Plasmid-Encoded Transferable mecB-Mediated Methicillin Resistance in Staphylococcus aureus

During cefoxitin-based nasal screening, phenotypically categorized methicillin-resistant Staphylococcus aureus (MRSA) was isolated and tested negative for the presence of the mecA and mecC genes as well as for the SCCmec-orfX junction region. The isolate was found to carry a mecB gene previously described for Macrococcus caseolyticus but not for staphylococcal species. The gene is flanked by β-lactam regulatory genes similar to mecR, mecI, and blaZ and is part of an 84.6-kb multidrug-resistance plasmid that harbors genes encoding additional resistances to aminoglycosides (aacA-aphD, aphA, and aadK) as well as macrolides (ermB) and tetracyclines (tetS). This further plasmidborne β-lactam resistance mechanism harbors the putative risk of acceleration or reacceleration of MRSA spread, resulting in broad ineffectiveness of β-lactams as a main therapeutic application against staphylococcal infections.

Novel SCCmec type XIII (9A) identified in an ST152 methicillin-resistant Staphylococcus aureus

We identified a novel staphylococcal cassette chromosome mec (SCCmec) element in an ST152 methicillin-resistant Staphylococcus aureus (MRSA) isolate by combining Illumina and MinION sequencing. The element contains a new ccrC allotype designated ccrC2. The mec complex resembles mec class A, but with an altered organization of the genes. The element was acknowledged as novel by the International Working Group on the Classification of Staphylococcal Cassette Chromosome Elements (IWG-SCC) and designated SCCmec type XIII (9A).

Characterization of Livestock-Associated Methicillin-Resistant Staphylococcus aureus CC398 and mecC-positive CC130 from Zoo Animals in the United Kingdom

Little is known about the characteristics and diseases associated with methicillin-resistant Staphylococcus aureus (MRSA) in nondomestic animals. Four presumptive MRSA isolates, obtained from clinical (n = 3) and surveillance specimens (n = 1) from dwarf (Helogale parvula) and yellow mongooses (Cynictis penicillata) from a United Kingdom zoo, were analyzed by PCR for detection of mecA and mecC-mediated methicillin resistance, and virulence genes. Isolates were genotyped by multilocus sequence typing (MLST) and staphylococcal cassette chromosome mec (SCCmec) and spa sequence typing. Three isolates, obtained from the dwarf mongooses, carried mecA, tetK, and fexA resistance and virulence genes (icaA, icaD, and sec) and were typed to SCCmec IVa, spa type t899, and clonal complex (CC) 398. The fourth MRSA isolate, obtained from the femoral bone marrow of a yellow mongoose showing postmortem findings consistent with septicemia, carried mecC and was oxacillin/cefoxitin susceptible, when tested at 37°C but showed a characteristic MRSA susceptibility profile at 25°C ± 2°C. Furthermore, this isolate exhibited a different genetic background (SCCmecXI/t843/CC130) and had biofilm-associated genes (bap, icaA, and icaD) and tetK tetracycline resistance genes. This work describes the first isolation of livestock-associated MRSA CC398 from two zoo mongoose species where it was associated with both clinical disease and colonization, and the first isolation of mecC MRSA from a zoo species in the United Kingdom. Both reports highlight the potential for zoo species to act as reservoirs for these zoonotic agents.

Nasal and pharyngeal carriage of methicillin-resistant Staphylococcus sciuri among hospitalised patients and healthcare workers in a Serbian university hospital

There has been a paucity of data on methicillin-resistant Staphylococcus sciuri (MRSS) epidemiology in European healthcare settings. The aim of the study was to determine the prevalence of nasal and pharyngeal carriage and diversity of MRSS among inpatients and healthcare workers (HCWs) in the largest healthcare centre in Serbia, and to assess performance of different methods for MRSS screening. Nasal and pharyngeal swabs were obtained from 195 patients and 105 HCWs in different departments. Each swab was inoculated directly onto MRSA-ID, oxacillin-resistance screening agar and mannitol salt agar (MSA) with 2 mg/L of oxacillin. After inoculation, each swab was dipped in Mueller-Hinton broth with 6.5% NaCl and after overnight incubation, subcultured onto oxacillin-MSA. Characterisation of isolated MRSS strains was determined by antimicrobial susceptibility testing, PFGE, SCCmec typing and antimicrobial resistance genes detection. MRSS nasal and pharyngeal carriage rate was high (5%) in our hospital and department-variable. PFGE revealed a possible cross-transmission of MRSS between a patient and an HCW, and dissemination across hospital wards. All analysed isolates were multidrug resistant. Fusidic acid resistance was discovered in 93.7% of isolates, but fusA mutations in EF-G and fusB/C genes were not detected. SCCmec regions of MRSS contained elements of classic methicillin-resistant S. aureus type III. Broth enrichment prior to isolation on oxacillin-MSA was superior to direct cultivation on different media with a sensitivity/specificity of 100% and 88.5%, respectively. MRSS is a significant coloniser of patients and HCWs in the hospital. Further research is needed to investigate the clinical significance of the bacterium in our settings.

Surveillance of variant mecA gene and analysis of efflux pump in mecA negative MRSA infections

It has been known from the medical records of Kancheepuram district hospitals that there is a prevalence of Methicillin resistant Staphylococcus aureus strain. Now the recent epidemiology has shown the presence of Methicillin Resistant Staphylococcus aureus even in school going children community individuals. In view of the above situation, Methicillin resistant Staphylococcus aureus screening was undertaken in the above said region. Both samples from hospital sources and school going children were screened for Methicillin Resistant Staphylococcus aureus. The results have shown that the samples had multiantibiotic resistant property along with methicillin resistance. The genomic studies have revealed that there were of positive genomic sequence for mec A and 5 isolates for mecA negative gene sequence. This would assume that even negative mec A genome has the resistant expression. Out of the 5 isolates two isolates have shown the presence of both nor A and qac A/B genes. The above results are discussed with efflux of drugs and future proposal on proteomics.

Biofilm formation of methicillin-resistant coagulase negative staphylococci (MR-CoNS) isolated from community and hospital environments

Methicillin-resistant coagulase negative staphylococci (MR-CoNS) are the major cause of infectious diseases because of their potential ability to form biofilm and colonize the community or hospital environments. This study was designed to investigate the biofilm producing ability, and the presence of mecA, icaAD, bap and fnbA genes in MR-CoNS isolates. The MR-CoNS used in this study were isolated from various samples of community environment and five wards of hospital environments, using mannitol salt agar (MSA) supplemented with 4 μg/ml of oxacillin. The specie level of Staphylococcus haemolyticus, Staphylococcus epidermidis, Staphylococcus hominis and Staphylococcus warneri was identified by specific primers of groESL (S. haemolyticus), rdr (S. epidermidis) and nuc (S. hominis and S. warneri). The remainder isolates were identified by tuf gene sequencing. Biofilm production was determined using Congo red agar (CRA) and Microtiter plate (MTP) assay. The mecA and biofilm associated genes (icaAD, fnbA and bap) were detected using PCR method. From the 558 samples from community and hospital environments, 292 MR-CoNS were isolated (41 from community environments, and 251 from hospital environments). S. haemolyticus (41.1%) and S. epidermidis (30.1%) were the predominant species in this study. Biofilm production was detected in 265 (90.7%) isolates by CRA, and 260 (88.6%) isolates were detected by MTP assay. The staphylococci isolates derived from hospital environments were more associated with biofilm production than the community-derived isolates. Overall, the icaAD and bap genes were detected in 74 (29.5%) and 14 (5.6%) of all isolates from hospital environments. When tested by MTP, the icaAD gene from hospital environment isolates was associated with biofilm biomass. No association was found between bap gene and biofilm formation. The MR-CoNS isolates obtained from community environments did not harbor the icaAD and bap genes. Conversely, fnbA gene presented in MR-CoNS isolated from both community and hospital environments. The high prevalence of biofilm producing MR-CoNS strains demonstrated in this study indicates the persisting ability in environments, and is useful in developing prevention strategies countering the spread of MR-CoNS.

Comparison of Oxacillin and Cefoxitin for the Detection of mecAGene to Determine Methicillin Resistance in Coagulase Negative Staphylococci(CoNs)

The aim of the study was to compare the effectiveness of Cefoxitin with that of Methicillin/Oxacillin in the determination of mecAgene in Methicillin resistant Coagulase-negative staphylococci(CoNS). We assessed 57 CoNS isolates for mecA gene via PCR, which were subsequently subjected to Methicillin/Oxacillin and Cefoxitin disc diffusion test. These methods are simple, inexpensive and easily available compared to PCR despite less specificity. Out of 41 mecApositive species, 33 (80.5%) were resistant to Methicillin/Oxacillin. Cefoxitin-resistance was seen in all 41 (100%) mecApositive samples. Two (12.5%) mecAnegative isolates of S.saprophyticuswere Methicillin/Oxacillin resistant, but were Cefoxitin sensitive. Four (9.7%) isolates of S.saprophyticus, three (7.3%) of S.epidermidisspecies, and one (2.4%) S.haemolyticusthat were mecApositive were sensitive to Methicillin/Oxacillin but resistant to Cefoxitin. Cefoxitin resistance provides a more accurate picture of mecAgene positivity as compared to Methicillin and Oxacillin.

Direct repeat unit (dru) typing and antimicrobial resistance of methicillin-resistant Staphylococcus pseudintermedius isolated from dogs in Atlantic Canada

There are few reports investigating the characterization of methicillin-resistant Staphylococcus pseudintermedius (MRSP) in dogs in Canada and none from Atlantic Canada. The objectives of this study were to strain type MRSP isolates cultured at a regional diagnostic laboratory using direct repeat unit (dru) typing and to describe their antimicrobial resistance profiles. Ninety-four isolates recovered from dogs between 2010 and 2012 had dru typing, cluster analysis, and antimicrobial susceptibility testing done. The majority of isolates belonged to type dt11a (30.9%), dt10h (24.5%), dt9a (18.1%), and dt11af (10.6%) with the remaining 15.9% of isolates distributed among 13 dru types. The predominant dru types identified were similar in Ontario; however, cluster 9a appears to be less common in Atlantic Canada. A significant difference in the distribution of clusters among Atlantic provinces was detected (P = 0.01). Resistance to ≥ 2 non-β-lactam antimicrobials was observed in 71.4% of the isolates. The MRSP isolates from this study were notably less resistant than those reported in the literature. A more comprehensive study of the MRSP dru types could help further elucidate the distribution of this pathogen in Canada.

The epidemiology of Staphylococcus aureus carriage in patients attending inner city sexually transmitted infections and community clinics in Calgary, Canada

BACKGROUND

Although the nares represent the most common carriage site for traditional hospital-associated strains of Staphylococcus aureus (SA), the predominant site of carriage of SA in the community is less certain.

METHODS

We conducted a cross-sectional study in 285 patients attending sexually transmitted diseases and inner-city clinics to evaluate the prevalence, body site colonisation and risk factors associated with carriage of methicillin susceptible SA (MSSA). All isolates were characterized by pulsed field gel electrophoresis, staphylococcal cassette chromosome mec, staphylococcal protein A and multilocus sequence typing.

RESULTS

The prevalence of colonisation with SA was 57.5% (164/285); 162 (56.8%) participants were colonized with MSSA, and 4 (1.4%) with methicillin-resistant SA (MRSA), 2 of them were co-colonised with both MRSA and MSSA. The most common sites of colonisation were the throat (73.1%), nares (65.2%) and interdigital web spaces of the hand (21.3%). Three out of 4 MRSA isolates were USA300-MRSA strains. Twelve MSSA isolates were closely related to the USA300 CA-MRSA. We identified sexual behaviours such as having more than 6 heterosexual sexual partners in the last 6 months and trimming pubic hair to be independently associated with MSSA colonisation, and more specifically practicing oral sex as a risk factor for throat colonisation.

CONCLUSION

There is a high prevalence of MSSA carriage in this population, with a low prevalence of MRSA. The throat was the most common site of carriage and sexual behaviours were found to be risk factors for MSSA colonisation. Close strain relatedness of MSSA and USA300-MRSA isolates suggests either gain or loss of the SCCmec element, respectively.

Molecular Epidemiology of Methicillin-Resistant and Methicillin-Susceptible Staphylococcus aureus in the Ovine Dairy Chain and in Farm-Related Humans

Staphylococcus aureus is a major cause of clinical infections in humans and its enterotoxins cause foodborne disease. In the present study, we tested a total of 51 isolates of S. aureus from small-ruminant dairy farms with artisan dairy facilities, all located in Latium, Italy. The farms have a known history of a high prevalence of methicillin-resistant S. aureus (MRSA). Most of the MRSA isolates (27 of 51) belonged to spa-type t127 (43.1%), followed by t2678 (3.9%), t044 (2%), t1166 (2%), and t1773 (2%). PFGE performed on mecA positive strains identified one cluster (≥ 80% of similarity), comprising 22 MRSA. Nine of twenty-two MRSA isolates were assigned human host origin, and 13 isolates did not belong to a specific host. During the characterization study, one strain isolated from bulk tank milk samples harbored the pvl gene; the strain was not enterotoxigenic with a non-specific host according to the biotyping scheme, highlighting the possible emerging risk of transmission of bacterial virulence factors by foods, the environment, and foodhandlers. These findings stress the importance of hygienic measures at all processing steps of the food production chain and underline that monitoring for the presence of MRSA throughout the food chain is essential for public health.

mecA-related structure in methicillin-resistant coagulase-negative staphylococci from street food in Taiwan

Antibiotic-resistant patterns, a mecA homologue complex, and staphylococcal cassette chromosome mec (SCCmec) were analysed in samples of ready-to-eat (RTE) street food in Taiwan. RTE food samples (270) were collected in three densely populated Taiwanese cities between June and November 2014. Among 14 strains being identified as methicillin-resistant coagulase-negative staphylococci (MRCoNS), genetic diversities was determined by PFGE analysis. SCCmec types IV, V, VIII and TXG-24 were detected in 9, and mecASs (a mecA homologue) detected in 8. The mecASs gene complex from S. sciuri subsp. sciuri TXG-24 was found to be closely related to those found in both S. sciuri subsp. sciuri (ATCC29062) and S. sciuri subsp. rodentium (ATCC700061). SCCmecTXG24 carries a class A mec complex, a ccrA5B3-like gene complex, a heavy metal gene complex, and an IS1216 mobile element carrying tet(S). Matching identity to ccrA5 was 84.5% for ccrA in S. pseudintermedius KM241. Matching identify to ccrB3 was 92.1% for ccrB in S. pseudintermedius AI16. Similar ccrA and SCCmec boundary sequences suggest that SCCmec is easily transmitted to coagulase-negative staphylococci (CoNS). Based on MRCoNS strains identified in this research, Taiwanese RTE food products likely carry multiple antibiotic resistance genes that can be transmitted to hospitals and other clinical settings.

Methicillin-Resistant Bacteria Inhabiting Surface Waters Monitored by mecA-Targeted Oligonucleotide Probes

Part of a 20-60 kb staphylococcal chromosome cassette called mecA encodes low-affinity penicillin-binding protein PBP2a and causes methicillin resistance. Among all methicillin-resistant bacteria, methicillin-resistant Staphylococcus aureus is a major pathogen and main concern worldwide. Although the origin of the mecA is not very well-defined, mecA homologues are also ubiquitous in methicillin-resistant non-staphylococcal bacteria. Due to the dissemination of methicillin resistance through the transmission of mecA gene among staphylococcal and non-staphylococcal bacteria inhabiting surface waters, there is a need to monitor mecA gene in these waters for public health safety. Therefore, this study aimed at monitoring mecA harboring bacteria inhabiting surface waters by using fluorescently labelled mecA-targeted oligonucleotide probes. Under the hybridization conditions of 55 % formamide and 0.020 M NaCl at 46°C, the oligonucleotide probe used in the study showed high hybridization stringency to the mecA gene targeted. The strong linear relationships observed between the signal intensity and the target gene were used to assess the population dynamics of mecA harboring isolates over a 2-year-period. The results indicated that mecA-targeted oligonucleotide probes can be effectively used for in situ monitoring of methicillin resistant isolates inhabiting surface waters.

Characteristics of Patients With Healthcare-Associated Infection Due to SCCmecType IV Methicillin-ResistantStaphylococcus aureus

Objective.

Methicillin-resistantStaphylococcus aureus(MRSA) with the staphylococcal cassette chromosomemec(SCCmec) type IV allele is most commonly associated with community-acquired MRSA (CA-MRSA) infection; however, such organisms have also been identified in the healthcare setting. The objective of the present study was to characterize the epidemiology of and clinical outcomes associated with SCCmec-IV MRSA infection acquired in the healthcare setting, compared with infection caused by MRSA of other SCCmectypes.

Design.

We evaluated a cohort of 100 inpatients with MRSA infection that met the Centers for Disease Control and Prevention definition for healthcare-associated infection and compared the patients' demographic characteristics, the antimicrobial susceptibilities of the MRSA isolates, the infection types, and the associated clinical and microbiological outcomes. For each MRSA isolate, the SCCmectype and the presence of Panton-Valentine leukocidin (PVL) were determined by polymerase chain reaction methods.

Results.

SCCmec-IV MRSA isolates were isolated from 53 patients (42% of these isolates were positive for PVL), and SCCmec-II or SCCmec-III MRSA was isolated from 47 patients (3% of these isolates were positive for PVL). No differences were noted between the patients in the SCCmec-II/III group and the patients in the SCCmec-IV group with respect to age (median, 55 vs 50 years); sex (77% vs 64% of patients were male); medical service (surgical service, 60% in both groups; ICU admission, 55% vs 53%), Acute Physiology and Chronic Health Evaluation II score (median, 8 vs. 7); infection type; or underlying comorbidities, except for presence of a burn wound (13% vs 2%;P< .04). Patients in the SCCmec-II/III group were more likely to have multiple sites of infection (P= .006) and a longer length of stay (LOS) prior to detection of MRSA than were patients in the SCCmec-IV group (median, 4 vs 1 days;P< .001). Total LOS was significantly greater for patients in the SCCmec-II/III, compared with those in the SCCmec-IV group (P= .006). Multiple logistic regression identified liver disease and longer LOS prior to detection of MRSA as predictors of infection with SCCmec-II/III MRSA. Rates of susceptibility to clindamycin, gentamicin, ciprofloxacin, levofloxacin, and tetracycline was significantly greater among SCCmec-IV MRSA isolates, compared with type II/III isolates (P⩽ .05). Compared with SCCmec-IV isolates acquired in the community, the susceptibility rates among healthcare-associated SCCmec-IV isolates was significantly less for clindamycin, gentamicin, and levofloxacin, indicating that these organisms may quickly acquire resistance to non-β-lactam antibiotics, as do SCCmec-II/III strains.

Conclusions.

SCCmec-IV MRSA appears to have become established in hospitals. The onset of infection caused by SCCmec-IV strains is earlier than the onset of infection with SCCmec-II/III strains; however, associated types of infection are similar. Infection with SCCmec- II/III MRSA is currently associated with an adverse impact on outcome, compared with infection with SCCmec-IV MRSA. Further research is warranted to determine the impact of SCCmectype IV strains in hospital settings.

Affinity Capillary Electrophoresis for the Screening of Novel Antimicrobial Targets

The increasing number of multiantibiotic-resistant organisms, including methicillin-resistant Staphylococcus aureus (MRSA), requires the development of novel chemotherapies that are structurally distinct and exempt from current resistance mechanisms. Bioinformatics data mining of microbial genomes has revealed numerous previously unexploited essential open reading frames (ORFs) of unknown biochemical function. The potential of these proteins as screening targets is not readily apparent because most screening technologies rely on knowledge of biological function. To address this problem, the authors employed affinity capillary electrophoresis (ACE) to identify antimicrobial compounds that bound the novel target YihA. Screening a small-molecule library of 44,000 compounds initially identified 115 binders, of which 76% were confirmed. Furthermore, the ACE assay distinguished diverse compounds that possessed drug-like properties and antimicrobial activity against drug-resistant clinical isolates. These data validate ACE as a valuable tool for the fast, efficient detection of specific binding molecules that possess biological activity.

Descriptive Epidemiology and Case-Control Study of Patients Colonized With Vancomycin-Resistant Enterococcus and Methicillin-ResistantStaphylococcus aureus

Background.

Patients colonized or infected with vancomycin-resistant enterococcus and methicillin-resistantStaphylococcus aureusmay be at risk of acquiring vancomycin-resistantS. aureusif the vanA gene is transferred from vancomycin-resistant enterococcus to methicillin-resistantS. aureus..

Objective.

Our goal was to identify risk factors for cocolonization or coinfection (CC/CI) with vancomycin-resistant enterococcus and methicillin-resistantS. aureus.

Design.

We conducted a descriptive, epidemiologic study of all patients with CC/CI identified from January 1998 to May 2003 and a nested case-control study of a cohort of patients hospitalized in the burn and wound unit.

Setting.

We conducted our study in a 813-bed tertiary care university teaching hospital.

Population.

The study population consisted of patients found to have CC/CI during the study period.

Methods.

Descriptive epidemiologic data were collected from hospital records of all patients identified as having CC/CI. A subset of patients hospitalized in the burn and wound unit were included in a case-control study.

Results.

CC/CI was detected in 71% of the patients during a single hospital stay. The burn and wound unit, which does active surveillance for both organisms, and the general medicine unit, which does not do active surveillance for either organism, cared for more than one-half of these patients. Among patients being cared for in the burn and wound unit, having exposure to 2 or more invasive devices (central venous catheters, indwelling urinary catheters, and enteral feeding tubes) and renal insufficiency were independent risk factors for CC/CI.

Conclusions.

Patients with CC/CI are the population at greatest risk for vancomycin-resistantS. aureuscolonization or infection. The number of invasive devices to which patients are exposed and, thus, possibly the patients' underlying severity of illness, as well as renal insufficiency, appear to be risk factors for CC/CI.

The Recombinant Bacteriophage Endolysin HY-133 Exhibits In Vitro Activity against Different African Clonal Lineages of the Staphylococcus aureus Complex, Including Staphylococcus schweitzeri

HY-133 is a recombinant bacteriophage endolysin with bactericidal activity againstStaphylococcus aureus Here, HY-133 showedin vitroactivity against major African methicillin-susceptible and methicillin-resistantS. aureuslineages and ceftaroline/ceftobiprole- and borderline oxacillin-resistant isolates. HY-133 was also active againstStaphylococcus schweitzeri, a recently described species of theS. aureuscomplex. The activity of HY-133 on the tested isolates (MIC50, 0.25 μg/ml; MIC90, 0.5 μg/ml; range, 0.125 to 0.5 μg/ml) was independent of the species and strain background or antibiotic resistance.

Plasmid-mediated resistance and virulence mechanisms in the private health sector in KwaZulu-Natal, South Africa: An investigation of methicillin resistant Staphylococcus aureus (MRSA) clinical isolates collected during a three month period

OBJECTIVES

Due to the lack of information on the plasmid content of MRSA strains in South Africa (SA), this study investigated the resistance and virulence mechanisms of 27 clinical isolates from the private health care sector over a period of 3 months.

METHODS

Plasmids were extracted and the presence of MRSA confirmed by the presence of mecA. The isolates were subjected to antimicrobial susceptibility testing and molecular characterization of common resistance encoding genes and frequently encountered virulence factors by PCR using plasmid DNA as the template. The genetic relatedness between the isolates was determined by pulsed field gel electrophoresis (PFGE).

RESULTS

All isolates were plasmid positive, and displayed ampillicin, ciprofloxacin, gentamicin, rifampicin, tetracycline, erythromycin, and clindamycin resistance. They were all fully susceptible to daptomycin, linezolid, vancomycin, tigecycline and fusidic acid. Multidrug resistance (MDR) was found in 74.1% (20/27) of the MRSA isolates. The frequency of the resistance and virulence genes ranged from 100% to 0%. PFGE analysis revealed 10 pulsotypes, designated A-J, which showed correlation with resistance profile of the isolates in each group. Of note, 85.2% (23/27) of the isolates clustered into six major PFGE types giving an indication of similar circulating MRSA clones.

CONCLUSIONS

This study highlights the genetic diversity and resistance mechanisms in MRSA strains from the private health sector in SA hence the need for implementing effective infection control programs.

Whole-Genome Sequencing of Methicillin-Resistant Staphylococcus aureus Resistant to Fifth-Generation Cephalosporins Reveals Potential Non-mecA Mechanisms of Resistance

Fifth-generation cephalosporins, ceftobiprole and ceftaroline, are promising drugs for treatment of bacterial infections from methicillin-resistant Staphylococcus aureus (MRSA). These antibiotics are able to bind native PBP2a, the penicillin-binding protein encoded by the mecA resistance determinant that mediates broad class resistance to nearly all other beta-lactam antibiotics, at clinically achievable concentrations. Mechanisms of resistance to ceftaroline based on mecA mutations have been previously described. Here we compare the genomes of 11 total parent-daughter strains of Staphylococcus aureus for which specific selection by serial passaging with ceftaroline or ceftobiprole was used to identify novel non-mecA mechanisms of resistance. All 5 ceftaroline-resistant strains, derived from 5 different parental strains, contained mutations directly upstream of the pbp4 gene (coding for the PBP4 protein), including four with the same thymidine insertion located 377 nucleotides upstream of the promoter site. In 4 of 5 independent ceftaroline-driven selections, we also isolated mutations to the same residue (Asn138) in PBP4. In addition, mutations in additional candidate genes such as ClpX endopeptidase, PP2C protein phosphatase and transcription terminator Rho, previously undescribed in the context of resistance to ceftaroline or ceftobiprole, were detected in multiple selections. These genomic findings suggest that non-mecA mechanisms, while yet to be encountered in the clinical setting, may also be important in mediating resistance to 5th-generation cephalosporins.

Synergistic, collaterally sensitive β-lactam combinations suppress resistance in MRSA

Methicillin-resistant Staphylococcus aureus (MRSA) is one of the most prevalent multidrug-resistant pathogens worldwide, exhibiting increasing resistance to the latest antibiotic therapies. Here we show that the triple β-lactam combination meropenem-piperacillin-tazobactam (ME/PI/TZ) acts synergistically and is bactericidal against MRSA subspecies N315 and 72 other clinical MRSA isolates in vitro and clears MRSA N315 infection in a mouse model. ME/PI/TZ suppresses evolution of resistance in MRSA via reciprocal collateral sensitivity of its constituents. We demonstrate that these activities also extend to other carbapenem-penicillin-β-lactamase inhibitor combinations. ME/PI/TZ circumvents the tight regulation of the mec and bla operons in MRSA, the basis for inducible resistance to β-lactam antibiotics. Furthermore, ME/PI/TZ subverts the function of penicillin-binding protein-2a (PBP2a) via allostery, which we propose as the mechanism for both synergy and collateral sensitivity. Showing in vivo activity similar to that of linezolid, ME/PI/TZ demonstrates that combinations of older β-lactam antibiotics could be effective against MRSA infections in humans.

A Plasmid-Borne System To Assess the Excision and Integration of Staphylococcal Cassette Chromosome mec Mediated by CcrA and CcrB

Resistance to methicillin and other β-lactam antibiotics in staphylococci is due to mecA, which is carried on a genomic island, staphylococcal cassette chromosome mec (SCCmec). The chromosomal excision and integration of SCCmec are mediated by the site-specific recombinase CcrAB or CcrC, encoded within this element. A plasmid-borne system was constructed to assess the activities of CcrA and CcrB in the excision and integration of SCCmec in Escherichia coli and Staphylococcus aureus. The excision frequency in E. coli mediated by CcrAB from methicillin-resistant S. aureus (MRSA) strain N315 was only 9.2%, while the integration frequency was 31.4%. In S. aureus the excision and integration frequencies were 11.0% and 18.7%, respectively. Truncated mutants identified the N-terminal domain of either CcrB or CcrA to be necessary for both integration and excision, while the C-terminal domain was important for recombination efficiency. Site-directed mutagenesis of the N-terminal domain identified S11 and R79 of CcrA and S16, R89, T149, and R151 of CcrB to be residues essential for catalytic activities, and the critical location of these residues was consistent with a model of the tertiary structure of the N terminus of CcrA and CcrB. Furthermore, CcrAB and CcrC, cloned from a panel of 6 methicillin-resistant S. aureus strains and 2 methicillin-resistant Staphylococcus epidermidis strains carrying SCCmec types II, IV, and V, also catalyzed integration at rates 1.3 to 10 times higher than the rates at which they catalyzed excision, similar to the results from N315. The tendency of SCCmec integration to be favored over excision may explain the low spontaneous excision frequency seen among MRSA strains.

IMPORTANCE

Spontaneous excision of the genomic island (SCCmec) that encodes resistance to beta-lactam antibiotics (methicillin resistance) in staphylococci would convert a methicillin-resistant strain to a methicillin-susceptible strain, improving therapy of difficult-to-treat infections. This study characterizes a model system by which the relative frequencies of excision and integration can be compared. Using a plasmid-based model for excision and integration mediated by the recombinases CcrA and CcrB, integration occurred at a higher frequency than excision, consistent with the low baseline excision frequency seen in most strains. This model system can now be used to study conditions and drugs that may raise the SCCmec excision frequency and generate strains that are beta-lactam susceptible.

[Homologous Analysis Using Repetitive-sequence-based PCR Typing of Exfoliative Toxin-producing Staphylococcus aureus Isolated from Our Hospital]

We examined staphylococcal coagulase types and homologous analysis using the DiversiLab repetitive-sequence-based PCR system in exfoliative toxin (ET)-producing Staphylococcus aureus. Twenty-two isolates (17 methicillin-sensitive Staphylococcus aureus (MSSA) and 5 methicillin-resistant Staphylococcus aureus (MRSA) isolates) obtained in our hospital from January 2012 and December 2013 were used. Three groups were classified according to the coagulase types and serotypes of ET. The first group (4 MSSA) showed coagulase type I and ET-A, and the second group (3 MSSA and 2 MRSA) showed coagulase type I and ET-B. The third group (10 MSSA and 3 MRSA) showed coagulase type V and ET-B. An analysis by DiversiLab demonstrated that homology was high in both the first and second groups. The homogenousness was high among the third group isolates except for the ocular isolates. In our hospital, three important groups were present according to a coagulase type and an ET type, and the homology of ocular isolates could be different from other materials isolates.

Evaluation of various staphylococcal cassette chromosome mec (SCCmec) types in Staphylococcus epidermidis invasive strains from hospitalised patients in Iran

Staphylococcus epidermidis is known to be a major cause of nosocomial infections particularly in catheter-associated bacteraemia, prosthetic valve endocarditis (PVE) and immunocompromised patients in different health care units. The emergence of multidrug-resistant strains, especially to β-lactam antibiotics such as methicillin, has increased the mortality due to S. epidermidis. A kind of low affinity penicillin-binding protein (PBP2α), which is encoded by the mecA gene that is located in the staphylococcal cassette chromosome mec (SCCmec), mediates the resistance to methicillin. The aim of this study was to investigate the prevalence of SCCmec types and evaluate the antibiotic profile assay in invasive strains isolated from clinical samples. The study focused on invasive strains, determining the antimicrobial resistance profile, designing new primers for detection of the mecA gene and SCCmec typing with the multiplex PCR method. By using the PCR molecular test, 87.1% of all isolates were found to be positive for the mecA gene. In SCCmec typing, different types (I-V) were identified, in which SCCmec type I was detected in 3 isolates, SCCmec type II in 5 isolates, SCCmec type III in 22 isolates, SCCmec type IV in 27 isolates and SCCmec type V was distinguished in 4 isolates. Since coagulase-negative staphylococci are reported as a major cause of hospital infections, molecular typing methods like SCCmec typing would be a helpful method to control and prevent bacterial infections.

Emergence of Staphylococcal Cassette ChromosomemecType IV Methicillin-ResistantStaphylococcus aureusas a Cause of Ventilator-Associated Pneumonia

Staphylococcal cassette chromosomemec(SCCmec) type IV methicillin-resistantStaphylococcus aureus(MRSA) strains were identified in 8 (19.5%) of 41 consecutive patients with MRSA ventilator-associated pneumonia (VAP) in this retrospective, observational study. There were no significant differences in VAP severity and crude mortality rates between patients with SCCmectype II strains and patients with SCCmectype IV strains.

Risk Factors for Methicillin-Resistant Staphylococcus Aureus Carriage in Residents of German Nursing Homes

Objectives:

To determine the prevalence of and the risk factors for methicillin-resistant Staphylococcus aureus (MRSA) carriage in nursing home residents in the Rhine-Neckar region of southern Germany.

Design:

Point-prevalence survey.

Setting:

Forty-seven nursing homes in the region.

Participants:

All residents of the approached nursing homes who agreed to participate.

Methods:

After informed consent was obtained, all participants had their nares swabbed, some personal data collected, or both. All swabs were examined for growth of MRSA All S. aureus isolates underwent oxacillin susceptibility testing and polymerase chain reaction for demonstration of the meek gene. All MRSA isolates were typed using pulsed-field gel electrophoresis after digestion with SmaI.

Results:

Swabs from 3,236 nursing home residents yielded 36 MRSA strains, contributing to a prevalence rate of 1.1%. Significant risk factors for MRSA carriage in the multivariate analysis were the presence of wounds or urinary catheters, limited mobility, admission to a hospital during the preceding 3 months, or stay in a medium-size nursing home. One predominant MRSA strain could be detected in 30 of the 36 MRSA carriers.

Conclusions:

The prevalence of MRSA in German nursing homes is still low. These residents seemed to acquire their MRSA in the hospital and transfer it to their nursing home. Apart from well-known risk factors for the acquisition of MRSA we identified the size of the nursing home as an independent risk factor. This might be due to an increased use of antimicrobials in nursing homes of a certain size.

Real-Time Polymerase Chain Reaction Detection of Methicillin-ResistantStaphylococcus aureus:Impact on Nosocomial Transmission and Costs

Objectives.

To assess the impact of real-time polymerase chain reaction (PCR) detection of methicillin-resistantStaphylococcus aureus(MRSA) on nosocomial transmission and costs.

Design.

Monthly MRSA detection rates were measured from April 1, 2000, through December 31, 2005. Time series analysis was used to identify changes in MRSA detection rates, and decision analysis was used to compare the costs of detection by PCR and by culture.

Setting.

A 1,200-bed, tertiary care hospital in Canada.

Patients.

Admitted patients at high risk for MRSA colonization. MRSA detection using culture-based screening was compared with a commercial PCR assay.

Results.

The mean monthly incidence of nosocomial MRSA colonization or infection was 0.37 cases per 1,000 patient-days. The time-series model indicated an insignificant decrease of 0.14 cases per 1,000 patient-days per month (95% confidence interval, —0.18 to 0.46) after the introduction of PCR detection (P= .39). The mean interval from a reported positive result until contact precautions were initiated decreased from 3.8 to 1.6 days (P<.001). However, the cost of MRSA control increased from Can$605,034 to Can$771,609. Of 290 PCR-positive patients, 120 (41.4%) were placed under contact precautions unnecessarily because of low specificity of the PCR assay used in the study; these patients contributed 37% of the increased cost. The modeling study predicted that the cost per patient would be higher with detection by PCR (Can$96) than by culture (Can$67).

Conclusion.

Detection of MRSA by the PCR assay evaluated in this study was more costly than detection by culture for reducing MRSA transmission in our hospital. The cost benefit of screening by PCR varies according to incidences of MRSA colonization and infection, the predictive values of the assay used, and rates of compliance with infection control measures.