In order to assess antimicrobial resistance in nontyphoid human Salmonella in Belgium, the six most important serovars, representing together more than 90% of laboratory confirmed cases, were randomly sampled. From June 2000 until December 2002, a total of 1756 isolates were screened for their antimicrobial resistance profile by the disc diffusion method. S. Hadar strains showed the highest level of antimicrobial resistance. Simultaneous resistance to ampicillin, nalidixic acid, tetracycline and streptomycin was observed in 81.5, 58 and 76.1% of these isolates in 2000, 2001 and 2002, respectively. All S. Hadar isolates resistant to nalidixic acid also displayed decreased susceptibility to ciprofloxacin (MIC50 values of 0.25 microg/mL in 2000-2001 and 0.19 microg/mL in 2002). In 2000, 2001 and 2002, respectively 44.6, 46 and 36.5% of S. Typhimurium isolates were multiresistant (resistant to 4 or more antimicrobial agents). These multiresistant isolates were preferably associated with a few phage types, such as DT104. Complete resistance to ciprofloxacin was detected in three S. Typhimurium isolates and sequencing of the gyrA gene revealed for each isolate two mutations at codons corresponding to Ser-83 and Asp-87. Multiresistance was also common in S. Virchow (7.7%, 15.9% and 29.7%, in 2000, 2001 and 2002, respectively). Resistance to nalidixic acid in S. Virchow isolates increased from 46.2% in 2000 to 80.9% in 2002 and six S. Virchow isolates were detected as cefotaxime resistant. In contrast, the vast majority of S. Enteritidis, S. Brandenburg and S. Derby isolates remained sensitive to almost all antimicrobial agents tested
