Antimicrobial susceptibility and serogroups of Salmonella isolates from Riyadh, Saudi Arabia

The antimicrobial susceptibility and serogroups of 153 Salmonella strains isolated during a period of 22 months from both children and adults at a major teaching hospital in Riyadh, Saudi Arabia were studied. Antimicrobial susceptibility testing by comparative disc method and MIC determination by E-test method were performed on selected antimicrobial agents. For nalidixic acid and trimethoprim only the comparative disc method was used. Discrepancy between the two methods were noticed only in 1.3% of isolates. The majority of isolates from children (41%) were serogroup B, while those from adults (43%) were serogroup C1. The overall resistance was 16% to ampicillin and ampicillin/sulbactam, 13% to nalidixic acid, and 11% to chloramphenicol and trimethoprim/sulphamethoxazole. The resistance of Salmonella isolates to the so-called first line anti-Salmonella agents, i.e. ampicillin, chloramphenicol and trimethoprim/sulphamethoxazole, has increased compared to that reported 4 years ago from this Institution. Almost all isolates were susceptible to the second, and third generation cephalosporins, fluoroquinolones, aztreonam, mecillinam and gentamicin. Multiple drug resistance to two or more drugs was noticed in 16% of isolates, most of which were serogroup B. The majority of these multiple drug resistant isolates (96%) were ampicillin resistant and β-lactamase producers. Although these isolates showed reduced MICs to ampicillin/sulbactam, their MICs were still higher than the susceptibility breakpoint for this combination. The nalidixic acid-resistant isolates showed higher MICs to the fluoroquinolones compared to the nalidixic acid-sensitive isolates. Isolates from children showed higher resistance to some of the antimicrobial agents compared to those from adults.