Meticillin-susceptible S. Aureus Research Articles

Emergence and spread of a mupirocin-resistant variant of the European epidemic fusidic acid-resistant impetigo clone of Staphylococcus aureus, Belgium, 2013 to 2023.

BackgroundAntimicrobial resistance to mupirocin and fusidic acid, which are used for treatment of skin infections caused by Staphylococcus aureus, is of concern.AimTo investigate resistance to fusidic acid and mupirocin in meticillin-susceptible S. aureus (MSSA) from community-acquired skin and soft tissue infections (SSTIs) in Belgium.MethodsWe collected 2013-2023 data on fusidic acid and mupirocin resistance in SSTI-associated MSSA from two large Belgian laboratories. Resistant MSSA isolates sent to the Belgian Staphylococci Reference Centre were spa-typed and analysed for the presence of the eta and etb virulence genes and the mupA resistance gene. In addition, we whole genome sequenced MSSA isolates collected between October 2021 and September 2023.ResultsMupirocin resistance increased between 2013 and 2023 from 0.5-1.5% to 1.7-5.6%. Between 2018 and 2023, 91.4% (64/70) of mupirocin-resistant isolates were co-resistant to fusidic acid. By September 2023, between 8.9% (15/168) and 10.1% (11/109) of children isolates from the two laboratories were co-resistant. Of the 33 sequenced isolates, 29 were sequence type 121, clonal and more distantly related to the European epidemic fusidic acid-resistant impetigo clone (EEFIC) observed in Belgium in 2020. These isolates carried the mupA and fusB genes conferring resistance to mupirocin and fusidic acid, respectively, and the eta and etb virulence genes.ConclusionWe highlight the spread of a mupirocin-resistant EEFIC in children, with a seasonal trend for the third quarter of the year. This is of concern because this variant is resistant to the two main topical antibiotics used to treat impetigo in Belgium.

Molecular epidemiology of meticillin-susceptible Staphylococcus aureus in the neonatal intensive care unit

Staphylococcus aureus - both meticillin-resistant S.aureus (MRSA) and meticillin-susceptible S.aureus (MSSA) - is a major cause of neonatal infections. Infection control measures have not lowered the incidence of MSSA infections to the same degree as that of MRSA infections. To investigate the transmission pathway of MSSA in neonatal intensive care unit (NICU) using genetic analysis. Neonatal patients, their parents, and healthcare workers were swab-tested in the NICU at our hospital at the time of hospitalization and then every month thereafter from October 1st, 2018 to March 31st, 2019. Whole-genome sequencing was performed to test for MSSA strains. Multi-locus sequence typing and single nucleotide polymorphism analysis were used to identify strains and understand their relatedness. There were 16 MSSA-positive patients. Four MSSA-positive patients shared strains from the same phylogenetic groups as those of healthcare workers. One presented the same strain as the parent. MSSA-positive twin neonates shared the same strain. Ten had sporadic strains; 32 of the 97 tested healthcare workers were MSSA positive. The findings of this study suggest that the route of transmission of MSSA in NICU may be through MSSA in the hospital environment in addition to horizontal transmission via healthcare workers. Along with hand hygiene with alcohol, thorough environmental maintenance and parental education are important for infection control in NICUs targeting MSSA.

Key predictors and burden of meticillin-resistant Staphylococcus aureus infection in comparison with meticillin-susceptible S. aureus infection in an Australian hospital setting

Staphylococcus aureus is associated with significant mortality and increased burden on the healthcare system. Relatively few reliable estimates are available regarding the impact of meticillin-resistant S.aureus (MRSA) infection compared with meticillin-susceptible S.aureus (MSSA) infection. To compare patients with MRSA infection and MSSA infection to identify differences in inpatient mortality, length of stay and costs of hospital services, and identify predictors of MRSA as a cause of S.aureus infection. An analytical, retrospective, longitudinal study using non-identifiable linked data on adults admitted to hospitals of a health district in Australia with a diagnosis of S.aureus infection over a 10-year period. The main outcome measure was 30-day inpatient mortality. Secondary endpoints included total overnight stays, all-cause inpatient mortality, and hospitalization costs within 1 year of index admission. Inpatient mortality at 30, 100 and 365 days was estimated to be significantly greater for patients with MRSA infection. The mean additional cost of MRSA infection when controlling for additional factors was $5988 and 4 nights of additional hospital stay per patient within 1 year of index admission. Key predictors of MRSA infection were: date of index admission; higher comorbidity score; greater socio-economic disadvantage; admission to hospital other than via the emergency department; older age; and prior admission to hospital within 28 days of index admission. MRSA infection is associated with increased inpatient mortality, costs and hospital length of stay compared with MSSA infection. Efforts are required to alleviate the additional burden of MRSA infection on patients and healthcare systems.

Staphylococcus aureus bloodstream infections: diverging trends of meticillin-resistant and meticillin-susceptible isolates, EU/EEA, 2005 to 2018.

BackgroundInvasive infections caused by Staphylococcus aureus have high clinical and epidemiological relevance. It is therefore important to monitor the S. aureus trends using suitable methods.AimThe study aimed to describe the trends of bloodstream infections (BSI) caused by meticillin-resistant S. aureus (MRSA) and meticillin-susceptible S. aureus (MSSA) in the European Union (EU) and the European Economic Area (EEA).MethodsAnnual data on S. aureus BSI from 2005 to 2018 were obtained from the European Antimicrobial Resistance Surveillance Network (EARS-Net). Trends of BSI were assessed at the EU/EEA level by adjusting for blood culture set rate (number of blood culture sets per 1,000 days of hospitalisation) and stratification by patient characteristics.ResultsConsidering a fixed cohort of laboratories consistently reporting data over the entire study period, MRSA percentages among S. aureus BSI decreased from 30.2% in 2005 to 16.3% in 2018. Concurrently, the total number of BSI caused by S. aureus increased by 57%, MSSA BSI increased by 84% and MRSA BSI decreased by 31%. All these trends were statistically significant (p < 0.001).ConclusionsThe results indicate an increasing health burden of MSSA BSI in the EU/EEA despite a significant decrease in the MRSA percentage. These findings highlight the importance of monitoring antimicrobial resistance trends by assessing not only resistance percentages but also the incidence of infections. Further research is needed on the factors associated with the observed trends and on their attributable risk.

Clinical and economic burden of bloodstream infections caused by meticillin-resistant and meticillin-susceptible Staphylococcus aureus in Chinese tertiary hospitals from 2010 to 2017: a multicentre, retrospective cohort study

BackgroundGlobally, antimicrobial resistance has become one of the most complex public health challenges. Accurate estimates of health and economic burden of antimicrobial resistance has not been available in China because of the absence of representative empirical data. We therefore aimed to estimate the effect of antimicrobial resistance attributable to Staphylococcus aureus bloodstream infections in Chinese hospitals, and to compare the effect of resistant strains with susceptible strains. MethodsWe did a multicentre, retrospective cohort study of 11 tertiary hospitals in China. The cohort consisted of all inpatients with S aureus bloodstream infections from the sample hospitals, and we randomly included individuals free of S aureus bloodstream infections. We stratified the patients into two categories of S aureus bloodstream infections: meticillin-susceptible S aureus (MSSA) or meticillin-resistant S aureus (MRSA). The primary outcome was the effect of antimicrobial resistance on excess length of stay in the hospital, hospital mortality, and cost of treatment. We adopted a multistate model to estimate the expected length of stay. Multi-level Cox proportional hazards models were adopted to compare the hazard of reaching hospital death, discharge alive, and the combined end-of-stay endpoint. Two bed-day values were adopted when estimating the cost of treatment. This study was approved by Peking University Health Science Center Ethics Committee, with a waiver for individual informed consent. FindingsFrom Jan 1, 2010, to June 31, 2017, we assessed 861 inpatients with S aureus bloodstream infections and 10 000 individuals free of S aureus bloodstream infections. Of the 861 admissions with an S aureus blood stream infection detected, 324 (38%) were due to MRSA and 537 (62%) were due to MSSA. Bloodstream infections caused by MRSA prolonged the length of stay in the hospital by 13·4 days (95% CI 11·1–16·3), and MSSA bloodstream infections prolonged the length of stay by 11·9 days (95% CI 9·7–14·1). However, bloodstream infections caused by MRSA did not increase the hazard of death (adjusted hazard ratio [aHR] 0·31, 95% CI 0·09–1·10). MSSA bloodstream infections also did not increase the hazard of death (aHR 0·59, 95% CI 0·14–2·54). Compared with MSSA, MRSA did not significantly increase the hazard of death (aHR 0·44, 95% CI 0·19–1·03), excess length of stay (1·5 days, 95% CI −0·4 to 2·8), and cost of treatment. Although the cost of MRSA treatment was higher per infection than MSSA treatment, the annual cost of MSSA was higher overall. InterpretationBy accounting for time-dependent bias and important confounders, this study added to the existing literature about the effect of antimicrobial resistance in China. Current results of the study implicate the importance of surveillance and infection control policies that target infections rather than resistance. FundingChina Medical Board (17-270).

Temporal trends and epidemiology of Staphylococcus aureus surgical site infection in the Swiss surveillance network: a cohort study

Staphylococcus aureus is the leading pathogen in surgical site infections (SSI). To explore trends and risk factors associated with S.aureus SSI. Risk factors for monomicrobial S.aureus SSI were identified from the Swiss multi-centre SSI surveillance system using multi-variate logistic regression. Both in-hospital and postdischarge SSI were identified using standardized definitions. Over a six-year period, data were collected on 229,765 surgical patients, of whom 499 (0.22%) developed monomicrobial S.aureus SSI; 459 (92.0%) and 40 (8.0%) were due to meticillin-susceptible S.aureus (MSSA) and meticillin-resistant S.aureus (MRSA), respectively. There was a significant decrease in the rate of MSSA SSI (P=0.007), but not in the rate of MRSA SSI (P=0.70). Independent protective factors for S.aureus SSI were older age [≥75 years vs <50 years: odds ratio (OR) 0.60, 95% confidence interval (CI) 0.44-0.83], laparoscopy/minimally invasive surgery (OR 0.68, 95% CI 0.50-0.92), non-clean surgery [OR 0.78 (per increase in wound contamination class), 95% CI 0.64-0.94] and correct timing of pre-operative antibiotic prophylaxis (OR 0.80, 95% CI 0.65-0.98). Independent risk factors were male sex (OR 1.38, 95% CI 1.14-1.66), higher American Society of Anesthesiologists' score (per one-point increment: OR 1.30, 95% CI 1.13-1.51), re-operation for non-infectious reasons (OR 4.59, 95% CI 3.59-5.87) and procedure type: cardiac surgery, laminectomy, and hip or knee arthroplasty had two-to nine-fold increased odds of S.aureus SSI compared with other procedures. SSI due to S.aureus are decreasing and becoming rare events in Switzerland. High-risk procedures that may benefit from specific preventive measures were identified. Unfortunately, many of the independent risk factors are not easily modifiable.

Route of transmission of Staphylococcus aureus

Route of transmission of Staphylococcus aureus

Staphylococcus aureus ST121: a globally disseminated hypervirulent clone.

Staphylococcus aureus is a leading cause of bacterial infections in hospitals and communities worldwide. With the development of typing methods, several pandemic clones have been well characterized, including the extensively spreading hospital-associated meticillin-resistant S. aureus (HA-MRSA) clone ST239 and the emerging hypervirulent community-associated (CA) MRSA clone USA300. The multilocus sequence typing method was set up based on seven housekeeping genes; S. aureus groups were defined by the sharing of alleles at ≥ 5 of the seven loci. In many cases, the predicted founder of a group would also be the most prevalent ST within the group. As a predicted founder of major S. aureus groups, approximately 90 % of ST121 strains was meticillin-susceptible S. aureus (MSSA). The majority of ST121 strains carry accessory gene regulator type IV, whereas staphylococcal protein A gene types for ST121 are exceptionally diverse. More than 90 % of S. aureus ST121 strains have Panton-Valentine leukocidin; other enterotoxins, haemolysins, leukocidins and exfoliative toxins also contribute to the high virulence of ST121 strains. Patients suffering from S. aureus ST121 infections often need longer hospitalization and prolonged antimicrobial therapy. In this review, we tried to summarize the epidemiology of the S. aureus clone ST121 and focused on the molecular types, toxin carriage and disease spectrum of this globally disseminated clone.

Evaluation of commercial chromogenic media for the detection of meticillin-resistant Staphylococcus aureus

Selective chromogenic media allowing one-step meticillin-resistant Staphylococcus aureus (MRSA) isolation and identification are widely used. However, the changing epidemiology of MRSA means that the suitability of these chromogenic media requires investigation. To evaluate the following chromogenic media - Colorex MRSA, MRSA Select II, ChromID MRSA, and MRSA Brilliance 2 - for the detection of divergent strain types. We used a diverse collection of S. aureus, including strains harbouring the mecC gene, strains expressing varying levels of meticillin resistance, and isolates recovered from patient samples. MRSA Select II, Colorex MRSA, and ChromID each grew at a density of 1.5 × 10(1)cfu/mL for each SCCmec type investigated. Brilliance 2 demonstrated growth at 1.5 × 10(1)cfu/mL for mecC MRSA but at a higher density (1.5 × 10(4)cfu/mL) for the three mecA MRSA strains. All four media demonstrated excellent sensitivity for MRSA detection (≥99%), but reduced levels of specificity (85-73%) when challenged with a range of meticillin-susceptible S. aureus (MSSA) isolates. High levels of false positives (∼50%) were also obtained with all chromogenic media when tested with mec-negative borderline oxacillin-resistant S. aureus (BORSA) isolates. Although false positives may be obtained with some strains of MSSA and BORSA, the high sensitivity of these media and their ability to recover almost all MRSA tested (including oxacillin-susceptible and mecC-positive strains) confirm the value of chromogenic agar in MRSA detection.

In vitro activity of oritavancin and comparator agents against staphylococci, streptococci and enterococci from clinical infections in Europe and North America, 2011–2014

Oritavancin is a lipoglycopeptide that has been approved for the treatment of acute bacterial skin and skin-structure infections (ABSSSIs) caused by susceptible organisms. Oritavancin causes cell death by inhibiting cell wall synthesis as well as depolarising and permeabilising the cellular membrane of Gram-positive pathogens. The activities of oritavancin in comparison with vancomycin, daptomycin and linezolid were determined against a collection of over 11000 recent clinical Gram-positive isolates from patient infections (2011–2014), including skin and skin-structure infections. A total of 7253 Staphylococcus aureus, 839 coagulase-negative staphylococci (CoNS), 1464 enterococci and 1637 β-haemolytic streptococci (βHS) were collected from the USA and Europe. Minimum inhibitory concentrations (MICs) were determined using Clinical and Laboratory Standards Institute (CLSI) broth microdilution methods, and susceptibility was determined using CLSI and US Food and Drug Administration (FDA) (for oritavancin) breakpoint criteria. Equivalent in vitro activity (MIC50/90, 0.015–0.03/0.06μg/mL) was observed for oritavancin against meticillin-resistant S. aureus (MRSA), meticillin-susceptible S. aureus (MSSA) and Enterococcus faecalis in both regions. Slightly higher oritavancin MICs were obtained against CoNS, Streptococcus agalactiae, Enterococcus faecium (MIC90, 0.12μg/mL) and against other βHS (MIC90, 0.25μg/mL). Oritavancin demonstrated comparatively lower MICs than daptomycin and vancomycin when tested against multidrug-resistant S. aureus, vancomycin-resistant enterococci and erythromycin-resistant βHS. Oritavancin exhibited potent in vitro activity against the most common pathogens associated with ABSSSIs in the USA and Europe.

Identification of genetic variation exclusive to specific lineages associated with Staphylococcus aureus bacteraemia

Meticillin-resistant Staphylococcus aureus (MRSA) bacteraemia cases have declined since 2003, and have mostly been due to two epidemic (E) strains, E15 (multi-locus sequence type clonal complex CC22) and E16 (CC30). By contrast, the incidence of meticillin-susceptible S. aureus (MSSA) bacteraemia has remained largely unchanged and our understanding of these isolates has remained poor. To investigate the distribution and nucleotide sequence of heterogeneous regions between successful lineages using the 2009 British Society for Antimicrobial Chemotherapy (BSAC) Bacteraemia Resistance Surveillance Programme collection of S. aureus. S. aureus isolates (N = 202) comprised of 103 MRSA and 99 MSSA isolates were analysed using fluorescent amplified fragment length polymorphism (FAFLP) to detect nucleotide variations due to lineage-specific sequence motifs as well as differences in the distribution of mobile genetic elements between lineages. E15 and E16 MRSA strains comprised 79% and 6% of the collection in 2009 respectively. Six lineages, including CC22 and CC30, were associated with MRSA bacteraemia in the UK and Ireland. MSSA isolates were more diverse with 19 different lineages detected. FAFLP revealed lineage-specific sequence variations in loci encoding factors such as proteases or factors involved in haem biosynthesis, both of which may affect the success of major S. aureus lineages. Proteins encoded on certain mobile genetic elements or involved in cobalamin biosynthesis were found to be exclusive to CC8, CC22, or CC30. Overall, the genetic diversity among regions of the core genome and mobile genetic elements may alter antimicrobial resistance and the production of virulence or fitness factors that may be linked to strain success.

Survival of vancomycin-intermediate Staphylococcus aureus on hospital surfaces

Contaminated surfaces play an important role in the transmission of certain pathogens that are responsible for healthcare-associated infections. Although previous studies have shown that meticillin-resistant Staphylococcus aureus (MRSA) can survive on dry surfaces at room temperature, no published data regarding vancomycin-intermediate S. aureus (VISA) are available to date. To compare the survival time on different types of surfaces, cell-surface hydrophobicity, adherence to abiotic surfaces and biofilm formation of meticillin-susceptible S. aureus (MSSA), MRSA and VISA. Survival of the S. aureus strains was tested on latex, cotton fabric, vinyl flooring and formica. Cell-surface hydrophobicity was determined using the hydrocarbon interaction affinity method. Adhesion to abiotic surfaces was tested on granite, latex (gloves), glass, vinyl flooring and formica. Biofilm formation was evaluated at 6, 12, 24 and 48 h. All of the samples survived on the vinyl flooring and formica for at least 40 days. VISA survived on both surfaces for more than 45 days. All of the strains were highly hydrophobic. VISA adhered to latex, vinyl flooring and formica. Biofilm formation increased for all of the tested strains within 6-24 h. VISA present high survival, adherence and cell-surface hydrophobicity. Therefore, as the treatment of patients with VISA is a significant challenge for clinicians, greater care with cleaning and disinfection of different types of surfaces in healthcare facilities is recommended because these may become important reservoirs of multi-resistant pathogens.

Meticillin-resistant Staphylococcus aureus and meticillin-susceptible Staphylococcus aureus screening in a cohort of haemodialysis patients: carriage, demographics and outcomes

Patients on haemodialysis are vulnerable to colonization with Staphylococcus aureus due to frequent hospital contact, indwelling devices, and impaired immunity. Additionally colonization is associated with increased risk of infection. To determine the prevalence of both meticillin-resistant S. aureus (MRSA) and meticillin-susceptible S. aureus (MSSA) carriage in our haemodialysis cohort and to identify any risk factors predisposing to carriage, recolonization, or persistent carriage following a decolonization programme. All haemodialysis patients screened for S. aureus carriage between June 2009 and May 2011 were retrospectively followed up for 18 months using hospital electronic records. Statistical analysis was performed using IBM SPSS version 19. Out of 578 patients screened, 288 patients (49%) had at least one positive swab (10% MRSA, 90% MSSA). Of these patients, 265 completed a course of decolonization therapy following which 36% successfully eradicated (eradicators) and 64% did not (non-eradicators). There was no statistically significant difference in patient demography, type of vascular access, 18-month patient mortality, or number of hospital admissions between the two groups. Those who failed to eradicate were more likely to have had an episode of S. aureus bacteraemia within the study period compared to those who successfully decolonized (P = 0.003). Half of our haemodialysis cohort was colonized with S. aureus at any one time over an 18-month period. Following decolonization, one-third of patients remained successfully eradicated for 18 months. Non-eradicators have an increased risk of bacteraemia, which is associated with poor mortality. We would recommend routine screening and aggressive attempts to decolonize.

Clonal relatedness is a predictor of spontaneous multidrug efflux pump gene overexpression in Staphylococcus aureus

Increased expression of genes encoding multidrug resistance efflux pumps (MDR-EPs) contributes to antimicrobial agent and biocide resistance in Staphylococcus aureus. Previously identified associations between norA overexpression and spa type t002 meticillin-resistant S. aureus (MRSA), and a similar yet weaker association between mepA overexpression and type t008 meticillin-susceptible S. aureus (MSSA), in clinical isolates are suggestive of clonal dissemination. It is also possible that related strains are prone to mutations resulting in overexpression of specific MDR-EP genes. Exposure of non-MDR-EP-overexpressing clinical isolates to biocides and dyes can select for MDR-EP-overexpressing mutants. spa types t002 and t008 isolates are predominated by multilocus sequencing typing sequence types (STs) 5 and 8, respectively. In this study, non-MDR-EP gene-overexpressing clinical isolates (MRSA and MSSA) representing ST5 and ST8 were subjected to single exposures of ethidium bromide (EtBr) to select for EtBr-resistant mutants. Measurements of active EtBr transport among mutants were used to demonstrate an efflux-proficient phenotype. Using quantitative reverse-transcription PCR, it was found that EtBr-resistant mutants of ST5 and ST8 parental strains predominantly overexpressed mepA (100%) and mdeA (83%), respectively, regardless of meticillin sensitivity. Associations between clonal lineage and MDR-EP gene overexpression differed from those previously observed and suggest the latter is due to clonal spread of efflux-proficient strains. The predilection of in vitro-selected mutants of related strains to overexpress the same MDR-EP gene indicates the presence of a consistent mutational process.

Pharmacodynamic profile of commonly utilised parenteral therapies against meticillin-susceptible and meticillin-resistant Staphylococcus aureus collected from US hospitals

Staphylococcus aureus is a well-recognised pathogen with an evolving phenotypic profile often limiting conventional β-lactam use. In vitro potency and pharmacodynamic profile of commonly utilised agents against 1238 meticillin-susceptible S. aureus (MSSA) and 1259 meticillin-resistant S. aureus (MRSA) from clinical specimens at 42 hospitals were assessed. Non-duplicate, non-urine isolates were tested by broth microdilution against cefazolin, ceftaroline, ceftriaxone, daptomycin, linezolid, nafcillin, tigecycline and vancomycin. Monte Carlo simulations were conducted using pharmacokinetic profiles from patients or volunteers to generate the probability of target attainment and determine the cumulative fraction of response (CFR), a modelling-based prediction tool of achieving pharmacokinetic/pharmacodynamic endpoints, for commonly used regimens. Of isolates tested, 62 MSSA (5.0%) were ceftriaxone-non-susceptible and 4 (0.3%) and 2 (0.2%) MRSA were ceftaroline- and daptomycin-non-susceptible, respectively. Against MSSA, cefazolin 1000mg q8h and nafcillin 2000mg q4h produced CFRs ≥90%. For ceftriaxone, only 2000mg q12h produced a CFR ≥90%. Against MSSA and MRSA, ceftaroline, daptomycin, linezolid and tigecycline provided CFRs ≥90%. Vancomycin produced similar CFRs against MSSA and MRSA. Vancomycin 1000mg and 15mg/kg q8h produced CFRs of 91% and 93%, respectively, whilst q12h doses were <90%. Against respiratory and blood isolates or ICU isolates, only vancomycin q8h produced desired CFRs, where the MIC90 was 2μg/mL. These data suggest cefazolin and nafcillin produce high CFRs against MSSA, whilst ceftriaxone at common doses may no longer be suitable. Vancomycin q8h is needed to optimise CFRs. Ceftaroline, daptomycin, tigecycline and linezolid produced sufficiently high CFRs against MSSA and MRSA utilising approved regimens.

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.

Selection and characterisation of Staphylococcus aureus mutants with reduced susceptibility to the investigational oxazolidinone MRX-I

MRX-I is a new oxazolidinone antimicrobial under development. In this study, the potential for development of resistance to MRX-I in Staphylococcus aureus was investigated and key mutations were characterised. Determination of spontaneous resistance frequency and the mutant selection window (MSW) were performed with meticillin-susceptible S. aureus (MSSA) ATCC 29213, meticillin-resistant S. aureus (MRSA) ATCC 33591 and two clinical MRSA isolates SA 0016 and SA 0017. Selected resistant mutants were sequenced for 23S rRNA as well as genes encoding the ribosomal proteins L3, L4 and L22. Resistance frequencies for the aforementioned strains were <8.25×10−12, <6.33×10−12, <2.96×10−13 and <4.52×10−13, respectively, and the MSW of MRX-I was 2–4, 1–4, 1–2 and 1–4mg/L, respectively. After 30 serial passages, MRX-I minimum inhibitory concentrations (MICs) increased up to 8- to 16-fold both against MSSA and MRSA, whilst linezolid MICs increased 128-fold against MSSA and 16- to 32-fold against MRSA. MRX-I resistance mutations were clustered mainly in 23S rRNA and L3 protein regions. The U2504A transversion in 23S rRNA dominated in MRX-I-resistant mutants. No mutations in L4 and L22 proteins were observed. MRX-I exhibits a low potential to develop resistance in S. aureus, with a reduced resistance propensity compared with linezolid.

Carriage of Staphylococcus species in the veterinary visiting dog population in mainland UK: Molecular characterisation of resistance and virulence

This study investigated the prevalence of nasal carriage of staphylococci in dogs and determined the characteristics of the isolates. A total of 724 dogs from 87 veterinary practices across the mainland UK were screened for carriage of Staphylococcus spp. All isolates were examined for meticillin resistance (MR) and the presence of the mecA gene investigated in those isolates showing resistance. All coagulase-positive staphylococci and MR coagulase-negative staphylococci (MRCoNS) were subjected to antimicrobial susceptibility testing. Spa typing and DNA microarray analysis of resistance and virulence genes was carried out on all MR S. aureus (MRSA) and a subset of meticillin susceptible S. aureus (MSSA). Staphylococci were isolated from 399 (55.1%) of the dogs; only seven (1%) carried MRSA, all of which were identified as the dominant UK healthcare-associated strain (EMRSA-15, ST22). MSSA was identified in 47 (6.5%) dogs, the sequence types of which have been suggested as precursors to successful MRSA clones. Forty (5.5%) dogs carried MRCoNS, while no dogs carried MR S. pseudintermedius, although this is increasingly reported in mainland Europe. Resistance to antimicrobials among the isolates varied between species, with multidrug resistance (MDR) in 87.5% of MRCoNS and 21.8% of coagulase positive staphylococci. Microarray analysis of MRSA and a subset of MSSA isolates identified numerous virulence genes associated with pathogenesis, which are commonly identified in isolates of human origin. However, no isolates carried Panton-Valentine leukocidin (PVL) genes. This study suggests that MRSA carriage is low in the vet visiting dog population, but there is a diverse range of virulence and resistance determinants in canine S. aureus and MRCoNS isolates.

Evaluation of the affinity of various species and strains of Staphylococcus to adhere to equine corneocytes

Meticillin-resistant Staphylococcus aureus (MRSA) strain USA 500 predominately colonizes horses and people working with them. Previous studies demonstrate that some Staphylococcus species exhibit higher affinity for corneocytes of specific mammalian species. The objective was to determine the relative affinities of various MRSA strains, meticillin-susceptible S.aureus (MSSA) strains and a meticillin-susceptible Staphylococcuspseudintermedius (MSSP) for equine corneocytes. We hypothesized that MRSA strain USA 500 would exhibit greater adhesion than other staphylococcal strains tested. Epidemic MRSA strains (USA 100, USA 300, USA 500 and USA 800), two MSSA control strains and an MSSP field strain were tested on corneocytes from 15 client-owned horses. Isolates were incubated with corneocytes in conditions (bacterial concentration of 10(8) colony-forming units/mL for 45min) recently shown to maximize adherence of S.aureus without competitive interference. A validated image-analysis system was used to quantify the cell surface density of bacterial adhesion. The MSSP strain adhered with significantly higher affinity (P<0.0015) to corneocytes than did MSSA strains. All MRSA strains other than USA 500 had significantly higher affinity than MSSA strains (P range <0.03 to <0.0015). There were no statistical differences in adhesion between strain USA 500 and the other MRSA strains tested. Meticillin-resistant S.aureus strain USA 500 did not adhere more robustly than other strains of Staphylococcus; therefore, its affinity to colonize horses may not be solely attributed to corneocyte adhesion. Additional studies are required to explain the epidemiological role of this strain as the predominant cause of colonization and infections of horses in North America.

Population structure and characterisation of Staphylococcus aureus from bacteraemia at multiple hospitals in China: association between antimicrobial resistance, toxin genes and genotypes

Staphylococcus aureus from bacteraemia at multiple hospitals in China were genetically characterised to improve understanding of its epidemiology. A total of 236 consecutive, non-duplicate S. aureus bacteraemia isolates were collected at 16 Chinese hospitals. Isolates were characterised by antimicrobial resistance, 19 toxin genes, agr alleles, multilocus sequence typing and spa typing. The prevalence of meticillin-resistant S. aureus (MRSA) was 47.5% (112/236). Forty-two sequence types (STs) and 63 spa types were identified, including 14 STs and 14 spa types for MRSA. Clonal complex (CC) 8, CC5, ST7 and CC188 accounted for 67.4% of the isolates. ST239–t030/t037–SCCmecIII–agrI was the predominant MRSA genotype (50%), followed by ST5–t002/t570–SCCmecII–agrII (8%). A vancomycin MIC ≥ 1mg/L was detected significantly more often in ST5–SCCmecII and ST239–t037–SCCmecIII, whereas rifampicin resistance was overwhelmingly associated with ST239–t030–SCCmecIII (P<0.001). Oxacillin MICs were relatively low for ST59-MRSA. Major genotypes of meticillin-susceptible S. aureus (MSSA) were ST7–t091/t796–agrI (16.1%), ST188–t189–agrI (12.1%) and ST398–t571/t034–agrI (5.6%). Toxin genes were identified in 95.8% of isolates and formed 89 toxin gene profiles. The toxin genes sea, selk, selq and sell were significantly more common in MRSA, whilst tsst-1, seb, sed, selm, seln, selp and selj were more prevalent in MSSA (P<0.001). The pvl gene was more commonly detected in CC59, whereas tsst-1 was more frequent in CC15, CC188 and ST398 (P<0.001). The major genotypes were associated with specific antimicrobial resistance and toxin gene profiles.