Direct antimicrobial susceptibility testing method for analysis of sputum collected from patients with cystic fibrosis.

Background: Chronic Pseudomonas aeruginosa colonisation and subsequent exacerbations in patients with cystic fibrosis (CF) require antimicrobial treatment. But since multiple morphotypes and other Gram-negative bacteria with different antibiotic susceptibilities are often isolated inside the same sputum sample, bacteriological analysis is difficult.

Methods: To simplify this analysis, we explored a direct sputum antimicrobial susceptibility testing (DSST) method by applying E test directly on plates inoculated with the sputum. A total of 316 samples collected from CF patients were analysed and compared with standard procedures (SP) for the identification and antimicrobial susceptibility testing of all Gram-negative bacterial species.

Results: DSST was as efficient as SP to detect P. aeruginosa including the mucoid morphotype in monomicrobial specimen, but was less sensible to detect all Gram-negative bacteria present in the same sample. It allowed the direct reading of the MIC inhibiting all Gram-negative bacteria. Agreements between these global MICs with the cumulative antibiotics susceptibility of all Gram-negative bacteria measured by SP were excellent for tobramycin and imipenem (>96%) and satisfactory for ticarcillin, ceftazidime, aztreonam and ciprofloxacin (90.4% to 94.3%). In conclusion, the DSST method is an efficient and easy antibiotic susceptibility testing method.