Phenotypic and Genotypic Characterization of Staphylococcus aureus Bloodstream Isolates in a Single Large Medical Center in Southeastern Minnesota

Background: 

Staphylococcus aureus is a leading cause of bacteremia often associated with high levels of morbidity and mortality. It is well known that S. aureus can produce superantigens as virulence factors, but it is unclear what role superantigens play in S. aureusbacteremia (SAB). The epidemiology of SAB varies by region. We investigated the clinical and molecular epidemiology of SAB isolates and the prevalence of their superantigen production.

Methods: 

154 SAB isolates collected throughout 2015 were studied; clinical data were analyzed. Molecular typing was performed using Staphylococcus protein A (spa) typing. Superantigen genes sea, seb, sec, sed, see, seg, seh, sei, sej, and tst were detected by PCR. Mitogenic activities of superantigens were assayed using splenocytes from HLA-DR3 transgenic mice stimulated by supernatants of planktonic cultures of the isolates.

Results: 

106 isolates (69%) were methicillin–susceptible S. aureus (MSSA). When MSSA and methicillin resistant S. aureus (MRSA) were compared, there were no differences in age, gender, underlying illness, primary site of infection, severity of infection, superantigen production, or mortality. More than 50% of MRSA isolates and 13% of MSSA isolates were resistant to clindamycin. Less than 3% of MRSA isolates were resistant to rifampin, mupirocin, or trimethoprim-sulfamethoxazole. spa typing showed 82 spa types clustered into 11 spa clonal complexes (CCs). Typing of MSSA revealed a wide genotypic diversity. Of the 48 MRSA isolates, 42 (88%) belonged to spa CC-002 and CC-008. The sej gene was the most prevalent, found in 122 S. aureus isolates (79%) and more than 50% of isolates had tst, sec, seg, and sei genes. Superantigen genes were equally distributed between MSSA and MRSA. The number of deaths attributed to SAB, deaths not attributed to SAB, and 30-day survival were 24 (16%), 2 (1%), and 128 (83%), respectively. Superantigens were not related with mortality.

Conclusion: 

The predominant strain was MSSA. Most MRSA isolates belonged to spa CC-002 and CC-008. Superantigen genes were prevalent in SAB and many isolates produced functional superantigens; however, superantigens were not related with mortality.