical detection of hVISA.

Vancomycin MICs (V-MIC) and the frequency of heteroresistant vancomycin-intermediate Staphylococcus aureus (hVISA) isolates are increasing among methicillin (meticillin)-resistant Staphylococcus aureus (MRSA) isolates, but their relevance remains uncertain. We compared the V-MIC (Etest) and the frequency of hVISA (Etest macromethod) for all MRSA blood isolates saved over an 11-year span and correlated the results with the clinical outcome. We tested 489 isolates: 61, 55, 187, and 186 isolates recovered in 1996-1997, 2000, 2002-2003, and 2005-2006, respectively. The V-MICs were ≤1, 1.5, 2, and 3 μg/ml for 74 (15.1%), 355 (72.6%), 50 (10.2%), and 10 (2.1%) isolates, respectively. We detected hVISA in 0/74, 48/355 (13.5%), 15/50 (30.0%), and 8/10 (80.0%) isolates with V-MICs of ≤1, 1.5, 2, and 3 μg/ml, respectively (P < 0.001). The V-MIC distribution and the hVISA frequency were stable over the 11-year period. Most patients (89.0%) received vancomycin. The mortality rate (evaluated with 285 patients for whose isolates the trough V-MIC was ≥10 μg/ml) was comparable for patients whose isolates had V-MICs of ≤1 and 1.5 μg/ml (19.4% and 27.0%, respectively; P = 0.2) but higher for patients whose isolates had V-MICs of ≥2 μg/ml (47.6%; P = 0.03). However, the impact of V-MIC and hVISA status on mortality or persistent (≥7 days) bacteremia was not substantiated by multivariate analysis. Staphylococcal chromosome cassette mec (SCCmec) typing of 261 isolates (including all hVISA isolates) revealed that 93.0% of the hVISA isolates were SCCmec type II. These findings demonstrate that the V-MIC distribution and hVISA frequencies were stable over an 11-year span. A V-MIC of ≥2 μg/ml was associated with a higher rate of mortality by univariate analysis, but the relevance of the V-MIC and the presence of hVISA remain uncertain. A multicenter prospective randomized study by the use of standardized methods is needed to evaluate the relevance of hVISA and determine the optimal treatment of patients whose isolates have V-MICs of ≥2.0 μg/ml.

Methicillin resistant Staphylococcus aureus (MRSA) is a major nosocomial pathogen that causes severe morbidity and mortality worldwide. MRSA strains are endemic in many American and European hospitals and account for 29%–35% of all clinical isolates. Recent studies have documented the increased costs associated with MRSA infection, as well as the importance of colonisation pressure. Surveillance strategies have been proposed especially in high risk areas such as the intensive care unit. Pneumonia and bacteraemia account for the majority of MRSA serious clinical infections, but intra-abdominal infections, osteomyelitis, toxic shock syndrome, food poisoning, and deep tissue infections are also important clinical diseases. The traditional antibiotic therapy for MRSA is a glycopeptide, vancomycin. New antibiotics have been recently released that add to the armamentarium for therapy against MRSA and include linezolid, and quinupristin/dalfopristin, but cost, side effects, and resistance may limit their long term usefulness.

Staphylococcus aureus colonization and infection occur more commonly among persons living or working in crowded conditions, but characterization of S. aureus colonization within medical communities in China is lacking. A total of 144 (15.4%, 144/935) S. aureus isolates, including 28 (3.0%, 28/935) MRSA isolates, were recovered from the nares of 935 healthy human volunteers residing on a Chinese medical college campus. All S. aureus isolates were susceptible to vancomycin, quinupristin/dalfopristin and linezolid but the majority were resistant to penicillin (96.5%), ampicillin/sulbactam (83.3%) and trimethoprim/sulfamethoxazole (93.1%). 82%, (23/28) of the MRSA isolates and 66% (77/116) of the MSSA isolates were resistant to multiple antibiotics, and 3 MRSA isolates were resistant to mupirocin—an agent commonly used for nasal decolonization. 16 different sequence types (STs), as well as SCCmec genes II, III, IVd, and V, were represented among MRSA isolates. We also identified, for the first time, two novel STs (ST1778 and ST1779) and 5 novel spa types for MRSA. MRSA isolates were distributed in different sporadic clones, and ST59-MRSA-VId- t437 was found within 3 MRSA isolates. Moreover, one isolate with multidrug resistance belonging to ST398-MRSA-V- t571 associated with animal infections was identified, and 3 isolates distributed in three different clones harbored PVL genes. Collectively, these data indicate a high prevalence of nasal MRSA carriage and molecular heterogeneity of S. aureus isolates among persons residing on a Chinese medical college campus. Identification of epidemic MRSA clones associated with community infection supports the need for more effective infection control measures to reduce nasal carriage and prevent dissemination of MRSA to hospitalized patients and health care workers in this community.

Background

Staphylococcus aureus is an important pathogen causing a wide range of infections in the hospital and community setting. In order to have adequate information for treatment of S. aureus infections, it is crucial to understand the trends in the antibiotic-resistance patterns. In addition, the occurrence and changes in types of S. aureus, clonal identities, and their geographic spread is essential for the establishment of adequate infection control programmes. In this study, 68 S. aureus isolates obtained from clinical and non-clinical sources in Nigeria between January and April 2009 were characterized using phenotypic and molecular methods.

Results

All the S. aureus isolates were susceptible to teicoplanin, vancomycin, phosphomycin, fusidic acid, rifampicin, daptomycin, mupirocin, linezolid and tigecycline. Sixteen percent of the isolates were resistant to oxacillin, while 55% and 72% of isolates were resistant to tetracycline and trimethoprim/sulphamethoxazole (cotrimoxazole), respectively (Table 1). There was excellent correlation between the broth microdilution assay and detection of antibiotic resistance genes by the multiplex PCR, in the determination of S. aureus resistance to erythromycin, gentamicin, methicillin and tetracycline. A total of 28 spa types were identified in the study, and the predominant spa type among the methicillin-susceptible S. aureus (MSSA) isolates was t084 (13 isolates). The t037-ST241-SCCmecIII type was the only clone identified in Maiduguri (North-East Nigeria) while in South-West Nigeria, diversity among the MRSA isolates (t451-ST8-SCCmecV; t008-ST94-SCCmecIV; t002-ST5-SCCmecV; t064-ST8-SCCmecV) was observed. The toxin genes seh and etd were detected in isolates affiliated with clonal complexes CC1, CC80 and sequence type ST25, respectively. The proportion of PVL-positive isolates among MSSA was high (40%). Most of the PVL-positive MSSA isolates were obtained from wound infections and associated with clonal complexes CC1, CC30, CC121 and with sequence type ST152.

Antibiotic resistance profile of S. aureus (MSSA and MRSA) from Nigeria

Conclusions

The use of phenotypic and molecular methods provided useful information on antibiotic resistance and molecular diversity of S. aureus in Nigeria. The high proportion of PVL-positive MSSA isolates affiliated to various clonal complexes and detected in all the health institutions is a major concern, both as a source of severe infections and as a potential reservoir that could lead to the emergence of PVL-positive MRSA. This study presents the first baseline information on the nature of the antibiotic resistance genes from S. aureus isolates in Nigeria. There is the need to curtail the spread and establishment of MRSA and PVL-positive MSSA clones in Nigerian health care institutions.

Strain D958, a methicillin-resistant Staphylococcus aureus strain with reduced susceptibility to vancomycin, was isolated from a 69-year-old Saudi male patient presenting with severe sepsis immediately after admission. Despite high serum levels of vancomycin, the same S. aureus strain was isolated from five blood culture sets during 1 week. Treatment failure under therapeutic levels of vancomycin prompted us to investigate the resistance profile of this strain in further detail. The MIC values for vancomycin as determined by Etest and microdilution were 3.0 and 2.0 mg/liter, respectively, and remained unchanged during the treatment course. The macro-Etest method showed a MIC of 4 mg/liter. The strain showed liquid vancomycin and lysostaphin MBCs of 2.0 and 5.0 mg/liter, respectively. The isolates were confirmed as heterogeneously vancomycin-intermediate S. aureus (hVISA) by vancomycin population analysis profile. The areas under these curves were similar for Mu3 and D958 for vancomycin and teicoplanin (ratio values were 1 and 1.1 for vancomycin and teicoplanin, respectively). Extensive genotyping and molecular characterization demonstrated that the strain harbored a staphylococcal cassette chromosome mec element (SCCmec) type III cassette and was of sequence type ST241, a single-locus variant of the successful multiresistant clone ST239. Microarray results demonstrated that D958 contained numerous resistance determinants (generally plasmid or phage encoded). These results suggest that this strain is constitutively expressing an altered susceptibility to vancomycin. Further studies are warranted to assess the clonal distribution of such strains displaying reduced susceptibility to vancomycin prior to any antimicrobial therapy.

To understand quinupristin-dalfopristin resistance among clinical isolates of gram-positive bacteria in Taiwan, where this agent is not yet available for clinical use, we evaluated 1,287 nonduplicate isolates recovered from January 1996 to December 1999 for in vitro susceptibility to quinupristin-dalfopristin and other newer antimicrobial agents. All methicillin-susceptible Staphylococcus aureus (MSSA) isolates were susceptible to quinupristin-dalfopristin. High rates of nonsusceptibility to quinupristin-dalfopristin (MICs, ≥2 μg/ml) were demonstrated for the following organisms: methicillin-resistant S. aureus (MRSA) (31%), coagulase-negative staphylococci (CoNS) (16%), Streptococcus pneumoniae (8%), viridans group streptococci (51%), vancomycin-susceptible enterococci (85%), vancomycin-resistant Enterococcus faecalis (100%), vancomycin-resistant Enterococcus faecium (66%), Leuconostoc spp. (100%), Lactobacillus spp. (50%), and Pediococcus spp. (87%). All isolates of MSSA, MRSA, S. pneumoniae, and viridans group streptococci were susceptible to vancomycin and teicoplanin. The rates of nonsusceptibility to vancomycin and teicoplanin were 5 and 7%, respectively, for CoNS, ranging from 12 and 18% for S. simulans to 0 and 0% for S. cohnii and S. auricularis. Moxifloxacin and trovafloxacin had good activities against these isolates except for ciprofloxacin-resistant vancomycin-resistant enterococci and methicillin-resistant staphylococci. In Taiwan, virginiamycin has been used in animal husbandry for more than 20 years, which may contribute to the high rates of quinupristin-dalfopristin resistance.

All 982 methicillin-resistant Staphylococcus aureus strains collected from August 2006 to December 2007 were tested for vancomycin susceptibility by using 3-μg/ml vancomycin brain heart infusion screening plates, a vancomycin Etest, and a vancomycin/teicoplanin macro Etest. Three vancomycin-intermediate Staphylococcus aureus (VISA) (0.3%) and two heterogeneous VISA (0.2%) isolates were identified. The screening method yielded 895 cases of ≤1 colony and 87 positive results (with growth of >1 colony after 48 h); further Etests showed 82/87 isolates with growth on screening plates to be false positive. Repeat testing showed a false-positivity rate of only 15 of the original 87 isolates by plate screening.