61 Comparison of two chromogenic media for isolation of Staphylococcus aureus from respiratory samples from patients with cystic fibrosis

Objective To evaluate two chromogenic culture media, chromID S. aureus (chromID) and chromID S. aureus Elite (chromID Elite) for isolation of Staphylococcus aureus from respiratory samples from patients with cystic fibrosis (CF). Methods The samples included 123 sputum samples and 108 cough swabs from 171 CF patients. Sputum samples were homogenized with sputasol and 10 µL was cultured onto each medium. The clinical material from cough swabs was dispersed in 2 mL of saline and 10 µL was cultured onto each medium. Media were incubated at 37°C and plates were read after 20 h, 48 h and 72 h. Any colonies showing expected coloration were identified using MALDI-TOF MS. Results S. aureus was isolated from 52 samples (22%). The sensitivity for chromID was 62% after 20 h of incubation rising to 79% after 72 h. chromID Elite showed a sensitivity of 79% after 20 h rising to 92% after 72 h ( P = 0.12). Fewer false positive colonies were encountered on chromID Elite (n = 68) compared with chromID ( n = 146). Conclusion chromID Elite is an effective medium for isolation of S. aureus with challenging clinical samples that may contain auxotrophic isolates of S. aureus (small colony variants) and a diversity of other species, including many with antibiotic resistance. It was notable that after 18 h incubation, chromID Elite showed equivalent performance to chromID that had been incubated for 72 h. A further advantage of chromID Elite was the fact that fewer false positive samples required investigation (P S. aureus from respiratory samples from patients with CF.