Characterization of antimicrobial resistance markers & their stability in Salmonella enterica serovar Typhi

Typhoid fever is a major cause of morbidity and mortality in the developing countries including India. Resistance to multiple antimicrobial agents is an emerging global problem that has serious impact on the treatment of disease. There are many factors associated with the emergence of resistance. Most important of them is the acquisition and further transmission and spread of resistance markers among various bacterial species. Therefore, we conducted this study to characterize the resistance plasmids in terms of their transferability and stability among Salmonella enterica serovar Typhi.

Six multidrug-resistant S. Typhi isolates were evaluated for the stability and transfer of resistance markers. The resistance plasmids were also checked for the presence of RepHI1A replicon.

All resistance markers were found to be transferred to the recipient through conjugation and transformation, except for nalidixic acid. None of the resistance plasmid was found to harbour RepHI1A replicon and therefore, did not belong to incompatibility group IncHI1. Resistance markers were found to be highly stable in all the isolates during serial passages and storage as stab cultures at different temperatures for different time periods.

Resistance markers for chloramphenicol, ampicillin, streptomycin and trimethoprim were transferred through conjugation as well as transformation whereas that for nalidixic acid was not transferred in any of the isolates. Markers for chloramphenicol and streptomycin resistance were found to be most stable during various storage conditions. Presence of small-sized non-IncHI1 resistance plasmids is a matter of concern due to their capability to exist inside the host, thereby increasing the possibility of their transmission and spread among S. Typhi and other bacterial species.