Background: Over a 19-month pilot phase, 93 multiply resistant Gram-negative isolates from Scottish cystic fibrosis patients were sent to a referral laboratory for further investigation.
Methods: In common with the referring diagnostic laboratories, disc diffusion testing was carried out. Antibiotic susceptibility testing was also established by MIC methodology. NCCLS methods were used throughout. Twenty antibiotics were tested.
Results: Comparing disc diffusion results against MIC results, there were 167 (14%) major errors. By MIC, Pseudomonas aeruginosa (n = 59), Stenotrophomonas maltophilia (n = 16), Burkholderia cepacia (n = 10) and Alcaligenes xylosoxidans (n = 7) were susceptible to 18%, 11%, 4% and 35% of the antibiotics tested, respectively. Colistin and tobramycin were the most active agents against P. aeruginosa with 60% and 49%, respectively, testing susceptible. Minocycline and gentamicin were most active against S. maltophilia with 58% and 18%, respectively, testing susceptible. B. cepacia were most susceptible to co-trimoxazole (10%) and ciprofloxacin (10%). Five and six of the seven A. xylosoxidans isolates were susceptible to piperacillin and imipenem, respectively.
Conclusions: Improved methods for susceptibility testing of such clinical isolates need to be employed in routine diagnostic laboratories. Levels of resistance in referred isolates were very high and similar to those described in the USA.