Multiplex polymerase chain reaction for detection of genes for Staphylococcus aureus thermonuclease and methicillin resistance and correlation with oxacillin resistance

Detecting mecA in Faecal Samples: A Tool for Assessing Carriage of Meticillin-Resistant Staphylococci in Pets and Owners in the Microbiological ‘Fast Age’?

Sampling animals for carriage of meticillin-resistant, coagulase-positive staphylococci (MRCoPS), considered zoonotic pathogens, can be challenging and time-consuming. Developing methods to identify mecA from non-invasive samples, e.g., faeces, would benefit AMR surveillance and management of MRS carrier animals. This study aimed to distinguish MRS carriers from non-carriers from faecal samples using quantitative polymerase chain reaction (qPCR) for mecA. Paired faecal and nasal swab samples (n = 86) were obtained from 13 dogs and 20 humans as part of a longitudinal study. Nasal MRCoPS carriage (either MR-Staphylococcus aureus or MR-Staphylococcus pseudintermedius was confirmed by identification of species (nuc) and meticillin resistance (mecA) (PCR). Faecal DNA (n = 69) was extracted and a qPCR method was optimised to provide a robust detection method. The presence of faecal mecA was compared between MRS carriers and non-carriers (Kruskal–Wallis test). Nasal swabbing identified seven canine and four human MRCoPS carriers. mecA was detected in 13/69 faecal samples, including four MRCoPS carriers and nine non-carriers. For dogs, there was no significant association (p = 1.000) between carrier status and mecA detection; for humans, mecA was more commonly detected in MRCoPS carriers (p = 0.047). mecA was detected in faeces of MRCoPS carriers and non-carriers by qPCR, but larger sample sizes are required to determine assay sensitivity. This rapid method enables passive surveillance of mecA in individuals and the environment.

Investigation of In Vitro Susceptibility and Resistance Mechanisms in Skin Pathogens: Perspectives for Fluoroquinolone Therapy in Canine Pyoderma

Fluoroquinolones (FQ) are commonly used in dogs with bacterial skin infections. Their use as first choice, along with the increased incidence of FQ-resistance, represents a risk to animal and public health. Our study determined minimum inhibitory (MIC) and bactericidal (MBC) concentrations of five FQs in Staphylococcus aureus, Staphylococcus pseudintermedius, and Escherichia coli, together with FQ-resistance mechanisms. MICs, efflux pump (EP) overexpression and MBCs were measured in 249 skin infection isolates following CLSI guidelines (CLSI VET01-A4, CLSI M26-A). Chromosomal and plasmid-mediated resistance genes were investigated after DNA extraction and sequencing. FQ-resistance was detected in 10% of methicillin-susceptible (MS), 90% of methicillin-resistant (MR) staphylococci and in 36% of E. coli. Bactericidal effect was observed except in 50% of MRSA/P for ciprofloxacin and in 20% of MRSPs for enrofloxacin. Highest MICs were associated with double mutation in gyrA (Ser83Leu + Asp87Asn), efflux pumps and three PMQR genes in E. coli, and grlA (Ser80Phe + Glu84Lys) in S. aureus. EP overexpression was high among E. coli (96%), low in S. aureus (1%) and absent in S. pseudintermedius. Pradofloxacin and moxifloxacin showed low MICs with bactericidal effect. Since in vitro FQ resistance was associated with MR, FQ use should be prudently guided by susceptibility testing.

Canine pyoderma: mecA persists autogenous bacterin formulation from meticillin-resistant Staphylococcus pseudintermedius (MRSP) and S. aureus (MRSA)

OBJECTIVE

Autogenous Staphylococcus pseudintermedius bacterins can reduce prescribing of antimicrobials in the management of canine recurrent pyoderma. However, increasing prevalence of meticillin-resistant, mecA-positive S. pseudintermedius (MRSP) raises concern over dispersal of mecA through bacterin therapy. We investigated the presence and integrity of mecA in bacterin formulations after manufacturing.

MATERIAL AND METHODS

Twenty clinical isolates (12 MRSP, 7 MR-S. aureus, 1 meticillin-susceptible SP) were investigated. Pellets from overnight growth were washed 3 times with 0.5 % phenol saline, followed by addition of 0.1 ml 10 % formal-saline to 10 ml phenol-saline. Sterility was confirmed, and DNA extracted using both a standard genomic extraction kit and one recommended for formalin-fixed tissue samples (FFPE). The presence of mecA was determined after PCR and its integrity examined in 5 randomly selected samples after sequencing.

RESULTS

In all bacterins from meticillin-resistant isolates, mecA was detected following FFPE extraction; products aligned fully to a reported mecA sequence. After standard DNA extraction, mecA was seen in 16/19 samples.

CONCLUSION

Persistence of mecA in MRSP bacterins suggests that dispersal of this important resistance mediator through therapy may be possible. While the ability of skin bacteria to uptake naked DNA remains unclear, it seems prudent to only formulate autogenous bacterins from mecA-negative S. pseudintermedius to avoid unnecessary spread of mecA.

Genetic resistance determinants to fusidic acid and chlorhexidine in variably susceptible staphylococci from dogs

BACKGROUND

Concern exists that frequent use of topically-applied fusidic acid (FA) and chlorhexidine (CHX) for canine pyoderma is driving clinically relevant resistance, despite rare description of FA and CHX genetic resistance determinants in canine-derived staphylococci. This study aimed to determine minimum inhibitory concentrations (MICs) and investigate presence of putative resistance determinants for FA and CHX in canine-derived methicillin-resistant (MR) and -susceptible (MS) staphylococci. Plasmid-mediated resistance genes (fusB, fusC, fusD, qacA/B, smr; PCR) and MICs (agar dilution) of FA and CHX were investigated in 578 staphylococci (50 MR S. aureus [SA], 50 MSSA, 259 MR S. pseudintermedius [SP], 219 MSSP) from Finland, U.S.A., North (NUK) and South-East U.K. (SEUK) and Germany. In all isolates with FA MIC ≥64 mg/L (n = 27) fusA and fusE were amplified and sequenced.

RESULTS

FA resistance determinants (fusA mutations n = 24, fusB n = 2, fusC n = 36) were found in isolates from all countries bar U.S.A. and correlated with higher MICs (≥1 mg/L), although 4 SP isolates had MICs of 0.06 mg/L despite carrying fusC. CHX MICs did not correlate with qacA/B (n = 2) and smr (n = 5), which were found in SEUK SA, and SP from NUK and U.S.A.

CONCLUSIONS

Increased FA MICs were frequently associated with fusA mutations and fusC, and this is the first account of fusB in SP. Despite novel description of qacA/B in SP, gene presence did not correlate with CHX MIC. Selection pressure from clinical use might increase prevalence of these genetic determinants, but clinical significance remains uncertain in relation to high skin concentrations achieved by topical therapy.

The Correlation between Airborne Methicillin-resistant Staphylococcus aureus with the Presence of MRSA Colonized Patients in a General Intensive Care Unit

Air sampling directly onto a methicillin-resistant Staphylococcus aureus (MRSA) selective agar was performed at six locations three times weekly over a period of 32 weeks in a new, initially MRSA-free Intensive Care Unit to examine if MRSA is present in air sample cultures and, if so, whether it is affected by the number of MRSA colonized patients present. A total of 480 air samples were collected on 80 days. A total of 39/480 (8.1%) samples were found to be MRSA positive of which 24/160 (15%) positive air samples were from the single rooms, where MRSA colonised patients were isolated, and 15/320 (4.7%) were from the open bed areas. A significant correlation was found between the daily number of MRSA colonized or infected patients in the Unit and the daily number of MRSA positive air samples cultures obtained (r2=0.128; P<0.005). The frequency of positive cultures was significantly higher in the single rooms than in the open bed areas (relative risk=3.2; P<0.001). The results from one of the single rooms showed a strong correlation between the presence of MRSA patients and MRSA positive air samples (relative risk=11.4; P<0.005). Our findings demonstrate that the presence of airborne MRSA in our unit is strongly related to the presence and number of MRSA colonized or infected patients in the Unit.

Complex ecological interactions of Staphylococcus aureus in tampons during menstruation

Menstrual toxic shock syndrome (mTSS) is a severe disease that occurs in healthy women vaginally colonized by Staphylococcus aureus producing toxic shock toxin 1 and who use tampons. The aim of the present study was to determine the impact of the composition of vaginal microbial communities on tampon colonisation by S. aureus during menses. We analysed the microbiota in menstrual fluids extracted from tampons from 108 healthy women and 7 mTSS cases. Using culture, S. aureus was detected in menstrual fluids of 40% of healthy volunteers and 100% of mTSS patients. Between class analysis of culturomic and 16S rRNA gene metabarcoding data indicated that the composition of the tampons' microbiota differs according to the presence or absence of S. aureus and identify discriminating genera. However, the bacterial communities of tampon fluid positive for S. aureus did not cluster together. No difference in tampon microbiome richness, diversity, and ecological distance was observed between tampon vaginal fluids with or without S. aureus, and between healthy donors carrying S. aureus and mTSS patients. Our results show that the vagina is a major niche of. S. aureus in tampon users and the composition of the tampon microbiota control its virulence though more complex interactions than simple inhibition by lactic acid-producing bacterial species.

Molecular analysis of the coagulase gene in clinical and nasal carrier isolates of methicillin-resistant Staphylococcus aureus by restriction fragment length polymorphism

OBJECTIVES

The coagulase enzyme, encoded by the coa gene, is an important virulence factor of Staphylococcus aureus and can be used for typing of S. aureus isolates. In this study, coa gene typing was used to study the epidemiology of S. aureus at a university hospital in Hamadān (Iran).

METHODS

A total of 200 S. aureus strains were analysed, among which 150 were isolated from clinical samples and 50 were from nasal swab specimens of carriers. Methicillin resistance was confirmed by presence of the mecA gene by PCR. For polymorphism analysis, the coa gene was amplified by PCR and the products were subjected to restriction digestion using the enzyme AluI.

RESULTS

Amplification of the coa gene produced five classes of bands based on size, ranging from 300bp to 800bp. The 600-bp amplicon included coa 3 genotype predominated in S. aureus isolated from clinical and carrier specimens (150/200; 75.0%). AluI digestion of the PCR products produced eight distinct restriction fragment length polymorphism (RFLP) patterns, designated coa 1-8. The results showed that the 700-bp and 800-bp amplicons formed two (coa 4a and 4b) and three (coa 5a, 5b and 5c) patterns following AluI digestion, respectively, whereas the 300-, 500- and 600-bp fragments generated unique patterns designated coa 1, coa 2 and coa 3, respectively.

CONCLUSIONS

This study performed coagulase typing, a technique used to determine the molecular epidemiology of S. aureus clinical isolates. coa gene amplification has been considered a simple and accurate method for typing of S. aureus.

Clinical, microscopic and microbial characterization of exfoliative superficial pyoderma-associated epidermal collarettes in dogs

BACKGROUND

The microscopic and microbial features of the spreading epidermal collarettes of canine exfoliative superficial pyodermas are poorly characterized.

OBJECTIVES

To characterize the clinical, cytological, microbial and histopathological features of epidermal collarettes in five dogs.

RESULTS

Cytology from the margins of collarettes identified neutrophils, extracellular and intracellular cocci within neutrophils but no acantholytic keratinocytes. Phenotypic and genotypic analyses identified all bacterial isolates from the centre and margin of five epidermal collarettes as Staphylococcus pseudintermedius. PCRs of collarette-associated Staphylococcus strains did not amplify genes encoding for the known exfoliative toxins expA and expB, whereas the predicted siet and speta amplification products were detected in all isolates. Microscopically, epidermal collarettes consisted of interfollicular, epidermal spongiotic pustules. Advancing edges of lesions consisted of peripheral intracorneal clefts in the deep stratum disjunctum above an intact stratum compactum; they contained lytic neutrophil debris, bacterial cocci and fluid, but no acantholytic keratinocytes. This intracorneal location of bacteria was confirmed using Gram stains and fluorescent in situ hybridization with eubacterial- and Staphylococcus-specific probes. The indirect immunofluorescence staining patterns of desmoglein-1, desmocollin-1, claudin-1, E-cadherin and corneodesmosin were discontinuous and patchy in areas of spongiotic pustules, whereas only that of corneodesmosin was weaker and patchy in advancing collarette edges.

CONCLUSION

Epidermal collarettes represent unique clinical and histological lesions of exfoliative superficial pyodermas that are distinct from those of impetigo and superficial bacterial folliculitis. The characterization of possible causative staphylococcal exfoliatin proteases and the role of corneodesmosin in collarette pathogenesis deserve further investigation.

Interaction of chlorhexidine with trisEDTA or miconazole in vitro against canine meticillin-resistant and -susceptible Staphylococcus pseudintermedius isolates from two UK regions

BACKGROUND

Topical therapy is an important alternative to systemic antibacterial therapy for treatment of canine superficial pyoderma in light of the emergence of multidrug-resistant staphylococci. Chlorhexidine is widely used in shampoo products alone or in combination with miconazole or tromethamine-ethylenediaminetetraacetic acid (trisEDTA). Comparisons of these combinations have not been made.

HYPOTHESIS/OBJECTIVES

To determine minimum inhibitory concentrations (MICs) of combinations of chlorhexidine/miconazole and chlorhexidine/trisEDTA in vitro in a collection of Staphylococcus pseudintermedius (SP) from northern (NUK) and southeastern (SEUK) United Kingdom (UK) sources.

METHODS

MICs of chlorhexidine, miconazole, trisEDTA and combinations of chlorhexidine/miconazole (1:1) or chlorhexidine/trisEDTA (80:16:1 and 80:5:1) were determined for 196 canine SP isolates from NUK [49 meticillin-resistant (MRSP), 50 meticillin-susceptible (MSSP)] and fom SEUK (48 MRSP, 49 MSSP) using agar dilution.

RESULTS

TrisEDTA alone did not inhibit growth. Chlorhexidine/miconazole MICs (median = 0.5 mg/L) were lower than those of either drug alone (P < 0.05) and lower than chlorhexidine/trisEDTA MICs (median = 1 mg/L; P < 0.0005) in each bacterial type and from both regions, except for miconazole in NUK MSSP. An additive interaction was noted between chlorhexidine and miconazole or trisEDTA (80:16:1 ratio) in 79 and 43 isolates, respectively, whereas antagonism between chlorhexidine and trisEDTA was noted for three isolates. NUK isolates were more susceptible than SEUK isolates (P < 0.05), except MRSP exposed to chlorhexidine and the chlorhexidine/trisEDTA (80:16:1) combination.

CONCLUSIONS AND CLINICAL IMPORTANCE

These low MICs are likely to be exceeded by topical therapy. Evaluation of the mechanisms by which chlorhexidine combinations interact to reduce MICs is warranted, in view of increasing concerns of biocide tolerance in staphylococci.

Strain-level Staphylococcus differentiation by CeO2-metal oxide laser ionization mass spectrometry fatty acid profiling

Background

The Staphylococcus genus is composed of 44 species, with S. aureus being the most pathogenic. Isolates of S. aureus are generally susceptible to β-lactam antibiotics, but extensive use of this class of drugs has led to increasing emergence of resistant strains. Increased occurrence of coagulase-negative staphylococci as well as S. aureus infections, some with resistance to multiple classes of antibiotics, has driven the necessity for innovative options for treatment and infection control. Despite these increasing needs, current methods still only possess species-level capabilities and require secondary testing to determine antibiotic resistance. This study describes the use of metal oxide laser ionization mass spectrometry fatty acid (FA) profiling as a rapid, simultaneous Staphylococcus identification and antibiotic resistance determination method.

Results

Principal component analysis was used to classify 50 Staphyloccocus isolates. Leave-one-spectrum-out cross-validation indicated 100 % correct assignment at the species and strain level. Fuzzy rule building expert system classification and self-optimizing partial least squares discriminant analysis, with more rigorous evaluations, also consistently achieved greater than 94 and 84 % accuracy, respectively. Preliminary analysis differentiating MRSA from MSSA demonstrated the feasibility of simultaneous determination of strain identification and antibiotic resistance.

Conclusion

The utility of CeO₂-MOLI MS FA profiling coupled with multivariate statistical analysis for performing strain-level differentiation of various Staphylococcus species proved to be a fast and reliable tool for identification. The simultaneous strain-level detection and antibiotic resistance determination achieved with this method should greatly improve outcomes and reduce clinical costs for therapeutic management and infection control.

PCR-based Approaches for the Detection of Clinical Methicillin-resistant Staphylococcus aureus

Staphylococcus aureus is an important pathogen that can cause a variety of infections, including superficial and systematic infections, in humans and animals. The persistent emergence of multidrug resistant S. aureus, particularly methicillin-resistant S. aureus, has caused dramatically economic burden and concerns in the public health due to limited options of treatment of MRSA infections. In order to make a correct choice of treatment for physicians and understand the prevalence of MRSA, it is extremely critical to precisely and timely diagnose the pathogen that induces a specific infection of patients and to reveal the antibiotic resistant profile of the pathogen. In this review, we outlined different PCR-based approaches that have been successfully utilized for the rapid detection of S. aureus, including MRSA and MSSA, directly from various clinical specimens. The sensitivity and specificity of detections were pointed out. Both advantages and disadvantages of listed approaches were discussed. Importantly, an alternative approach is necessary to further confirm the detection results from the molecular diagnostic assays.

Case-control risk factor study of methicillin-resistant Staphylococcus pseudintermedius (MRSP) infection in dogs and cats in Germany

Methicillin-resistant Staphylococcus pseudintermedius (MRSP) has emerged as a highly drug-resistant small animal veterinary pathogen. Although often isolated from outpatients in veterinary clinics, there is concern that MRSP follows a veterinary-hospital-associated epidemiology. This study's objective was to identify risk factors for MRSP infections in dogs and cats in Germany. Clinical isolates of MRSP cases (n=150) and methicillin-susceptible S. pseudintermedius (MSSP) controls (n=133) and their corresponding host signalment and medical data covering the six months prior to staphylococcal isolation were analysed by multivariable logistic regression. The identity of all MRSP isolates was confirmed through demonstration of S. intermedius-group specific nuc and mecA. In the final model, cats (compared to dogs, OR 18.5, 95% CI 1.8-188.0, P=0.01), animals that had been hospitalised (OR 104.4, 95% CI 21.3-511.6, P<0.001), or visited veterinary clinics more frequently (>10 visits OR 7.3, 95% CI 1.0-52.6, P=0.049) and those that had received topical ear medication (OR 5.1, 95% CI 1.8-14.9, P=0.003) or glucocorticoids (OR 22.5, 95% CI 7.0-72.6, P<0.001) were at higher risk of MRSP infection, whereas S. pseudintermedius isolates from ears were more likely to belong to the MSSP-group (OR 0.09, 95% CI 0.03-0.34, P<0.001). These results indicate an association of MRSP infection with veterinary clinic/hospital settings and possibly with chronic skin disease. There was an unexpected lack of association between MRSP and antimicrobial therapy; this requires further investigation but may indicate that MRSP is well adapted to canine skin with little need for selective pressure.

Risk factors for methicillin-resistant Staphylococcus aureus (MRSA) infection in dogs and cats: a case-control study

Risk factors for methicillin-resistant Staphylococcus aureus (MRSA) infection in dogs and cats were investigated in an unmatched case-control study. A total of 197 animals from 150 veterinary practices across the United Kingdom was enrolled, including 105 MRSA cases and 92 controls with methicillin-susceptible S. aureus (MSSA) infection. The association of owners and veterinarian staff with the human healthcare sector (HCS) and animal-related characteristics such as signalment, antimicrobial and immunosuppressive therapy, and surgery were evaluated as putative risk factors using logistic regression. We found that significant risk factors for MRSA infection were the number of antimicrobial courses (p=0.005), number of days admitted to veterinary clinics (p=0.003) and having received surgical implants (p=0.001). In addition, the odds of contact with humans which had been ill and admitted to hospital (p=0.062) were higher in MRSA infected pets than in MSSA controls. The risk factors identified in this study highlight the need to increase vigilance towards identification of companion animal groups at risk and to advocate responsible and judicious use of antimicrobials in small animal practice.

Meticillin-resistant Staphylococcus aureus carriage in UK veterinary staff and owners of infected pets: new risk groups

Meticillin-resistant Staphylococcus aureus (MRSA) nasal carriage on admission to hospital remains one of the most important risk factors for subsequent infection. Identification of high risk groups for MRSA carriage is vital for the success of infection control programmes. Veterinary staff may be one such risk group but little is known about pet owners and the role of contact with infected pets. As part of a UK-wide case-control study investigating risk factors for MRSA infection in dogs and cats between 2005 and 2008, 608 veterinary staff and pet owners in contact with 106 MRSA and 91 meticillin-susceptible S. aureus (MSSA)-infected pets were screened for S. aureus nasal carriage. Laboratory isolation and characterisation included salt broth enrichment, standard and automated microbiological tests, demonstration of the S. aureus-specific thermonuclease gene (nuc) and of mecA, and polymerase chain reaction-based lineage characterisation. MRSA carriage was 12.3% in veterinarians attending MRSA-infected animals and 7.5% in their owners. In the MSSA control group, MRSA carriage was 4.8% in veterinary staff and 0% in owners. Veterinary staff carried MRSA more frequently than owners (odds ratio: 2.33; 95% confidence interval: 1.10-4.93). All MRSA from humans and all but one animal MRSA were CC22 or CC30, typical for hospital MRSA in the UK. This study indicates for the first time an occupational risk for MRSA carriage in small animal general practitioners. Veterinary staff and owners of MRSA-infected pets are high risk groups for MRSA carriage despite not having direct hospital links. Strategies to break the cycle of MRSA infection must take these potential new reservoirs into account.

Molecular Diagnostics and Comparative Genomics in Clinical Microbiology

Initially, the availability of molecular diagnostics was considered a panacea, but replacement of conventional tests for detection and identification of microorganisms by molecular procedures eventually gathered momentum. This chapter describes current state-of-the-art molecular diagnostics and comparative genomics in medical microbiology to provide an understanding of infectious disease over the coming years. Nucleic acid-based tests are being introduced with increasing speed into routine clinical microbiology laboratories. Some of the problems remaining to be solved prior to general acceptance of nucleic acid-mediated detection and identification of microbial pathogens are reviewed. Historic objections are slowly being taken apart, and an accelerated introduction of molecular diagnostics is being pursued in many cases. Clear improvement in clinical testing is achieved by introducing molecular tests. Therefore, swift introduction of such tests into clinical practice is important to be pursued. Several PCR tests show increased sensitivity, excellent specificity, and cost effectiveness highlighting the success of the novel applications in the field of bacterial infections. Finally, some of the problems remaining to be solved prior to general acceptation of nucleic acid-mediated detection and identification of microbial pathogens are also reviewed.

Lack of transmission of methicillin-resistant Staphylococcus aureus (MRSA) between apparently healthy dogs in a rescue kennel

Although it is widely accepted that methicillin-resistant Staphylococcus aureus (MRSA) can be transmitted between humans and animals in both directions, little is known about the dynamics of animal-to-animal transfer. This study aimed to investigate aspects of dog-to-dog MRSA transfer in a rescue facility in the South-East of England during an MRSA outbreak. One hundred and twenty-nine apparently healthy dogs, mostly housed in pairs, were swabbed at nasal, oral, axillary and perianal sites. Swabs were enriched in selective broth and staphylococci identified using standard biological methods. MRSA isolates were confirmed by demonstration of the thermonuclease gene (nuc) and mecA. After initial swabbing, a dog excluded from the study design but housed at the same facility was discovered to have a wound infection due to MRSA. MRSA carriage was identified in 10/129 dogs (7.8%) and all isolates were of the same lineage as the one isolated from the infected dog. All carrier dogs lived in shared kennels and their 16 kennel partners sampled negative on two occasions. Concurrently with successful antimicrobial treatment of the infected patient, MRSA carriage resolved spontaneously in all dogs within two weeks. In conclusion, MRSA did not transmit readily between apparently healthy dogs, MRSA carriage was not supported for long periods in a regularly cleaned environment and exposure alone may not lead to MRSA acquisition by dogs without the presence of additional risk factors.

Susceptibility to quinupristin-dalfopristin and linezolid in 839 clinical isolates of gram-positive cocci from Norway

A total of 839 clinical isolates of Gram-positive cocci from Norway including Staphylococcus aureus (n = 214), coagulase negative Staphylococcus spp. (n = 100), Streptococcus pyogenes (n = 99), Streptococcus agalactiae (n = 80), Streptococcus pneumoniae (n = 127), Streptococcus spp. viridans group (n = 70), Enterococcus faecalis (n = 75), and Enterococcus faecium (n = 74), were tested by E-test for susceptibility to a range of antimicrobials including the novel antibiotics quinupristin-dalfopristin and linezolid. Subgroups of oxacillin resistant S. aureus and coagulase negative Staphylococcus spp., penicillin non-susceptible S. pneumoniae and vancomycin resistant Enterococcus spp. were specifically included as they are the intended targets for these new drugs. All isolates were susceptible to linezolid (MIC5o and MIC9o 0.25-2.0 mg/l, MIC range 0.12-2 mg/l). Staphylococcal and streptococcal isolates were also susceptible to quinupristin-dalfopristin except for some intermediately susceptible viridans group isolates (MIC54, and MIC90 0.25-2 mg/l, MIC range 0.125-2 mg/l). Enterococcus faecium (MIC90 = 4.0 mg/l) and Enterococcus faecalis (MIC50 = 8.0 mg/l, MIC90 > or = 32 mg/l) were less susceptible to this substance. There was no linkage between reduced susceptibility to linezolid or quinupristin-dalfopristin and resistance to other classes of antimicrobials. The study demonstrated a high prevalence of in vitro susceptibility to linezolid and quinupristin-dalfopristin, which is necessary for their use in the treatment of infections with resistant Gram-positive pathogens. The results were used to evaluate the appropriateness of breakpoints and to define a baseline for monitoring possible future emergence of resistance to quinupristin-dalfopristin and linezolid in Norway.

Clinical features, epidemiology, antimicrobial resistance, and exotoxin genes (including that of Panton-Valentine leukocidin) of gentamicin-susceptible methicillin-resistant Staphylococcus aureus (GS-MRSA) isolated at a paediatric teaching hospital in New South Wales, Australia

AIMS

To describe the epidemiological, clinical, and laboratory features of gentamicin-susceptible methicillin-resistant Staphylococcus aureus (GS-MRSA) seen at a paediatric teaching hospital.

METHODS

Patients from whom GS-MRSA was isolated between 1 January 2001 and 31 December 2002 were enrolled. Retrospective chart review was performed. Susceptibility testing was performed with the Vitek2 system; PCR confirmed methicillin resistance. Phage typing and pulsed field gel electrophoresis (PFGE) was performed (utilising MLST/SCCmec-defined control strains). PCR detection of tst, luk-PV, and entA-entE was performed.

RESULTS

Eighty-five per cent of all Staphylococcus aureus isolates during the study period were methicillin-sensitive, and 15% were MRSA (9% GS-MRSA, 6% gentamicin resistant-MRSA). 100 GS-MRSA infections in 98 children were identified: 59 cases of skin/soft tissue, four bone and joint, four surgical site infections, three pneumonia, eight other types, and 22 represented colonisation. Ninety-nine isolates were non-multidrug resistant, but 17 strains were resistant to erythromycin, 7 to tetracyclines, 12 to ciprofloxacin, 11 to fusidic acid, 1 each to rifampicin and mupirocin. 44 isolates were Oceania strain (ST30-MRSA-IV), 20 were Queensland strain (ST93-MRSA-IV), ten were UK EMRSA-15 (ST22-MRSA-IV), eight were WA MRSA-1 (ST1-MRSA-IV), two were WA MRSA-5 (ST8-MRSA-IV), one was WA MRSA-2 (ST78slv-MRSA-IV), one was WA MRSA-15 (ST59-MRSA-IV), and the remainder were sporadics. Twenty patients were Pacific Islanders, of whom 12 had the Oceania strain; ten were Aboriginal, of whom eight had the Queensland strain. Sixty-eight isolates possessed luk-PV, including all Queensland strains and 91% of Oceania strains. Enterotoxin genes were detected in 25% of the isolates, and tst was detected in four isolates.

CONCLUSIONS

GS-MRSA is a significant paediatric problem in New South Wales: two minority groups are over-represented, multiple epidemic strains were detected, most community strains possess luk-PV, and many isolates are multidrug resistant.

Incidence, antibiotic resistance and clonal relations of MRSA strains isolated from a Romanian university hospital

Our aim was to estimate the frequency and characteristics ofmethicillin-resistant Staphylococcus aureus (MRSA) strains occurring in a Romanian teaching hospital. We retrospectively studied isolates from infected or colonized patients treated at the intensive care and surgical units during January 2004-December 2005. The antibiotic susceptibility of MRSA strains and the presence of mecA gene were determined. Consecutively occurring strains isolated through a three-month period were typed using pulsed field gel electrophoresis. A total of 423 S. aureus strains were identified, methicillin-resistance was detected in 211 (49.9%) strains. Most of them were multiresistant. One of the MRSA genotypes identified by PFGE was commonly recovered from patients treated in the intensive care unit. According to our results, MRSA strains were frequently isolated pathogens in our hospital and there is an urgent need to enhance infection control efforts.

First report of multiresistant, mecA-positive Staphylococcus intermedius in Europe: 12 cases from a veterinary dermatology referral clinic in Germany

Resistance to cephalosporins and/or fluoroquinolones by Staphylococcus intermedius has remained low in Europe, with effective drugs generally available for systemic therapy in pets. However, multiresistant, mecA-positive S. intermedius isolated from dogs and cats is now emerging in Europe. Twelve S. intermedius isolates, highly resistant to at least five antimicrobial classes, were isolated from skin and ear infections in 11 dogs and a cat. The 12 isolates represented 23% of all S. intermedius submissions from one veterinary dermatology referral clinic in northern Germany to veterinary diagnostic laboratories during an 18-month period and resistance included cefalexin, methicillin and enrofloxacin. The animals had been referred to the clinic with recurrent superficial pyoderma, deep pyoderma, pododermatitis or chronic otitis, all unresponsive to systemic beta-lactam-antibiotics or fluoroquinolones. Infection resolved in 10 dogs and the cat on a combination of antimicrobial treatment and correction of underlying causes. Four dogs and a cat required systemic and topical therapy; in six dogs topical antimicrobial therapy alone was successful. Phenotypic and genotypic characteristics of the S. intermedius isolates were determined; species identification was confirmed by polymerase chain detection of thermonuclease genes (nuc) and the presence and expression of the gene conferring resistance to all beta-lactam antibiotics (mecA) were demonstrated in all; based on pulsed-field gel electrophoresis, six were indistinguishable, the others closely or possibly related. The emergence of multiresistant, mecA-positive S. intermedius in Europe is alarming. Zoonotic implications, awareness among veterinary laboratories and strategies for the use of antimicrobials in small animal practice need to be considered.

Susceptibility to daptomycin, quinupristin-dalfopristin and linezolid and some other antibiotics in clinical isolates of methicillin resistant and methicillin sensitive S.aureus from the Oslo area

Our study compared the susceptibility of 136 clinical isolates of Staphylococcus aureus and 119 multidrug-resistant Staphylococcus aureus (MRSA) isolates from Oslo to a range of antibiotics, including the novel antibiotics quinupristin-dalfopristin, linezolid and daptomycin. All isolates were susceptible to daptomycin, linezolid and quinupristin-dalfopristin, although a subgroup was less susceptible to the latter. There was no linkage between reduced susceptibility to daptomycin, linezolid or quinupristin-dalfopristin and resistance to other classes of antimicrobials. In addition, MRSA strains from 2004 have become more sensitive to fucidin and rifampicin. The results can be used to evaluate the appropriateness of breakpoints and to define a baseline for monitoring possible future emergence of resistance to daptomycin, quinupristin-dalfopristin and linezolid in Staphylococcus aureus in Norway.

Antimicrobial resistance predicts death in Tanzanian children with bloodstream infections: a prospective cohort study

Background

Bloodstream infection is a common cause of hospitalization, morbidity and death in children. The impact of antimicrobial resistance and HIV infection on outcome is not firmly established.

Methods

We assessed the incidence of bloodstream infection and risk factors for fatal outcome in a prospective cohort study of 1828 consecutive admissions of children aged zero to seven years with signs of systemic infection. Blood was obtained for culture, malaria microscopy, HIV antibody test and, when necessary, HIV PCR. We recorded data on clinical features, underlying diseases, antimicrobial drug use and patients' outcome.

Results

The incidence of laboratory-confirmed bloodstream infection was 13.9% (255/1828) of admissions, despite two thirds of the study population having received antimicrobial therapy prior to blood culture. The most frequent isolates were klebsiella, salmonellae, Escherichia coli, enterococci and Staphylococcus aureus. Furthermore, 21.6% had malaria and 16.8% HIV infection. One third (34.9%) of the children with laboratory-confirmed bloodstream infection died. The mortality rate from Gram-negative bloodstream infection (43.5%) was more than double that of malaria (20.2%) and Gram-positive bloodstream infection (16.7%). Significant risk factors for death by logistic regression modeling were inappropriate treatment due to antimicrobial resistance, HIV infection, other underlying infectious diseases, malnutrition and bloodstream infection caused by Enterobacteriaceae, other Gram-negatives and candida.

Conclusion

Bloodstream infection was less common than malaria, but caused more deaths. The frequent use of antimicrobials prior to blood culture may have hampered the detection of organisms susceptible to commonly used antimicrobials, including pneumococci, and thus the study probably underestimates the incidence of bloodstream infection. The finding that antimicrobial resistance, HIV-infection and malnutrition predict fatal outcome calls for renewed efforts to curb the further emergence of resistance, improve HIV care and nutrition for children.

International dissemination of a high virulence rabbit Staphylococcus aureus clone

High virulence rabbit Staphylococcus aureus strains cause chronic and spreading problems of mastitis, pododermatitis and subcutaneous abscesses on rabbit flock level, whereas infections with low virulence strains are limited to individual rabbits. In the present report, 13 high virulence rabbit S. aureus strains, selected out of a large collection of strains isolated in five European countries between 1983 and 2004, were genotyped using pulsed-field gel electrophoresis, spa typing, multilocus sequence typing (MLST) and accessory gene regulator (agr) group typing. Two low virulence rabbit S. aureus strains were also included in the study. The results indicate the clonal origin of high virulence rabbit S. aureus strains present in Europe. Furthermore, the results of MLST and spa typing form a basis for international epidemiology of rabbit S. aureus strains, as these DNA sequence-based typing techniques can easily be used for intercentre comparisons.

Use of broth enrichment and real-time PCR to exclude the presence of methicillin-resistant Staphylococcus aureus in clinical samples: a sensitive screening approach

A rapid and sensitive method for excluding the presence of methicillin-resistant Staphylococcus aureus (MRSA) in clinical samples was developed and evaluated. The method utilised an MRSA-selective enrichment broth for 16 h, followed by PCR quantification of the nuc gene. Samples below a quantitative PCR threshold were reported as MRSA-negative. Broths from PCR-positive samples were subcultured for MRSA isolation. Clinical samples (n = 334) in a constructed high prevalence population were analysed in parallel with a selective plating method. The new broth-PCR assay increased the number of positive samples by 35% (49 vs. 66), and 94% of negative samples were reported within 24 h. To reduce costs and workload, 665 clinical samples were grown separately in enrichment broth and then pooled in the PCR step. The broth-PCR assay increased the number of MRSA positive samples from 11 to 15 compared with selective plating. Most (89%) of the culture-negative samples were also PCR-negative and could be reported within 24 h. The growth of 25 European EMRSA strains was tested in the selective enrichment broth. On average, the MRSA strains showed a 300 000-fold increase in CFU, compared with 30-fold for the eight methicillin-sensitive Staphylococcus aureus strains tested.

Guidelines for the laboratory diagnosis and susceptibility testing of methicillin-resistant Staphylococcus aureus (MRSA)

These evidence-based guidelines have been produced after a literature review of the laboratory diagnosis and susceptibility testing of methicillin-resistant Staphylococcus aureus (MRSA). We have considered the detection of MRSA in screening samples and the detection of reduced susceptibility to glycopeptides in S. aureus. Recommendations are given for the identification of S. aureus and for suitable methods of susceptibility testing and screening for MRSA and for S. aureus with reduced susceptibility to glycopeptides. These guidelines indicate what tests should be used but not when the tests are applicable, as aspects of this are dealt with in guidelines on control of MRSA. There are currently several developments in screening media and molecular methods. It is likely that some of our recommendations will require modification as the new methods become available.

Rapid detection of antibacterial resistance in emerging Gram-positive cocci

Effective infection control efforts obviously depend on the performance of the laboratory to detect emerging resistant pathogens accurately and confirm resistance patterns by additional methods to conventional or automated systems. Conventional methods still remain the predominant approaches for detection and identification of bacteria and resistance patterns. However, the estimated time for conventional tests to detect resistance is at least 24-48 h for methicillin-resistant Staphylococcus aureus, vancomycin-resistant enterococci and other epidemiologically important pathogens. Most of the tests used for rapid detection require bacterial growth in culture. There is an important clinical need for rapid detection of bacteria directly from patient samples. Rapid methods based on immunological or molecular technologies have contributed significantly. Molecular assays for several resistance markers are reliable, such as for mecA in staphylococci and vanA in enterococci. However, for other resistance markers, there is a lack of field testing. Cost-effectiveness of rapid detection of antibacterial resistance is another concern. Molecular assays would be useful for tertiary hospitals considering the investment costs and requirement of expert laboratory staff. For smaller centres, rapid tests based on immunological techniques may be a better choice.

Evaluation of two new chromogenic media, CHROMagar MRSA and S. aureus ID, for identifying Staphylococcus aureus and screening methicillin-resistant S. aureus

Thirty-nine methicillin-resistant Staphylococcus aureus (MRSA) isolates with diverse genetic backgrounds and two reference strains were correctly identified as S. aureus on CHROMagar MRSA and S. aureus ID media. Growth inhibition on CHROMagar MRSA was noted. A combination of cefoxitin disk and S. aureus ID was found suitable for rapid MRSA screening.

Outbreak of methicillin-resistant Staphylococcus aureus in two nursing homes in Central Norway

Until recently, infections with methicillin-resistant Staphylococcus aureus (MRSA) have mainly been associated with hospital outbreaks in Norway. However, increasingly cases are contracted outside hospitals. This paper reports the first two outbreaks of MRSA in two nursing homes in central Norway, affecting 23 residents and five staff members. Pulsed-field gel electrophoresis analysis showed that all strains from nursing home A were identical and that the strains from nursing home B were genotypically similar with one or two band differences. Multi-locus sequence typing (MLST) showed that the strains from the two nursing homes belong to clonal complex 45, with each strain being a single-locus variant of sequence type 45 (ST 45), a well-known European epidemic strain. No evident source of the two outbreaks was found, and there was no obvious connection between the two outbreaks. The latter is also supported by the minor differences observed by MLST, suggesting a connection at some time in the past. The outbreaks led to a heavier workload and economic strain on both nursing homes. The outbreak in nursing home A was brought to an end, whereas two residents remained colonized in nursing home B despite several eradication attempts. These outbreaks show the potential for MRSA spread in a nursing home. If the prevalence of MRSA in Norway continues to increase, nursing home staff and residents may have to be included in the groups to be screened for MRSA upon hospital admission.

Evaluation of the Vitek 2 system for identification and antimicrobial susceptibility testing of Staphylococcus spp

The Vitek 2 system was assessed against reference methods with 197 methicillin-resistant Staphylococcus aureus from Belgian hospitals and 121 clinically significant blood culture isolates of Staphylococcus spp. Vitek 2 identified 95% of staphylococcal isolates correctly, detected oxacillin resistance with a sensitivity/specificity of 99/96%, and showed acceptable accuracy for susceptibility testing of five of eight other evaluable antibiotics. The median time for reporting results was 2 h 45 min for identification and 7 h for susceptibility tests.

Rapid detection of mecA and nuc genes in staphylococci by real-time multiplex polymerase chain reaction

A multiplex real-time polymerase chain reaction (RT-PCR) targeting the mecA and nuc genes was developed for the detection of methicillin resistance and identification of Staphylococcus aureus. Novel mecA and nuc primers and fluorescence resonance energy transfer hybridization probes specific for the mecA and nuc genes were evaluated. The assay was performed using the LightCycler system (Roche Molecular Biochemicals, Mannheim, Germany) and evaluated against the traditional gel-based multiplex PCR (PCR-gel) method currently used at Royal Perth Hospital. Clinical isolates (n = 222) and isolates from a culture collection library (n = 206) were tested by both assays in parallel. The RT-PCR assay was 100% sensitive and specific for the detection of methicillin resistance and for the identification of S. aureus when compared with the PCR-gel assay. Results from the RT-PCR assay showed 5 isolates with lower efficiency fluorescence curves for the nuc gene PCR fragment. DNA sequencing showed mutations within the region of the probe-binding sites compared with the reference strain. The results of the RT-PCR assay were available within 2 h. This rapid mecA/nuc RT-PCR assay is a suitable and practical tool for the routine detection of methicillin resistance and identification of S. aureus, which can be easily incorporated into the diagnostic molecular microbiology laboratory work flow.

Cefoxitin resistance as a surrogate marker for the detection of methicillin-resistant Staphylococcus aureus

OBJECTIVES

To evaluate the usefulness of cefoxitin when used as a surrogate marker for the detection of methicillin resistance.

PATIENTS AND METHODS

Eight hundred and seventy-one strains of Staphylococcus aureus, collected from eight tertiary referral centres serving diverse socio-economic populations, were included in the study using NCCLS disc diffusion and the agar dilution methods.

RESULTS

Using cefoxitin and NCCLS criteria for disc diffusion, the sensitivity and specificity for recognizing methicillin resistance were both 100%. Similar results were obtained when the strains were tested by the agar dilution method. The cefoxitin MICs for methicillin-susceptible strains were < or = 4 mg/L.

CONCLUSIONS

Testing with cefoxitin as a surrogate marker for the detection of methicillin resistance was very accurate with both disc diffusion and agar dilution methods. Such testing clearly distinguished methicillin-resistant strains of S. aureus from methicillin-susceptible strains.

Genetic methods for detection of antimicrobial resistance

Accurate and rapid diagnostic methods are needed to guide antimicrobial therapy and infection control interventions. Advances in real-time PCR have provided a user-friendly, rapid and reproducible testing platform catalysing an increased use of genetic assays as part of a wider strategy to minimize the development and spread of antimicrobial-resistant bacteria. In this review we outline the principal features of genetic assays in the detection of antimicrobial resistance, their advantages and limitations, and discuss specific applications in the detection of methicillin-resistant Staphylococcus aureus, glycopeptide-resistant enterococci, aminoglycoside resistance in staphylococci and enterococci, broad-spectrum resistance to beta-lactam antibiotics in gram-negative bacteria, as well as genetic elements involved in the assembly and spread of antimicrobial resistance.

Antimicrobial resistance and resistance genes in Staphylococcus aureus strains from rabbits

Fifty-six Staphylococcus aureus isolates recovered between 1998 and 2003 from 31 rabbit farms with and without problems of chronic staphylococcosis, were screened for resistance to enrofloxacin, erythromycin, gentamicin, lincomycin, neomycin, penicillin and tetracyclines using the agar dilution test. For penicillin, a disk diffusion test was also performed. The detection of tetP(B), tet(K), tet(L), tet(M), tet(O), tet(T), tet(W), erm(A), erm(B), erm(C) and mec(A) genes was done via a PCR assay. Four isolates showed resistance to erythromycin and lincomycin. These isolates were positive for the erm(C) gene in the PCR. Eleven strains were resistant to tetracyclines and all harboured the tet(K) gene. In the agar dilution test, five isolates showed resistance to penicillin, whereas in the disk diffusion test 12 isolates showed resistance. None of these 12 resistant isolates carried the mec(A) gene. Only one strain showed resistance to gentamicin, and all strains were susceptible to enrofloxacin and neomycin. This study demonstrates that resistance to antimicrobial agents in S. aureus isolates originating from rabbits is relatively rare compared to resistance in S. aureus isolates originating from other animals and humans.

Evaluation of a triplex PCR assay to discriminate Staphylococcus aureus from coagulase-negative Staphylococci and determine methicillin resistance from blood cultures

A triplex PCR targeting the 16S rRNA, mecA, and nuc genes was developed for identification of staphylococci and detection of methicillin resistance. After validation of the assay with a collection of strains of staphylococci and enterococci (n = 169), the assay was evaluated with cultures of blood with gram-positive cocci from 40 patients. Accurate results were obtained for 59 (98%) of 61 cultures within 6 h of growth detection.

Evaluation of the MRSA-Screen Test in detecting oxacillin resistance in community and hospital isolates of Staphylococcus aureus

The MRSA-Screen Test (Denka Seiken Co., Japan), a latex agglutination test to detect penicillin-binding protein 2a, was compared with PCR for the detection of oxacillin resistance in Staphylococcus aureus. A total of 77 oxacillin-sensitive and 269 oxacillin-resistant (ORSA) isolates were evaluated. Of the ORSA isolates, 186 were non-multiresistant (NORSA), defined as being resistant to two or fewer antibiotics other than beta-lactams. Eighty-three were multiresistant ORSA (MORSA) strains. If PCR is considered the gold standard test, then the sensitivity, specificity, positive and negative predictive values of the MRSA-Screen Test were 100, 99, 99 and 100%, respectively. The endpoint was hard to read with NORSA strains that took longer than 60 s to react. MORSA strains took a median 12 s (range 5-60 s) to give a positive reaction with the MRSA-Screen Test, whereas NORSA strains took a median 30 s (range 5-180 s), a difference which was significantly different (P < 0.0001, two-tailed Mann-Whitney unpaired two sample test). NORSA strains had an MIC50 of 128 mg/l and MIC90 of 256mg/l, whereas MORSA strains had an MIC50 and MIC90 of >256mg/l. The time that the MRSA-Screen Test took to agglutinate with ORSA strains correlated weakly with the MIC (r2 = 0.26). Detection of methicillin resistance cost AUD$9 per isolate with the MRSA-Screen Test, compared with AUD$13 per isolate with mecA PCR. The MRSA-Screen Test gave excellent sensitivity and specificity, and was quicker and cheaper than PCR. The full 3 min should be allowed to elapse before calling a test negative. Organisms giving indeterminate reactions should be tested for the mecA gene by PCR.

Molecular detection of antimicrobial resistance

The determination of antimicrobial susceptibility of a clinical isolate, especially with increasing resistance, is often crucial for the optimal antimicrobial therapy of infected patients. Nucleic acid-based assays for the detection of resistance may offer advantages over phenotypic assays. Examples are the detection of the methicillin resistance-encoding mecA gene in staphylococci, rifampin resistance in Mycobacterium tuberculosis, and the spread of resistance determinants across the globe. However, molecular assays for the detection of resistance have a number of limitations. New resistance mechanisms may be missed, and in some cases the number of different genes makes generating an assay too costly to compete with phenotypic assays. In addition, proper quality control for molecular assays poses a problem for many laboratories, and this results in questionable results at best. The development of new molecular techniques, e.g., PCR using molecular beacons and DNA chips, expands the possibilities for monitoring resistance. Although molecular techniques for the detection of antimicrobial resistance clearly are winning a place in routine diagnostics, phenotypic assays are still the method of choice for most resistance determinations. In this review, we describe the applications of molecular techniques for the detection of antimicrobial resistance and the current state of the art.

Optimal inoculation methods and quality control for the NCCLS oxacillin agar screen test for detection of oxacillin resistance in Staphylococcus aureus

To define more precisely the inoculation methods to be used in the oxacillin screen test for Staphylococcus aureus, we tested agar screen plates prepared in house with 6 microg of oxacillin/ml and 4% NaCl using the four different inoculation methods that would most likely be used by clinical laboratories. The organisms selected for testing were 19 heteroresistant mecA-producing strains and 41 non-mecA-producing strains for which oxacillin MICs were near the susceptible breakpoint. The inoculation method that was preferred by all four readers and that resulted in the best combination of sensitivity and specificity was a 1-microl loopful of a 0.5 McFarland suspension. A second objective of the study was to then use this method to inoculate plates from five different manufacturers of commercially prepared media. Although all commercial media performed with acceptable sensitivity compared to the reference lot, one of the commercial lots demonstrated a lack of specificity. Those lots of oxacillin screen medium that fail to grow heteroresistant strains can be detected by using S. aureus ATCC 43300 as a positive control in the test and by using transmitted light to carefully examine the plates for any growth. However, lack of specificity with commercial lots may be difficult to detect using any of the current quality control organisms.

Performance of eight methods, including two new rapid methods, for detection of oxacillin resistance in a challenge set of Staphylococcus aureus organisms

Using a set of 55 Staphylococcus aureus challenge organisms, we evaluated six routine methods (broth microdilution, disk diffusion, oxacillin agar screen, MicroScan conventional panels, MicroScan rapid panels, and Vitek cards) currently used in many clinical laboratories and two new rapid methods, Velogene and the MRSA-Screen, that require less than a day to determine the susceptibility of S. aureus to oxacillin. The methods were evaluated by using the presence of the mecA gene, as detected by PCR, as the "gold standard." The strains included 19 mecA-positive heterogeneously resistant strains of expression class 1 or 2 (demonstrating oxacillin MICs of 4 to >16 microg/ml) and 36 mecA-negative strains. The oxacillin MICs of the latter strains were 0.25 to 4 microg/ml when tested by broth microdilution with 2% NaCl-supplemented cation-adjusted Mueller-Hinton broth as specified by the NCCLS. However, when tested by agar dilution with 4% salt (the conditions used in the oxacillin agar screen method), the oxacillin MICs of 16 of the mecA-negative strains increased to 4 to 8 microg/ml. On initial testing, the percentages of correct results (% sensitivity/% specificity) were as follows: broth microdilution, 100/100; Velogene, 100/100; Vitek, 95/97; oxacillin agar screen, 90/92; disk diffusion, 100/89; MicroScan rapid panels, 90/86; MRSA-Screen, 90/100; and MicroScan conventional, 74/97. The MRSA-Screen sensitivity improved to 100% if agglutination reactions were read at 15 min. Repeat testing improved the performance of some but not all of the systems.

Development of a PCR assay for identification of staphylococci at genus and species levels

We have developed a PCR-based assay which allows the detection of staphylococci at the genus level by targeting the tuf gene, which encodes the elongation factor Tu. Degenerate PCR primers derived from consensus regions of several tuf genes were used to amplify a target region of 884 bp from 11 representative staphylococcal species. Subsequently, the entire nucleotide sequence of these amplicons was determined. The analysis of a multiple alignment of these sequences revealed regions conserved among staphylococci but distinct from those of other gram-positive bacteria genetically related to staphylococci. PCR primers complementary to these regions could amplify specifically and efficiently a DNA fragment of 370 bp for all of 27 different staphylococcal species tested. There was no amplification with genomic DNA prepared from 53 nonstaphylococcal species tested to verify the specificity of the assay (20 gram positive and 33 gram negative). Furthermore, this assay amplified efficiently all 27 American Type Culture Collection (ATCC) staphylococcal reference strains as well as 307 clinical isolates of staphylococci from the Québec City region. Analysis of the multiple sequence alignment for the 884-bp fragment for the 11 staphylococcal species as well as comparison of the sequences for the 370-bp amplicon from five unrelated ATCC and clinical strains for each of the species S. aureus, S. epidermidis, S. haemolyticus, S. hominis, and S. saprophyticus demonstrated sufficient interspecies polymorphism to generate genus- and species-specific capture probes. This sequence information allowed the development of Staphylococcus-specific and species-specific (targeting S. aureus, S. epidermidis, S. haemolyticus, S. hominis, or S. saprophyticus) capture probes hybridizing to the 370-bp amplicon. In conclusion, this PCR assay is suitable for detection of staphylococci at both genus and species levels.

Non-multiresistant and multiresistant methicillin-resistant Staphylococcus aureus in community-acquired infections

OBJECTIVE

To survey Staphylococcus aureus strains isolated from patients presenting from the community, comparing clinical features and antibiotic sensitivity profiles between multiresistant and non-multiresistant methicillin-resistant and methicillin-sensitive isolates.

DESIGN

Retrospective case series.

PARTICIPANTS AND SETTING

Patients who presented to emergency or dermatology departments in hospitals served by the South Western Sydney Area Health Service between 1 May 1998 and 30 April 1999. All patients with methicillin-resistant S. aureus (MRSA) and the first 100 with methicillin-sensitive S. aureus were eligible.

MAIN OUTCOME MEASURES

Patient demographic characteristics; risk factors; clinical presentation; treatment; outcome; and isolate antibiotic susceptibility.

RESULTS

139 patients were eligible, and 122 had clinical records available. Ten of these 122 (8%) had multiresistant MRSA, 26 (21%) non-multiresistant MRSA and 86 (70%) methicillin-sensitive S. aureus. Among patients with non-multiresistant MRSA, 29% (7/24) were born in New Zealand, Samoa or Tonga, a higher proportion than among those with multiresistant MRSA or methicillin-sensitive S. aureus (P= 0.03). Nearly half (44%) of non-multiresistant MRSA strains were community-acquired in patients with no risk factors. Two-thirds of patients with non-multiresistant MRSA (17/26) presented with cellulitis or abscess, and 58% (11/19 evaluable patients) required surgical treatment.

CONCLUSIONS

Non-multiresistant MRSA strains are common, especially among people born in New Zealand, Samoa or Tonga, and are usually community acquired. Medical practitioners should routinely swab all staphylococcal lesions for culture and sensitivity.

In vivo deletion of the methicillin resistance mec region from the chromosome of Staphylococcus aureus strains

Two sets of Staphylococcus aureus isolates recovered from two patients exhibited similar susceptibility profiles except for oxacillin susceptibility (MSSA) or resistance (MRSA). SMA:I macrorestriction and inter-IS256 PCR analysis showed patterns closely related to the Belgian epidemic MRSA clone 1 in each pair of MSSA/MRSA strains. Loss of one large SMA:I DNA fragment and concurrent gain of a smaller fragment in the MSSA isolates was observed. The mecA sequence present in the MRSA was absent in the MSSA variant. Therefore, in vivo deletion of the mec region may occur in some lineages of S. aureus more frequently than previously thought.