ANALYSIS OF THE MECHANISM OF QUINOLONE RESISTANCE IN NALIDIXIC ACID-RESISTANT CLINICAL ISOLATES OF SALMONELLA SEROTYPE TYPHIMURIUM

Over a period of 2.5 years, 42 cases of gastro-enteritis caused by nal idixic acid-resistant Salmonella serotype Typhimurium occurred in Mala ga. The epidemiological relationship among the strains involved was in vestigated by analysis of plasmid profile and of chromosomal DNA by pu lsed-field gel electrophoresis (PFGE), Despite having different plasmi d profiles, all 42 nalidixic-acid resistant Typhimurium isolates had e volved from one clone as shown by analysis of chromosomal DNA by PFGE, The mechanism of quinolone resistance in these Typhimurium isolates w as also investigated, Analysis of outer-membrane proteins and lipopoly saccharide from quinolone-susceptible and -resistant clinical isolates tested showed no differences, All nalidixic acid-resistant isolates h ad MICs for ciprofloxacin of 0.25 mg/L and for nalidixic acid of 1024 mg/L, Polymerase chain reaction fragments of 285 bp, containing the qu inolone resistance-determining region of the gyrA gene, and of 237 bp, containing the region of parC homologous to the quinolone resistance- determining region of the gyrA gene, were sequenced, All resistant iso lates presented a change at Ser-83 to Phe in the GyrA protein, but no changes were observed in the ParC protein, These findings indicated th at this mutation in gyrA plays a major role in the acquisition of nali dixic-acid resistance in clinical isolates of Typhimurium.