Molecular epidemiology of an outbreak caused by methicillin-resistant Staphylococcus aureus in a health care ward and associated nursing home

Molecular characteristics and risk factor analysis of Staphylococcus aureus colonization put insight into CC1 colonization in three nursing homes in Shanghai

Research indicates that Staphylococcus aureus colonization in the elderly with predisposing risks is associated with subsequent infection. However, the molecular epidemiology and risk factors for S. aureus colonization among residents and staff in nursing homes (NHs) in China remain unclear. A multicenter study was conducted in three NHs in Shanghai between September 2019 and October 2019. We explored the prevalence, molecular epidemiology, and risk factors for S. aureus colonization. All S. aureus isolates were characterized based on antimicrobial resistance, virulence genes, multilocus sequence typing (MLST), staphylococcus protein A (spa) typing, and staphylococcal cassette chromosome mec (SCCmec) typing. NH records were examined for potential risk factors for S. aureus colonization. S. aureus and methicillin-resistant S. aureus (MRSA) isolates were detected in 109 (100 residents and 9 staff, 19.8%, 109/551) and 28 (24 residents and 4 staff, 5.1%, 28/551) subjects among 496 residents and 55 staff screened, respectively. Compared to methicillin-susceptible S. aureus isolates, all 30 MRSA isolates had higher resistance rates to most antibiotics except minocycline, rifampicin, linezolid, vancomycin, and teicoplanin. Sequence type (ST) 1 (21.3%) was the most common sequence type, and t127 (20.5%) was the most common spa type among 122 S. aureus isolates. SCCmec type I (70%) was the dominant clone among all MRSA isolates. CC1 (26/122, 21.3%) was the predominant complex clone (CC), followed by CC398 (25/122, 20.5%), CC5 (20/122, 16.4%) and CC188 (18/122, 14.8%). Female sex (OR, 1.70; 95% CI, 1.04-2.79; P = 0.036) and invasive devices (OR, 2.19; 95% CI, 1.26-3.81; P = 0.006) were independently associated with S. aureus colonization.

Methicillin Resistant Staphylococcus aureus and public fomites: a review

Staphylococcus genus is a Gram-positive coccus normally associated with skin and mucous membranes of warm-blooded animals. It is part of the commensal human microflora, or found in animals, or contaminating surfaces in the community and hospital settings. Staphylococcus aureus is the most pathogenic species belonging to this genus, as it possesses a collection of virulence factors that are expressed solely to evade the immune system. The increase in the misuse of antimicrobial agents predisposed S. aureus to develop antibiotic resistance, including the resistance to methicillin which led to the emergence of Methicillin-Resistant S. aureus (MRSA). MRSA is considered one of the most dangerous nosocomial pathogens causing many hard to treat infections in hospitals and was named as Hospital Associated MRSA (HA-MRSA). Over the past 20-25 years, MRSA was isolated from community settings and thus Community Associated MRSA (CA-MRSA) has emerged. Inside hospitals, MRSA has been isolated from fomites in contact with patients, as well as staff's protective and personal items. This review highlights the worldwide prevalence of MRSA on fomites within the contexts of hospital and community settings.

Prevalence and Molecular Genetics of Methicillin-Resistant Staphylococcus aureus Colonization in Nursing Homes in Saudi Arabia

Objective

Methicillin-resistant Staphylococcus aureus (MRSA) is one of the main causative agents of nosocomial infections that has posed a major threat to those with compromised immune systems such as nursing home residents. The aim of this study was to determine the rates of MRSA strains and the types of Staphylococcal Cassette Chromosome mec (SCCmec)in nursing homes in Saudi Arabia.

Methods

A total of 188 nasal swabs were collected from the residents and nursing staff in two nursing homes in Riyadh, Saudi Arabia. All MRSA isolates were tested for antimicrobial susceptibility and analyzed for mecA and SCCmec typing by multiplex PCR assay. Detection of the Panton–Valentine leukocidin (PVL) gene was also tested in all positive MRSA isolates by multiplex PCR using specific primers.

Results

Among the 188 collected nasal swabs (105 males and 83 females), MRSA colonization rate was 9.04% (11 (5.85%) females and 6 (5.71%) males). About 47% of MRSA were multidrug resistant (MDR) as acquired resistance to beta-lactam, macrolide, and aminoglycoside antibiotics. However, all the MRSA isolates showed susceptibility to vancomycin, tigecycline, and linezolid. All the MRSA isolates (n = 17) were mecA-positive with the SCCmec IVc (n = 7, 41.18%) as the most common SCCmec type followed by SCCmec V (n = 5, 29.41%) and SCCmec IVa (n = 2, 11.76%). The remaining isolates (n = 3) were nontypeable (17.65%). In addition, the PVL toxin gene was only detected in four of the male samples.

Conclusion

MRSA nasal colonization is a common incident among nursing home residents. The prevalence of community-associated (CA) MRSA (SCCmec IV and V) was more common than hospital-associated (HA) MRSA in our study samples. It is crucial to investigate such rate of incidence, which is a key tool in preventive medicine and would aid in determining health policy and predict emergent outbreaks.

Multiclonal methicillin-resistant Staphylococcus aureus (MRSA) outbreak and its control after use of the Veterans Affairs (VA) MRSA bundle in a VA long-term care facility, 2004-2014

A multiclonal methicillin-resistant Staphylococcus aureus (MRSA) outbreak with 91 infections occurred in our Veterans Affairs (VA) community living center over 46 months. Both similar and unique strains were shown by repetitive polymerase chain reaction to contribute to the outbreak, including 1 strain causing infections over a 33-month period. Most infections were soft tissue infections (67%). For 21 months after the initiation of the VA MRSA bundle, no infections were identified, and low rates of infection have been sustained an additional 4 years. The average annual rate of MRSA infection decreased by 62% (P < .001) from 0.6 per 1,000 resident days for 4 years prior to the bundle implementation to 0.09 per 1,000 resident days for 4 years after the bundle implementation.

Infection control intervention on meticillin resistant Staphylococcus aureus transmission in residential care homes for the elderly

BACKGROUND

The objectives were to evaluate the effectiveness of an infection control bundle in controlling the meticillin resistant Staphylococcus aureus (MRSA) transmission in residential care homes for the elderly (RCHEs) in Hong Kong.

METHODS

This was a two-arm cluster randomised controlled trial. Infection control bundles focused on hand hygiene (HH), environmental hygiene, and modified contact precautions were applied to the intervention arm. Nasal swabs from residents; staff HH compliance and effectiveness; and environmental hygiene were assessed by microbiological sampling or observation at the baseline and quarterly after the intervention.

RESULTS

A total of 2776 residents from 36 RCHEs were recruited. The overall MRSA prevalence was 20.4% (95% confidence interval, 18.9%-21.9%). The intervention elicited an immediate effect of 2.4% absolute decrease in the prevalence and 3.7% in the intra-facility transmission, though the difference between the two arms was insignificant. Staff HH compliance increased substantially from 5.9% to 45.6% post-intervention (p < 0.001).

CONCLUSIONS

We initiated the infection control culture into the RCHEs and gained their acceptance. However, this behavioural change takes time to emerge. Our study shows that relying on the bundle alone could not bring sustainable MRSA reduction. Administrative control for strengthening infection control infrastructure is important for continuous compliance and improvement.

Association between ward-specific antimicrobial use density and methicillin-resistant Staphylococcu aureus surveillance: a 60-month study

It is not known whether or not ward-specific antimicrobial use density (AUD) affects the ratio of methicillin-resistant Staphylococcus aureus (MRSA) in culture-positive S. aureus. A 60-month study was attempted to ascertain the association between inpatient MRSA ratio and ward-specific AUDs as well as the former and latter study intervals, specimen types, and ward specialty. During the study, the professionals in infection control regulated the use of broad-spectrum antimicrobials and those for MRSA. By both month and ward, the ratio of inpatients positive for MRSA to those positive for S. aureus was calculated. Factors associated with MRSA ratio included AUDs averaged for the sampling month and its previous month, outpatient MRSA ratio by age, ward specialty, specimen type, and half intervals to represent historical changes. Of a total of 4,245 strains of S. aureus isolated during the 5-year study, 2,232 strains (52.6%) were MRSA. By year, outpatient MRSA ratio at age ≥15 decreased in later years, as did inpatient MRSA ratio. Multivariate analysis for inpatient MRSA ratio revealed a positive risk in AUDs for meropenem (odds ratio [OR] 1.761; 95% confidence interval [CI] 1.761–2.637, P = 0.01), imipenem-cilastatin (OR 1.583; 95% CI 1.087–2.306, P = 0.02), ampicillin-sulbactam (OR 1.623; 95% CI 1.114–2.365, P = 0.01), and minocycline (OR 1.680; CI 1.135–2.487, P = 0.01), respiratory care ward (OR 2.292; 95% CI 1.085–4.841, P = 0.03), and outpatient MRSA ratio (OR 1.536; 95% CI 1.070–2.206, P = 0.02). Use of broad-spectrum antimicrobials, such as meropenem, imipenem-cilastatin, and ampicillin-sulbactam may increase inpatient MRSA ratio. Ward factor should be included in MRSA surveillance because of the possible effect on AUD and considering patients’ backgrounds.

Meticillin-resistant Staphylococcus aureus in elderly residents of care homes: colonization rates and molecular epidemiology

BACKGROUND

Meticillin-resistant Staphylococcus aureus (MRSA) is a significant cause of mortality and morbidity in healthcare and community settings; however, there is a paucity of large-scale, longitudinal studies monitoring the occurrence of MRSA in the care home setting.

AIM

To determine the molecular epidemiology of MRSA colonizing elderly residents of care homes.

METHODS

Residents in 65 care homes in Leeds, UK, were screened for MRSA nasal colonization in four consecutive years (2006-2009). Isolates were characterized using antibiotic susceptibility testing, detection of the Panton-Valentine leucocidin (PVL) locus, accessory gene regulator allotyping, characterization of the staphylococcal cassette chromosome mec element, spa-typing and pulsed-field gel electrophoresis.

FINDINGS

MRSA was recovered from 888 nasal swabs of 2492 residents and prevalence was similar (19-22%) throughout the study. Resistance to ≥3 antibiotic classes was common (34%), but resistance to only β-lactam agents was rare (3%); no PVL-positive isolates were identified. Most isolates were related to healthcare-associated epidemic-MRSA type 15 (EMRSA-15, ST22-IV); such isolates decreased in prevalence during the study (86-72%; P < 0.0001, χ(2)-test). The remainder belonged to five different multi-locus sequence type clonal complexes (CC). Most notably, CC59 strains increased in prevalence (10-25%; P < 0.0001, χ(2)-test) and were associated with high-level mupirocin resistance.

CONCLUSIONS

The molecular epidemiology of MRSA in care homes is complex and dynamic. There was a high, consistent prevalence of MRSA nasal colonization, dominated by healthcare-associated strains. Vigilance is recommended; however, as high-level mupirocin resistance was associated with a single clonal group (CC59) that significantly increased in prevalence during the study.

Clinical significance of methicillin-resistantStaphylococcus aureuscolonization in residents in community long-term-care facilities in Spain

Methicillin-resistantStaphylococcus aureus(MRSA) is highly prevalent in Spanish hospitals and community long-term-care facilities (LTCFs). This longitudinal study was performed in community LTCFs to determine whether MRSA colonization is associated with MRSA infections and overall mortality. Nasal and decubitus ulcer cultures were performed every 6 months for an 18-month period on 178 MRSA-colonized residents (86 490 patient-days) and 196 non-MRSA carriers (97 470 patient-days). Fourteen residents developed MRSA infections and 10 of these were skin and soft tissue infections. Two patients with respiratory infections required hospitalization. The incidence rate of MRSA infection was 0·12/1000 patient-days in MRSA carriers and 0·05/1000 patient-days in non-carriers (P=0·46). No difference in MRSA infection rate was found according to the duration of MRSA colonization (P=0·69). The mortality rate was 20·8% in colonized residents and 16·8% in non-carriers; four residents with MRSA infection died. Overall mortality was statistically similar in both cohorts. Our results suggest that despite a high prevalence of MRSA colonization in LTCFs, MRSA infections are neither frequent nor severe while colonized residents remain at the facility. The epidemiological impact of an MRSA reservoir is more relevant than the clinical impact of this colonization for an individual resident and supports current recommendations to control MRSA spread in community LTCFs.

Epidemiology of methicillin-resistant Staphylococcus aureus (MRSA) among residents of nursing homes in Belgium

OBJECTIVES

A national survey was conducted to determine the prevalence, risk factors and molecular epidemiology of methicillin-resistant Staphylococcus aureus (MRSA) carriage among nursing home (NH) residents in Belgium.

METHODS

A random stratified, cross-sectional prevalence survey was conducted in NH residents who were screened for MRSA carriage by multisite enriched culture. Characteristics of NHs and residents were collected by a questionnaire survey and analysed by two-stage logistic regression modelling. MRSA isolates were genotyped by PFGE, staphylococcal cassette chromosome mec (SCCmec) typing, multilocus sequence typing (MLST) and resistance genes.

RESULTS

Of 2953 residents screened in 60 NHs, 587 (19.9%) were MRSA carriers. Risk factors included hospital contact, antibiotic exposure, impaired mobility and skin lesions at the resident level, and lack of MRSA surveillance, lack of antibiotic therapeutic formulary and the combination of less-developed infection control activities and a high ratio of physicians to residents at the institution level. MRSA isolates showed eight major types, three of which were predominant: B2-ST45-SCCmec IV (49%; where ST stands for sequence type); A21-ST8-SCCmec IV (13%); and A20-ST8-SCCmec IV (10%). Each was recovered in 55, 21 and 25 NHs, respectively. The geographical distribution of NH genotypes paralleled that of acute-care hospitals.

CONCLUSIONS

A high prevalence of MRSA carriage in NH residents was associated with hospital care, co-morbidities and less-developed coordination of institutional care. The predominant MRSA strains from NH residents and hospitalized patients of the same area were identical. Strengthening and coordination of MRSA surveillance and control activities are warranted within and between NHs and hospitals.

Methicillin-resistant Staphylococcus aureus USA300 clone in long-term care facility

We performed a longitudinal analysis of 661 methicillin-resistant Staphylococcus aureus (MRSA) isolates obtained from patients in a long-term care facility. USA300 clone increased from 11.3% of all MRSA isolates in 2002 to 64.0% in 2006 (p<0.0001) and was mostly recovered from skin or skin structures (64.3% vs. 27.0% for non-USA300 MRSA; p<0.0001).

Molecular characterization of methicillin-resistant Staphylococcus epidermidis strains from bacteraemic patients

In order to study the clonality of clinical methicillin-resistant Staphylococcus epidermidis (MRSE) strains and their staphylococcal cassette chromosome mec (SCCmec) elements, 60 isolates of MRSE from bacteraemic patients in three units of the Helsinki University Hospital, Finland were selected, covering the periods 1990-1993 and 1997-1998. The MRSE strains were analysed by pulsed-field gel electrophoresis (PFGE), multilocus sequence typing and SCCmec typing. Eleven PFGE types (FIN-SE-1-11) with sequence type ST2 (clonal complex 2; CC2) were identified. The previously established methicillin-resistant Staphylococcus aureus SCCmec criteria were applied to name the MRSE SCCmec complexes, and it was found that 7% of the isolates carried SCCmec type IA (ccrA1, class B), whereas the majority (93%) yielded six non-typeable SCCmec PCR patterns (P1-P6). Within each SCCmec PCR pattern, two ccr recombinase genes (ccrA2 and ccrA3) and two mec gene complexes (class A and class B) were detected. In addition, the ccrC gene was associated with three of the six patterns. In conclusion, the MRSE strains were genetically related to each other (ST2) but their SCCmec complexes were unique combinations of elements previously recognized among SCCmec types III and IV.

Carriage of methicillin-resistant Staphylococci and their SCCmec types in a long-term-care facility

Following an outbreak caused by staphylococcal cassette chromosome mec (SCCmec) type V methicillin (meticillin)-resistant Staphylococcus aureus (MRSA), a point-prevalence survey of the nasal carriage of staphylococci was conducted in a long-term-care facility in northern Finland in 2004. The focus was directed at methicillin-resistant coagulase-negative staphylococci (MR-CNS) and their SCCmec elements. A nasal swab was taken from 76 of the 80 residents 6 months after the onset of the outbreak. Staphylococcal isolates were identified by conventional methods and the GenoType Staphylococcus test, and their SCCmec elements were analyzed. Of the 76 individuals, 24 (32%) carried S. aureus and 67 (88%) CNS in their nostrils. Of the CNS carriers, 41 (61%) had at least one mecA-positive MR-CNS, and two individuals (3%) had both MRSA and methicillin-resistant Staphylococcus epidermidis (MRSE). Among the 61 MR-CNS isolates identified, 49 (80%) were MRSE. The distribution of the SCCmec types was diverse: 20 (33%) were of type IV, 11 (18%) of type V, 4 (6%) of type I or IA, 3 (4%) of type II, and 23 (38%) of new types (with six different combinations of ccr and other mec genes or only mecA). Both of the individuals with MRSA and MRSE shared SCCmec type V among their isolates. Nasal MR-CNS carriage was common among the residents of this long-term-care facility. A variety of SCCmec types, including many new types, were identified among the MR-CNS strains. The horizontal transfer of SCCmec elements is speculated based on the sharing of SCCmec type V between MRSA and MRSE.

Epidemiology of Staphylococcus aureus colonization in nursing home residents

BACKGROUND

We sought to characterize the clinical and molecular epidemiologic characteristics of Staphylococcus aureus colonization (especially extranasal colonization) and to determine the extent to which community-associated methicillin-resistant S. aureus (MRSA) has emerged in community nursing homes.

METHODS

The study enrolled a total of 213 residents, with or without an indwelling device, from 14 nursing homes in southeastern Michigan. Samples were obtained from the nares, oropharynx, groin, perianal area, wounds, and enteral feeding tube site. Standard microbiologic methods were used to identify methicillin-susceptible S. aureus and MRSA. Molecular epidemiologic methods included pulsed-field gel electrophoresis, PCR detection of Panton-Valentine leukocidin, and SCCmec and agr typing.

RESULTS

One hundred thirty-one residents (62%) were colonized with S. aureus (MRSA colonization in 86). S. aureus colonization occurred in 80 (76%) of 105 residents with indwelling devices and in 51 (47%) of 108 residents without indwelling devices (P<.001). Of the 86 residents who were colonized with MRSA, nares culture results were positive for only 56 (65%). Residents with devices in place were more likely to be colonized at multiple sites. Eleven different strains of MRSA were identified by pulsed-field gel electrophoresis. Seventy-three residents (85%) were colonized with hospital-associated SCCmec II strains, and 8 (9%) were colonized with community-associated SCCmec IV strains, 2 of which carried Panton-Valentine leukocidin.

CONCLUSIONS

Extranasal colonization with MRSA is common among nursing home residents-particularly among residents with an indwelling device. We documented the emergence of community-associated SCCmec IV MRSA strains in the community nursing home setting in southeastern Michigan.

Management of a large healthcare-associated outbreak of Panton-Valentine leucocidin-positive meticillin-resistant Staphylococcus aureus in Germany

We report the largest documented healthcare-associated outbreak of Panton-Valentine leucocidin-positive meticillin-resistant Staphylococcus aureus (PVL(+) MRSA) in Europe. Six index patients from three long-term care facilities (LTCFs) were screened positive for PVL(+) MRSA in 2004 on admission to a community hospital in Germany. The purpose of this prospective study was to describe the prevalence of PVL(+) MRSA in the LTCFs before and after infection control interventions. Screening for MRSA with or without PVL was performed in all three LTCFs in 2004 [453 residents, 240 healthcare workers (HCWs)] and 2005 (440 residents, 192 HCWs). Swabs from anterior nares and wounds, if applicable, were collected. Colonised residents and staff were treated with mupirocin nasal ointment and topical antiseptics, and staff were provided with hygiene education. Total MRSA carrier rate of residents and HCWs in 2004 was 11.3% (PVL(+) MRSA 9.1%, PVL(-) MRSA 2.2%). There were comparable carrier rates between residents and HCWs in each LTCF. All PVL(+) MRSA isolates were of clonal origin (MLST 22) representing a novel spa sequence type t310. A decrease in total MRSA prevalence (from 11.3 to 5.5%) and PVL(+) MRSA (from 9.1 to 3.3%) was observed in 2005. The rate of PVL(-) MRSA remained unaffected. No symptomatic skin infections were noted among residents or HCWs. In this outbreak incomplete control of PVL(+) MRSA presumably resulted from difficult and delayed detection and decolonisation of carriers, incomplete compliance with control measures and lack of enforcement by public health authorities.

Staphylococcus aureus colonization among nursing home residents in a large Finnish nursing home

We studied colonization with methicillin-resistant and -sensitive Staphylococcus aureus (MRSA, MSSA) in the second largest nursing home in Finland, in which the residents volunteered had their nostrils, throats, perineums, skin lesions, and catheter exit sites swabbed, and catheter urines cultured. The specimens were cultured onto non-selective and selective agar, with or without enrichment in salt-containing trypticase soy broth (TSB). S. aureus was identified by routine methods, methicillin resistance was detected by oxacillin and cefoxitin disk diffusion and MIC E-tests, and GenoType MRSA -test was used for mecA gene confirmation. A total of 663 cultures were obtained from 213 residents. Of those, 165 specimens (25%) from 94 residents (44%) were positive for S. aureus, and 3 specimens (0.4%) from 2 (0.9%) residents were positive for MRSA. Of the 165 S. aureus isolates, 31 (19%) from 25 (27%) residents were found only from sites other than nostrils (30 MSSA and 1 MRSA). TSB enrichment detected additional 33 (5%) S. aureus isolates (32 MSSA and 1 MRSA), resulting in 8 (5%) additional residents. None of the MRSA strains would have been found if only nostrils and throat had been screened, and no enrichment broth had been used.

Epidemiology and genetic diversity of methicillin-resistant Staphylococcus aureus strains in residential care homes for elderly persons in Hong Kong

OBJECTIVE

To determine the prevalence and molecular epidemiology of methicillin-resistant Staphylococcus aureus (MRSA) strains among residents in residential care homes for the elderly in Hong Kong.

DESIGN

Cross-sectional and descriptive study.

PARTICIPANTS

A total of 949 residents in 13 residential care homes for elderly persons in Hong Kong in January 2005.

METHODS

MRSA colonization was assessed by culture of swab specimens from anterior nares and active skin lesions. Characteristics of residents were obtained by a standard questionnaire. All MRSA isolates were analyzed by polymerase chain reaction for their staphylococcal cassette chromosome (SCC) mec content and were typed by pulsed-field gel electrophoresis (PFGE) and multilocus sequencing.

RESULTS

MRSA colonization was detected in 27 residents (2.8%). No MRSA was found in 2 facilities. The rate of MRSA carriage in the other 11 facilities ranged from 1.9% to 4.2%. In univariate analysis, functional immobility (odds ratio [OR], 1.4), history of hospital admission (OR, 2.3), and the use of nebulized medication (OR, 5.4) were significantly associated with MRSA colonization. The isolates had 11 unique antibiograms, with 14 isolates susceptible to all but 1 or 2 of the non- beta -lactam antimicrobial agents tested. The isolates exhibited SCCmec types I (1 isolate), II (2 isolates), III (1 isolate), IV/IVA (10 isolates), and V (13 isolates). No isolates had the Panton-Valentine leukocidin genes. PFGE analysis clustered all except 1 isolate into 7 PFGE types, designated HKU10 to HKU70. Between 1 and 4 unique PFGE types were found in the individual residential care facilities.

CONCLUSION

This study documented the emergence of SCCmec types IV and V among genetically diverse MRSA strains in residential care homes for elderly persons in Hong Kong.

The molecular evolution of methicillin-resistant Staphylococcus aureus

Staphylococcus aureus is a potentially pathogenic bacterium that causes a broad spectrum of diseases. S. aureus can adapt rapidly to the selective pressure of antibiotics, and this has resulted in the emergence and spread of methicillin-resistant S. aureus (MRSA). Resistance to methicillin and other beta-lactam antibiotics is caused by the mecA gene, which is situated on a mobile genetic element, the Staphylococcal Cassette Chromosome mec (SCCmec). To date, five SCCmec types (I-V) have been distinguished, and several variants of these SCCmec types have been described. All SCCmec elements carry genes for resistance to beta-lactam antibiotics, as well as genes for the regulation of expression of mecA. Additionally, SCCmec types II and III carry non-beta-lactam antibiotic resistance genes on integrated plasmids and a transposon. The epidemiology of MRSA has been investigated by pulsed-field gel electrophoresis (PFGE), multilocus sequence typing (MLST), spa typing and SCCmec typing. Numerous MRSA clones have emerged and disseminated worldwide. SCCmec has been acquired on at least 20 occasions by different lineages of methicillin-sensitive S. aureus. Although most MRSA strains are hospital-acquired (HA-MRSA), community-acquired MRSA (CA-MRSA) strains have now been recognised. CA-MRSA is both phenotypically and genotypically different from HA-MRSA. CA-MRSA harbours SCCmec types IV or V, and is associated with the genes encoding Panton-Valentine leukocidin. The prevalence of MRSA ranges from 0.6% in The Netherlands to 66.8% in Japan. This review describes the latest developments in knowledge concerning the structure of SCCmec, the molecular evolution of MRSA, the methods used to investigate the epidemiology of MRSA, and the risk-factors associated with CA-MRSA and HA-MRSA.