Molecular Characterization Of Chromosomal And Extrachromosomal Elements Related To Drug Resistance Against Salmonella Enterica Serovar Typhi Isolates From Pakistan

Salmonella enterica serovar Typhi the causative agent of typhoid, the 4th largest killer disease in Pakistan, is becoming a major threat due to emerging antimicrobial resistance.Mechanisms underlying bacterial drug resistance involve developing and/or acquiring of genetic elements in the form of variety of genes, gene complexes or mutations.These genetic elements are either chromosomal and/or extrachromosomal in the form of plasmids, transposons, gene cassettes and genomic islands.Understanding the genetic mechanism(s) responsible for acquisition and spread of antimicrobial drug resistance could facilitate development of effective prevention and control strategies.The present study was conducted to investigate the pattern of drug resistance and possible genetic elements involved in this phenomenon in the local isolates of S.Typhi obtained from NIBGE stock culture.A total of 46 isolates were tested for antimicrobial susceptibility by disk diffusion and were subjected to minimum inhibitory concentration (MIC) determination of third generation cephalosporin and fluoroquinolones by the E-test method.The results showed that 58.7% of S. Typhi isolates were multi-drug resistant (MDR), a phenotype of bacteria showing resistance against at least three different groups of drugs. The most common resistance (R) type was AmChStrTeSxtSmzTmp that is frequently observed in isolates from India and Pakistan. MIC determination against the current therapeutic options revealed ceftriaxone, gatifloxacin and ofloxacin to be effective for the treatment of MDR typhoid fever.The molecular mechanism of resistance among the S. Typhi isolates was evaluated by identifying drug resistance related genes, finding mutation in drug targeted genes as well as detecting the occurrence and cassette content of integrons.There was good correlation between the presences of resistance genes and corresponding resistance phenotypes with sul2 the most commonly detected gene among our isolates. Only class 1 integrons associated with sul1 and having a single gene cassette conferring resistance to trimethoprim were identified.Reduced susceptibility to fluoroquinolone ciprofloxacin, which is a forefront drug for treatment of MDR typhoid, was found in 4 isolates. It was associated with point mutation in GyrA subunit of DNA gyrase.However, no resistance was detected towards ofloxacin and gatifloxacin. Among 3rd generation cephalosporins ceftriaxone was found to be effective against all isolates although low level resistance was detected against other drugs of this generation.This study provides a significant baseline profile of antimicrobial drug resistance of local S.Typhi isolates and associated molecular element