Harmonization of antibiotic susceptibility testing for Salmonella: Resultsof a study by 18 national reference laboratories within the European Union-funded enter-net group

For the effective surveillance of antimicrobial drug resistance within Salm onella organisms from humans, harmonization of methods used for sensitivity testing by laboratories responsible for the typing of such organisms is es sential. A study of resistance or sensitivity to a panel of 11 antimicrobia ls by the Enter-net international surveillance network was therefore undert aken. There are 18 national Salmonella reference laboratories within this E uropean Union-funded network. Forty-eight strains of Salmonella enterica we re distributed to each laboratory for testing for resistance or sensitivity to ampicillin, cefotaxime, chloramphenicol, gentamicin, kanamycin, strepto mycin, sulfonamides, tetracyclines, trimethoprim, nalidixic acid, and cipro floxacin. Over 8,500 tests were assessed involving disk diffusion (DD), aga r breakpoint (BP), or full minimum inhibitory concentration (MIC). Results indicated that whichever method was used, there was a high degree of concor dance for the detection of resistance to most antimicrobials; only for decr eased sensitivity to ciprofloxacin was there substantial nonconcordance. Be cause all isolates with decreased sensitivity to ciprofloxacin were resista nt to nalidixic acid, it is suggested that, if required, MICs to ciprofloxa cin could be determined for isolates resistant to nalidixic acid. For the d etection of sensitivity, the main area of nonconcordance was in the detecti on of sensitivity to streptomycin. With the exception of decreased sensitiv ity to ciprofloxacin, we are confident that a database of antimicrobial sus ceptibilities can now be established and harmonized antibiogram data for Sa lmonella can now be exchanged for national Salmonella reference laboratorie s within the European Union.