Increase in incidence of resistance to ampicillin, chloramphenicol and trimethoprim in clinical isolates of Salmonella serotype Typhimurium with investigation of molecular epidemiology and mechanisms of resistance

Antimicrobial resistance patterns of Salmonella serotype Typhimurium isolat es obtained during the period 1987-1994 were examined and the molecular epi demiology and the mechanisms of resistance to ampicillin, chloramphenicol a nd trimethoprim were investigated in 24 strains isolated during 1994. Resis tance to ampicillin increased from 18% to 78%, to chloramphenicol from 15% to 78%, to tetracycline from 53% to 89% and to co-trimoxazole from 3% to 37 %, whereas resistance to norfloxacin remained at 0%. Of Salmonella serotype Typhimurium strains isolated during 1994, all ampicillin-resistant strains had an MIC > 256 mg/L, except one strain in which the MIC was 64 mg/L. Twe lve strains (52%) had a TEM-type beta-lactamase, nine (39%) a CARE-type bet a-lactamase and two strains (8%) had an OXA-type beta-lactamase, Chloramphe nicol acetyl-transferase activity was detected in only nine (47%) of 19 chl oramphenicol resistant strains, whereas all eight trimethoprim-resistant st rains produced a dihydrofolate reductase type Ia enzyme. Three different ep idemiological groups were defined by either low-frequency restriction analy sis of chromosomal DNA and pulsed-held gel electrophoresis or repetitive ex tragenic palindromic-PCR. The latter technique provided an alternative, rap id and powerful genotyping method for S. Typhimurium. Although quinolones p rovide a good therapeutic alternative, the multiresistance of S. Typhimuriu m is of public health concern and it is important to continue surveillance of resistance levels and their mechanisms.