Sensitivity index of antimicrobial agents as a simple solution for multidrug resistance in Salmonella Typhi

Multi drug-resistant (MDR) typhoid in India is an escalating problem. MDR isolates of Salmonella Typhi are on rise and are becoming a challenge for timely and appropriate treatment. Occurrence of per cent sensitive (%S), per cent resistant (%R) and per cent intermediate (%I) isolates may vary geographically and treatment decided on the basis of only one of these three subpopulations may lead to selection of inappropriate drug for treatment and thus treatment failure. Determination of sensitivity index (SI) of antimicrobial agents, instead of %S or %R subpopulations, may give clearer insight regarding selection of appropriate antimicrobial for treatment of typhoid. In present work, the data of sensitivity testing were analysed and interpreted both in terms of SI as well as %S, %I and %R.

A total of 205 isolates of Salmonella Typhi were collected during June 2000 and August 2002 from a network of five institutes- Lady Hardinge Medical College (LHMC, N=110), Ram Manohar Lohia Hospital (RML, N=14), Majeedia Hospital (MH, N=48), Lal's Pathology Lab (LAL, N=28) and All India Institute of Medical Sciences (AIIMS, N=5) on nutrient agar slopes. Of these, 142 isolates were subjected to phage typing and biotyping at National Salmonella Phage Typing Centre, New Delhi. Five isolates resistant to 3-7 and one isolate susceptible to all of total 12 antimicrobial tested were subjected to plasmid analysis. SI for various antimicrobials was determined as the ratio of %S and %R values derived form %RIS analysis using WHONET5.

18 (8.7%) isolates were susceptible to all tested antimicrobials and 124 (60%) were MDR. Of the 142 isolates, 103 were phage type E1 and biotype I. SI of antimicrobials rather than individual %S or %R or %I population presents a better criterion for interpretation of sensitivity testing data as well as selection of the most appropriate antimicrobial for timely treatment. Presence of 140, 48 and 23 Kb size plasmids in all 5 MDR isolates and none in susceptible isolate was observed.

Re-emergence of chloramphenicol sensitivity in Salmonella typhi was observed in the present study. Interpretation in terms of SI criteria warrants that reintroduction of chloramphenicol at present for treatment of typhoid may rebound resistance. Current empiric therapy used for treatment of typhoid may soon become ineffective. SI being a ratio will not only eliminate geographical variation of %RIS data but also its interpretation. SI can provide guidelines for clinicians in remote areas where facilities for sensitivity testing are not available.