2605. Mixed Subpopulation of Hemolytic and Non-Hemolytic Phenotype in Clinical Staphylococcus aureus Blood Isolates

BACKGROUND: Agr is a key regulator that controls expression of secreted exoproteins and surface protein in Staphylococcus aureus. It has been reported that mixed status of two different phenotypes including agr functional and nonfunctional subpopulations can coexist in vitro and in vivo. However, data on the natural course and clinical implication of the mixed agr status is limited. We thus investigated the frequency and characteristics of the mixed agr in clinical settings. METHODS: We evaluated isogenic paired MRSA isolates collected from patients with persistent S. aureus bacteremia (SAB) between October 2010 and April 2016, and then prospectively performed surveillance for the presence of mixed agr function in MRSA isolates from patients with SAB between May 2016 and December 2017. The mixed agr status was evaluated by single colony evaluation on sheep blood agar plate containing RN4220 supernatant (β-hemolysin) (Figure 1). Cross-streaking with RN4220 and RNAIII measurement were performed to confirm the agr functionality of each of hemolytic and non-hemolytic colonies, separately. The expression levels of RNAIII, hla, and saeS/saeR were measured by real-time reverse transcription polymerase chain reaction. RESULTS: A total of 161 first blood isolates were collected during study period, and 6 isolates (4%) displayed mixed phenotype by single colony test. The mixed hemolytic pattern was observed in 5 out of 52 ST72 isolates (10%) and 1 out of 82 ST5 isolates (1%) (Figure 1). No difference was found in the genotypes between hemolytic and non-hemolytic colonies from each isolate. Of the 6 isolates, three lost mixed hemolytic features in the follow-up blood cultures (Table 1). One ST72 and one ST5 isolate showed agr mixed pattern determined by different RNAIII levels, but remaining four ST72 isolates had mixed hemolytic pattern due to different expression of hla correlated with saeS/saeR expression (Figure 2). CONCLUSION: The mixture of agr function status among the clinical blood isolates of MRSA was rarely observed and isolates displaying heterogeneous hemolytic phenotype were largely due to differential expression of α-hemolysin. Further investigation is needed to unveil the clinical significance of mixture of different hemolytic phenotypes. [Image: see text] [Image: see text] [Image: see text] DISCLOSURES: All authors: No reported disclosures.