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The scourge of hospitals and other healthcare facilities, S. aureus is responsible for skin and tissue infections, pneumonia, septicemia, and device-associated infections. The nature and severity of these staph infections is highly dependent on the strain, which variably express virulence factors including coagulase, hyaluronidase, and toxins such as TSST-1 (toxic shock syndrome), exfoliatin (scalded-skin syndrome), and Panton-Valentine leukocidin. In 20% of healthy adults, S. aureus is among the normal flora of the skin or anterior nares. All humans are susceptible to infection, but the risk is increased in newborns, the immunocompromised, those with chronic conditions (e.g., diabetes, vascular disease, lung disease), breastfeeding women, and those with skin disorders or injuries.
As a gram positive bacterium, S. aureus was the initial target of the penicillins. However, with increasing use resistance to these agents steadily increased, which was countered by the development of new beta-lactams such as methicillin. This approach eventually proved futile when MRSA (methicillin-resistant S. aureus) strains emerged in the 1960’s, some of which exhibit multidrug resistance and additional virulence factors. With few alternative treatments available (depending on strain), invasive MRSA infections are associated with high mortality. Vancomyin is the “drug of last resort”, but the recent emergence of vancomycin-resistant S. aureus (VRSA) has cast doubt on its future use.
MRSA strains carry one of many variants of the mobile element SCCmec (staphylococcal cassette chromosome mec), with the common component being the mecA gene encoding methicillin-refractory transpeptidase PBP2a. With respect to MRSA origin, it is believed that SCCmec elements were transferred into a limited number of methicillin-susceptible strains. MRSA strains associated with U.S. healthcare facilities (HA-MRSA) include ST5:USA100 and ST36:USA200. Community-acquired (CA-MRSA) strains include ST8:USA300, and less commonly ST1:USA400, ST8:USA500, and ST59:USA1000. S. aureus is a moving target, however, and these patterns continually shift.
Since S. aureus is highly contagious, and the nature, severity, and response to treatment of a staph infection are strongly strain-dependent, typing is an important adjunct to infection control. Pulsed-field gel electrophoresis has been widely used for strain typing (yielding the “USA” classifications above), but due to significant limitations (e.g., turnaround time and portability) it is increasingly complemented or replaced by DNA sequence-based approaches. Primary among these is multilocus sequence typing (MLST), requiring analysis of 7 housekeeping genes. Over 2700 MLST types have been identified to date, such as ST5 and ST8 noted above. However, increased resolution and substantially decreased cost are provided by single locus DNA-sequencing of polymorphic repeat regions of the staphylococcal protein A gene spa. Over 12,000 spa types have been identified; common ones include t002 and t008, respectively, for ST5:USA100 and ST8:USA300 strains. Both of these sequence-based typing methods have been extensively validated as reliable targets for typing S. aureus strains including MRSA, and large public databases are available for each (saureus.mlst.net; spa.ridom.de).
MicrobiType services for S. aureus strain identification include SaMTspa and SaMLST, based on our own genomics optimized primers. Note that SaMTspa goes beyond conventional spa repeat typing since the analysis includes additional upstream sequence. Consequently, strains which have identical spa types may be distinquished by this expanded spa sequence analysis. For example, as shown in the SaMT-spa dendrogram, four strains which type as t002-ST5 form two very distinct clusters (Mu3/Mu50 and ECT-R2/ED98) upon SaMTspa analysis. Results delivered include a numerical type, along with dendrogram and sequence alignment illustrating the relatedness of your isolate to database isolates and to previously or concurrently submitted isolates from your lab.