WS19.3 Prevalence of small colony variants of Staphylococcus aureus from lower respiratory tract specimens

Abstract

Background Staphylococcus aureus , one of the first bacteria isolated from cystic fibrosis (CF) airways, is the most common cause of respiratory infections. Small-colony phenotypic variants (SCVs) of S. aureus are selected by long-term exposure to antibiotics or co-culture with Pseudomonas aeruginosa and are often difficult to detect. The prevalence of this pathogen is largely unknown. Aim To establish the prevalence of SCV S. aureus by screening lower airway specimens from children with CF. Methods SCV S. aureus were obtained from the hospital clinical laboratory from lower airway specimens of children with CF. Each visible morphotype was subcultured onto Horse blood agar (HBA) to enable identification of SCVs. If identified, isolates required confirmation by PCR amplification of the S. aureus -specific gene, nuc and metabolic defects determined by growing isolates in the presence of haemin, menadione, thymidine and CO 2 . Paired normal and SCV S. aureus isolates were to be genotyped using pulse-field gel electrophoresis. Results 26 children (mean age 11.35(5.05) years) from the AREST CF study (Melbourne) were identified with a positive S. aureus culture from lower respiratory tract specimens collected in 2014. All children were on at least one or multiple antibiotics prior to or at time of collection, namely Augmentin (46%), Tobramycin (15%), Bactrim (11%), or other (27%). 57% of samples were visible on large MSA plates and subcultured onto wild-type HBA plates, but no SCVs of S. aureus were isolated. Conclusion SCV's were not isolated from the lower airways specimens and therefore not identified as contributing to lung infections in our cohort of young subjects with CF.