Increasing minimum inhibitory concentration of cefazolin for Salmonella typhi

Cefazolin resistance was observed in the disc diffusion method of antibiotic susceptibility testing of Salmonella typhi strains isolated from January to June 1999. Eighty six multi drug resistant (MDR) isolates were studied. Of these, 46 (53.5%) were found resistant to cefazolin by the disc diffusion test. The minimum inhibitory concentration (MIC) of cefazolin was determined for the 86 isolates of S. typhi by the agar dilution method. MIC of these strains ranged from 1.0 to 64.0 micrograms/ml. In comparison, 25 MDR S. typhi strains isolated between 1994-1998 (5 per year) also tested by the same method, gave MIC values of 0.5 to 4.0 micrograms/ml. Our findings indicate a definite upward shift in MIC of cefazolin to MDR S. typhi since 1999. This needs to be monitored since the second, third and fourth generation cephalosporins are therapeutic options for typhoid fever.