MOLECULAR ANALYSIS OF AND IDENTIFICATION OF ANTIBIOTIC-RESISTANCE GENES IN CLINICAL ISOLATES OF SALMONELLA-TYPHI FROM INDIA

A representative sample of 21 Salmonella typhi strains isolated from c ultures of blood from patients at the Christian Medical College and Ho spital, Vellore, India, were tested for their susceptibilities to vari ous antimicrobial agents. Eleven of the S. typhi strains possessed res istance to chloramphenicol (256 mg/liter), trimethoprim (64 mg/liter), and amoxicillin (>128 mg/liter), while four of the isolates were resi stant to each of these agents except for amoxicillin. Six of the isola tes were completely sensitive to all of the antimicrobial agents teste d. All the S. typhi isolates were susceptible to cephalosporin agents, gentamicin, amoxicillin plus clavulanic acid, and imipenem, The antib iotic resistance determinants in each S. typhi isolate were encoded by one of four plasmid types, Plasmid-mediated antibiotic resistance gen es were identified with specific probes in hybridization experiments; the genes responsible for chloramphenicol, trimethoprim, and ampicilli n resistance were chloramphenicol acetyltransferase type I, dihydrofol ate reductase type VII, and TEM-1 beta-lactamase, respectively. Pulsed -field gel electrophoresis analysis of XbaI-generated genomic restrict ion fragments identified a single distinct profile (18 DNA fragments) for all of the resistant isolates, In comparison, six profiles, differ ent from each other and from the resistance profile, were recognized a mong the sensitive isolates, It appears that a single strain containin g a plasmid conferring multidrug-resistance has emerged within the S. typhi bacterial population in Vellore and has been able to adapt to an d survive the challenge of antibiotics as they are introduced into cli nical medicine.