Columbia 5 sheep red blood agar plates

S. aureus clinical isolates were isolated from clinical specimen referred to the routine medical microbiology laboratory of the Department of Infectious Diseases, University Hospital Heidelberg, Germany. Clinical isolates were numbered according to their origin, e.g. isolates cultured from patients with invasive diseases were categorized as “INV” and those obtained from respiratory specimen of cystic fibrosis patients as “CF”. Storage of the bacterial strains at −80°C was performed in skim milk (Becton Dickinson, Heidelberg, Germany) or with cryobeads (Cryobank™, Mast Diagnostica, Reinfeld, Germany). Reference and mutant S. aureus strains are listed in Table 1. For each experiment, bacteria were prepared, thawed and cultured freshly on Columbia/5% sheep red blood agar plates (Becton Dickinson, Heidelberg, Germany), Mueller Hinton agar plates (BioMerieux, Nuertingen, Germany), or CASO-bouillon (Becton Dickinson, Heidelberg, Germany) and incubated at 37°C overnight.

All samples were stored at 4°C during transport to the laboratory. All specimen were inoculated within 48 hours. All swabs were streaked on Columbia / 5% sheep red blood agar plates (Becton Dickinson, Heidelberg, Germany) and selective agar plates, i.e. CHROMagarMRSA (MAST Diagnostica GmbH) for nasal swabs and CHROMagarESBL (MAST Diagnostica GmbH) for intrarectal swabs and feces. Plates were incubated at 37 ± 1°C for 24 h. Incubation of air sample plates was started on-site. Sealed plates were incubated for 48 h at 37 ± 1°C. The colonies were counted as total number of CFU/m³ and reported after statistical correction with the species-specific correction factor (Pr / r) according to Feller [38 ].