Prior vancomycin use is a risk factor for reduced vancomycin susceptibility in methicillin-susceptible but not methicillin-resistant Staphylococcus aureus bacteremia

Trends in and correlations between antibiotic consumption and resistance of Staphylococcus aureus at a tertiary hospital in China before and after introduction of an antimicrobial stewardship programme

The overuse of antibiotics and the rapid emergence of antibiotic resistance prompted the launch of an antimicrobial stewardship programme in 2011. This study aimed to investigate the trends and correlations between antibiotic consumption and resistance of Staphylococcus aureus in a tertiary hospital of northwest China from 2010 to 2016. Trends were analysed by linear regression, and correlations were assessed by an autoregressive integrated moving average model. The total consumption of antibiotics halved during the 7-year study period, while the rates of resistance of S. aureus decreased significantly or remained stable; methicillin-resistant S. aureus (MRSA) declined markedly, from 73.3% at the beginning of the study to 41.4% by the end. This latter decrease was significantly correlated with the consumption of several classes of antibiotics. In conclusion, reduction in antibiotic use impacted significantly on resistance rates and contributed to a decline in MRSA prevalence.

Impact of Time to Appropriate Therapy on Mortality in Patients with Vancomycin-Intermediate Staphylococcus aureus Infection

Despite the increasing incidence of vancomycin-intermediate Staphylococcus aureus (VISA) infections, few studies have examined the impact of delay in receipt of appropriate antimicrobial therapy on outcomes in VISA patients. We examined the effects of timing of appropriate antimicrobial therapy in a cohort of patients with sterile-site methicillin-resistant S. aureus (MRSA) and VISA infections. In this single-center, retrospective cohort study, we identified all patients with MRSA or VISA sterile-site infections from June 2009 to February 2015. Clinical outcomes were compared according to MRSA/VISA classification, demographics, comorbidities, and antimicrobial treatment. Thirty-day all-cause mortality was modeled with Kaplan-Meier curves. Multivariate logistic regression analysis (MVLRA) was used to determine odds ratios for mortality. We identified 354 patients with MRSA (n = 267) or VISA (n = 87) sterile-site infection. Fifty-five patients (15.5%) were nonsurvivors. Factors associated with mortality in MVLRA included pneumonia, unknown source of infection, acute physiology and chronic health evaluation (APACHE) II score, solid-organ malignancy, and admission from skilled care facilities. Time to appropriate antimicrobial therapy was not significantly associated with outcome. Presence of a VISA infection compared to that of a non-VISA S. aureus infection did not result in excess mortality. Linezolid use was a risk for mortality in patients with APACHE II scores of ≥14. Our results suggest that empirical vancomycin use in patients with VISA infections does not result in excess mortality. Future studies should (i) include larger numbers of patients with VISA infections to confirm the findings presented here and (ii) determine the optimal antibiotic therapy for critically ill patients with MRSA and VISA infections.

Effects of vancomycin versus nafcillin in enhancing killing of methicillin-susceptible Staphylococcus aureus causing bacteremia by human cathelicidin LL-37

Recent studies have demonstrated that anti-staphylococcal beta-lactam antibiotics, like nafcillin, render methicillin-resistant Staphylococcus aureus (MRSA) more susceptible to killing by innate host defense peptides (HDPs), such as cathelicidin LL-37. We compared the effects of growth in 1/4 minimum inhibitory concentration (MIC) of nafcillin or vancomycin on the LL-37 killing of 92 methicillin-susceptible S. aureus (MSSA) isolates. For three randomly selected strains among these, we examined the effects of nafcillin, vancomycin, daptomycin, or linezolid on LL-37 killing and autolysis. Growth in the presence of subinhibitory nafcillin significantly enhanced LL-37 killing of MSSA compared to vancomycin and antibiotic-free controls. Nafcillin also reduced MSSA production of the golden staphylococcal pigment staphyloxanthin in 39 % of pigmented strains vs. 14 % for vancomycin. Among the antibiotics tested, only nafcillin resulted in significantly increased MSSA autolysis. These studies point to additional mechanisms of anti-staphylococcal activity of nafcillin beyond direct bactericidal activity, properties that vancomycin and other antibiotic classes do not exhibit. The ability of nafcillin to enhance sensitivity to innate HDPs may contribute to its superior effectiveness against MSSA, as suggested by studies comparing clinical outcomes to vancomycin treatment.

High vancomycin minimum inhibitory concentration is associated with poor outcome in patients with methicillin-susceptible Staphylococcus aureus bacteremia regardless of treatment

We retrospectively investigated the impact of high vancomycin minimum inhibitory concentration (MIC > 2 μg/ml) on the outcome of 53 patients with bacteremia caused by methicillin-susceptible Staphylococcus aureus (MSSA). Vancomycin MIC was determined by broth microdilution according to CLSI methods. The primary outcome was 30-day all-cause mortality from the date of the first positive blood culture. The mortality rate was 22.6% (12 of 53 patients). High vancomycin MIC (odds ratio (OR) = 9.3; 95% confidence interval (95% CI) = 1.31-63.20; p = 0.027), Charlson comorbidity index ≥ 3 (OR = 10.3; 95% CI = 1.3-102.04; p = 0.03), advanced age (OR = 35.8; 95% CI = 2.3-659.2; p = 0.01), and severe sepsis (OR = 8.5; 95% CI = 1.2-61.4; p = 0.03) were associated with mortality.

Evaluation of in vitro susceptibility trends to vancomycin and daptomycin by strain type of Staphylococcus aureus causing bloodstream infections

In total, 718 consecutive clinical meticillin-resistant Staphylococcus aureus (MRSA) isolates from 2006 to 2010 and 417 clinical meticillin-susceptible S. aureus (MSSA) isolates from mid-2007 to 2010 were evaluated. Isolates were from blood cultures obtained from separate patients in Detroit, MI, and were tested for in vitro susceptibility trends to vancomycin and daptomycin by molecular strain type. The MRSA pulsed-field gel electrophoresis (PFGE) results showed that 290 (40.4%) were USA100, 296 (41.2%) were USA300 and the remaining isolates were non-USA100/300. Vancomycin minimum inhibitory concentrations (MICs) by Etest [mean±standard deviation (S.D.) 1.55±0.26mg/L] in MRSA isolates showed no significant change over the 5-year period within all strain types, whilst daptomycin MICs by Etest (mean±S.D. 0.51±0.25mg/L) showed a significant downward trend across time (r=-0.243; P<0.001), with this trend occurring among all PFGE groups. For MSSA, a significant decrease in MICs to vancomycin was found by Etest (r=-0.160; P=0.001) and conversely a significant increase in daptomycin MICs by Etest was found (r=0.146; P=0.028). The results of this study showed that changes in MIC were not specific to strain molecular type. For vancomycin, there was no change in MRSA MICs and a decrease in MSSA MICs for blood isolates. For daptomycin, MICs decreased in MRSA and increased in MSSA blood isolates over the study period.

Reduced vancomycin susceptibility and staphylococcal cassette chromosome mec (SCCmec) type distribution in methicillin-resistant Staphylococcus aureus bacteraemia

OBJECTIVES

Recent epidemiological evidence suggests that genotypic and phenotypic characteristics that have typically distinguished community-associated methicillin-resistant Staphylococcus aureus (MRSA) and healthcare-associated MRSA strains may be evolving. The objective of this study was to examine the association between reduced vancomycin susceptibility (RVS) and staphylococcal cassette chromosome mec (SCCmec) type in MRSA bloodstream isolates.

METHODS

A cohort study of patients who were hospitalized from 2007 to 2009 with S. aureus bacteraemia was conducted within a university health system. Bivariable analyses were conducted to determine the association between RVS and SCCmec type, as well as other microbiological characteristics including Panton-Valentine leucocidin, accessory gene regulator (agr) dysfunction and vancomycin heteroresistance.

RESULTS

A total of 188 patients with MRSA bacteraemia were identified: 116 (61.7%) and 72 (38.3%) patients had infections due to healthcare-associated MRSA and community-associated MRSA, respectively. As defined by a vancomycin Etest MIC > 1.0 mg/L, the prevalence of RVS was 40.4%. Isolates with RVS were significantly more likely to be associated with SCCmec II compared with isolates without RVS (74.7% and 47.3%, respectively, P < 0.001), but not with Panton-Valentine leucocidin (P = 0.10), agr dysfunction (P = 0.19) or healthcare-associated infection (P = 0.36).

CONCLUSIONS

The results of our study demonstrate important microbiological characteristics among MRSA isolates characterized by RVS, including a significant association between SCCmec II and elevated vancomycin MIC. It is clear that the clinical and molecular epidemiology of MRSA is evolving, and further understanding of factors determining virulence will be important for the elucidation of optimal treatment approaches for associated infections.

Derivation and validation of clinical prediction rules for reduced vancomycin susceptibility in Staphylococcus aureus bacteraemia

Reduced vancomycin susceptibility (RVS) may lead to poor clinical outcomes in Staphylococcus aureus bacteraemia. We conducted a cohort study of 392 patients with S. aureus bacteraemia within a university health system. The association between RVS, as defined by both Etest [vancomycin minimum inhibitory concentration (MIC) >1·0 μg/ml] and broth microdilution (vancomycin MIC ≥1·0 μg/ml), and patient and clinical variables were evaluated to create separate predictive models for RVS. In total, 134 (34·2%) and 73 (18·6%) patients had S. aureus isolates with RVS by Etest and broth microdilution, respectively. The final model for RVS by Etest included methicillin resistance [odds ratio (OR) 1·51, 95% confidence interval (CI) 0·97-2·34], non-white race (OR 0·67, 95% CI 0·42-1·07), healthcare-associated infection (OR 0·56, 95% CI 0·32-0·96), and receipt of any antimicrobial therapy ≤30 days prior to the culture date (OR 3·06, 95% CI 1·72-5·44). The final model for RVS by broth microdilution included methicillin resistance (OR 2·45, 95% CI 1·42-4·24), admission through the emergency department (OR 0·54, 95% CI 0·32-0·92), presence of an intravascular device (OR 2·24, 95% CI 1·30-3·86), and malignancy (OR 0·51, 95% CI 0·26-1·00). The availability of an easy and rapid clinical prediction rule for early identification of RVS can be used to help guide the timely and individualized management of these serious infections.