Molecular epidemiology and clinical characteristics of Staphylococcus aureus bacteremia in Japanese adults

Staphylococcus aureus bacteremia (SAB), especially when caused by methicillin-resistant S. aureus (MRSA), is of considerable clinical importance. In recent years, the proportion of MRSA among S. aureus has decreased, and a relative increase in the proportion of methicillin-susceptible S. aureus (MSSA) has been observed. It is therefore necessary to consider both MRSA and MSSA when assessing the microbiological and clinical significance of SAB.

We included SAB cases from the Nara Medical University Hospital between January 2015 and February 2017. We performed drug susceptibility testing, toxicity gene analysis, multilocus sequence typing (MLST), and polymerase chain reaction-based open reading frame typing (POT) of stored strains to integrate clinical and bacteriological characteristics.

There were 90 cases during the experimental period (42 MRSA and 48 MSSA), with 30-day mortality rates of 19 % for MRSA and 10.4 % for MSSA. Deaths were more frequently complicated by septic shock and disseminated intravascular coagulation. MLST studies showed that ST8, ST764, ST1, and ST15 were prevalent in the MRSA group, whereas ST5, ST188, and ST12 were prevalent in MSSA. Infective endocarditis cases had a long time from onset to the initiation of effective antimicrobials and were all MSSA. MLST and POT results correlated well, and POT appeared to have better discriminatory power.

The severity and mortality of SAB, along with the microbiological characteristics of causative isolates, vary by location and time. Continued studies integrating clinical and microbiological investigations are thus needed.