Genome sequence of a recently emerged, highly transmissible, multi-antibiotic- and antiseptic-resistant variant of methicillin-resistant Staphylococcus aureus, sequence type 239 (TW).

The 3.1-Mb genome of an outbreak methicillin-resistant Staphylococcus aureus (MRSA) strain (TW20) contains evidence of recently acquired DNA, including two large regions (635 kb and 127 kb). The strain is resistant to a wide range of antibiotics, antiseptics, and heavy metals due to resistance genes encoded on mobile genetic elements and also mutations in housekeeping genes.

Difference in virulence between Staphylococcus aureus isolates causing gangrenous mastitis versus subclinical mastitis in a dairy sheep flock.

Staphylococcus aureus mastitis in dairy sheep ranges from subclinical mastitis to lethal gangrenous mastitis. Neither the S. aureus virulence factors nor the host-factors or the epidemiological events contributing to the different outcomes are known. In a field study in a dairy sheep farm over 21 months, 16 natural isolates of S. aureus were collected from six subclinical mastitis cases, one lethal gangrenous mastitis case, nasal carriage from eight ewes and one isolate from ambient air in the milking room. A genomic comparison of two strains, one responsible for subclinical mastitis and one for lethal gangrenous mastitis, was performed using multi-strain DNA microarrays. Multiple typing techniques (pulsed-field-gel-electrophoresis, multiple-locus variable-number, single-nucleotide polymorphisms, randomly amplified polymorphic DNA, spa typing and sas typing) were used to characterise the remaining isolates and to follow the persistence of the gangrenous isolate in ewes' nares. Our results showed that the two strains were genetically closely related and they shared 3 615 identical predicted open reading frames. However, the gangrenous mastitis isolate carried variant versions of several genes (sdrD, clfA-B, sasA, sasB, sasD, sasI and splE) and was missing fibrinogen binding protein B (fnbB) and a prophage. The typing results showed that this gangrenous strain emerged after the initial subclinical mastitis screening, but then persisted in the flock in the nares of four ewes. Although we cannot dismiss the role of host susceptibility in the clinical events in this flock, our data support the hypothesis that S. aureus populations had evolved in the sheep flock and that S. aureus genetic variations could have contributed to enhanced virulence.

An outbreak of severe infections with community-acquired MRSA carrying the Panton-Valentine leukocidin following vaccination.

BACKGROUND: Infections with community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) are emerging worldwide. We investigated an outbreak of severe CA-MRSA infections in children following out-patient vaccination. METHODS AND FINDINGS: We carried out a field investigation after adverse events following immunization (AEFI) were reported. We reviewed the clinical data from all cases. S. aureus recovered from skin infections and from nasal and throat swabs were analyzed by pulse-field gel electrophoresis, multi locus sequence typing, PCR and microarray. In May 2006, nine children presented with AEFI, ranging from fatal toxic shock syndrome, necrotizing soft tissue infection, purulent abscesses, to fever with rash. All had received a vaccination injection in different health centres in one District of Ho Chi Minh City. Eight children had been vaccinated by the same health care worker (HCW). Deficiencies in vaccine quality, storage practices, or preparation and delivery were not found. Infection control practices were insufficient. CA-MRSA was cultured in four children and from nasal and throat swabs from the HCW. Strains from children and HCW were indistinguishable. All carried the Panton-Valentine leukocidine (PVL), the staphylococcal enterotoxin B gene, the gene complex for staphylococcal-cassette-chromosome mec type V, and were sequence type 59. Strain HCM3A is epidemiologically unrelated to a strain of ST59 prevalent in the USA, although they belong to the same lineage. CONCLUSIONS: We describe an outbreak of infections with CA-MRSA in children, transmitted by an asymptomatic colonized HCW during immunization injection. Consistent adherence to injection practice guidelines is needed to prevent CA-MRSA transmission in both in- and outpatient settings.

Rapid determination of hospital-acquired meticillin-resistant Staphylococcus aureus lineages.

Multilocus sequence typing (MLST) and multi-strain microarray analysis have shown that most human Staphylococcus aureus strains belong to ten dominant clonal complexes (CCs) or lineages, each with unique surface architecture. Meticillin-resistant S. aureus (MRSA) strains currently belong to six of these lineages (CC1, CC5, CC8, CC22, CC30 and CC45), each of which has independently acquired mobile genetic elements (MGEs) carrying antibiotic resistance genes. MLST and microarrays are expensive and time consuming methods for routine determination of S. aureus lineage. A restriction-modification (RM) test has now been developed that is rapid, simple, inexpensive and accurately determines lineage of hospital-acquired MRSA. The RM test is based on three PCRs for hsdS gene variants, as hsdS genes likely control the independent evolution of S. aureus lineages. The RM test correctly identified 102 MRSA isolates as belonging to one of the six lineages/CCs. Real-time MRSA typing can be used to identify and track changes in local MRSA outbreaks, and provide support for targeting infection control strategies. Simple and accurate typing methods will also support large scale epidemiological studies, and could lead to greater understanding of the carriage, spread and virulence of different MRSA lineages.

An outbreak in an intensive care unit of a strain of methicillin-resistant Staphylococcus aureus sequence type 239 associated with an increased rate of vascular access device-related bacteremia.

BACKGROUND: Patients in intensive care units are at high risk of developing methicillin-resistant Staphylococcus aureus (MRSA) bacteremia. We report an epidemiological and bacterial genomic analysis of a 2-year outbreak in an intensive care unit of a variant of MRSA sequence type 239 (hereafter designated TW). METHODS: A cohort study was conducted to compare risk factors for MRSA bacteremia in patients who acquired TW versus patients who acquired non-TW strains of MRSA. Genetic analysis of TW was performed using multilocus sequence typing and microarray analysis. RESULTS: Patients who acquired TW were more likely than patients who acquired non-TW strains of MRSA to have MRSA isolated from blood samples (47% vs. 13%; P<.001) and to have MRSA-positive vascular access device-sample cultures (59% vs. 26%; P<.001), but less likely to have MRSA isolated from screening swab samples (30% vs. 71%; P<.001). This increased rate of TW bacteremia was confined to the first week after acquisition of TW infection. Using Cox regression analysis, the adjusted hazard ratio for bacteremia with TW was 4.5 times that of non-TW strains of MRSA (95% confidence interval, 2.25-9.00; P<.001). Microarray analysis revealed that TW had accumulated all detectable mobile genetic elements that were variably expressed by other epidemic strains of MRSA sequence type 239 in the United Kingdom. CONCLUSIONS: To our knowledge, this is the first report to provide direct evidence that strains of MRSA can differ in their ability to cause bacteremia. Further genetic and in vitro analysis of the TW strain may provide insight into the mechanism of vascular access device-related bacteremia in the intensive care unit environment.