Chromogenic Detection Increases Diagnostic Sensitivity for Staphylococcus Aureus in Cystic Fibrosis Respiratory Cultures

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Rationale: Staphylococcus aureus and Pseudomonas aeruginosa both establish long-term infections in the CF airways. However, P. aeruginosa is more commonly identified on routine cultures than S. aureus in patients who have chronic infections with both organisms. This observation suggested that there may be low diagnostic sensitivity for S. aureus. We hypothesized that culture techniques applying selective and differential agar media would increase sensitivity for S. aureus in CF respiratory cultures. Method: We prospectively enrolled patients with CF in a study to identify risk factors for persistent S. aureus infection. We collected paired respiratory culture specimens (either sputum or throat swabs) from patients with CF. One specimen was sent to the clinical microbiology laboratory for standard diagnostic testing. The paired specimen was cultured in the research laboratory using Aureus Chromoselect agar with egg yolk tellurite. On this medium, S. aureus colonies appear black. Catalase-positive, coagulase-positive colonies were analyzed by MALDI-TOF to confirm the presence of S. aureus. Results: We tested 59 paired respiratory cultures from 54 unique patients (median age 21, IQR 7 -28) from August -December 2020. Specimens included 52 oropharyngeal cultures and 7 sputa. Clinical laboratory testing revealed 33 cultures (55.9%) were positive for S. aureus by standard culture methods, similar to the observed prevalence in laboratory specimens over the past 5 years. With chromogenic media, 48 cultures (81.4%) were positive. Chromogenic testing identified S. aureus in every culture that tested positive on standard media in the clinical laboratory. The clinical laboratory did not detect S. aureus in 15 specimens, including 8 cases of methicillin resistant S. aureus. Conclusions: Chromogenic agar increases the detection of S. aureus in CF respiratory cultures. Standard culture methods could underestimate the prevalence of S. aureus in CF. Improvements in diagnostic methodology are needed for this prevalent CF pathogen.

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