Antimicrobial susceptibility of non-fermenting Gram-negative pathogens isolated from cystic fibrosis patients

Non-fermenting Gram negative bacteria are increasingly being cultured in respiratory samples from people with cystic fibrosis (CF). This study aimed to determine the susceptibility of clinical CF respiratory isolates from distinct geographical regions to a range of antimicrobials. A total of 286 isolates (106 Stenotrophomonas maltophilia, 100 Burkholderia spp., 59 Achromobacter spp., 12 Pandoraea spp. and 9 Ralstonia spp.) from The Netherlands, Northern Ireland, Spain, USA and Australia were tested. The MIC50, MIC90 and susceptibility categorization were determined. Cotrimoxazole was the most active compound for all the microorganisms (MIC50=0.12-4 mg/L, MIC90=1-16 mg/L). For S. maltophilia, 47% and 62% of isolates were susceptible to cotrimoxazole according to CLSI and EUCAST breakpoints, respectively. Ceftazidime presented a lower level of susceptibility (35%, MIC50=32 mg/L, MIC90=256 mg/L). Tobramycin and colistin MIC90 were >128 mg/L and >16 mg/L, respectively. Regarding Burkholderia isolates, 72%, 56% and 44% were susceptible to cotrimoxazole, ceftazidime and meropenem, respectively. For both ceftazidime and meropenem, the MIC50 and MIC90 values were within the intermediate or resistant category. The most active antibiotics for A. xylosoxidans were cotrimoxazole (MIC50=2, MIC90=8 mg/L) and imipenem (MIC50=2, MIC90=8 mg/L). Cotrimoxazole, imipenem, and ciprofloxacin were active against the 12 Pandoraea species (MIC50= 0.12-4 mg/L, MIC90= 1-8 mg/L). Ciprofloxacin (MIC50=4 mg/L) and cotrimoxazole (MIC50=1 mg/L) were the only active antibiotics for Ralstonia spp. There were no statistically significant differences in susceptibility rates between countries. Non-fermenting Gram-negative bacteria other than P. aeruginosa are potential pathogens in CF and cotrimoxazole has demonstrated to be the most active compound against them.