Antimicrobial susceptibility of salmonella typhi and staphylococcus aureus isolates and the effect of some media on susceptibility testing results

Abstract

Aim: (i) to determine the antibiogram of Salmonella typhi and Staphylococcus aureus isolates. (ii) to quantify the effect of Nutrient and Tryptone Soy agars on susceptibility testing results. Methodology: The Kirby Bauer method was used to evaluate the susceptibility of 30 isolates each of Salmonella typhi and Staphylococcus aureus to various antimicrobial agents on Mueller-Hinton agar (recommended medium). Subsequently, the susceptibility testing procedure was repeated on the isolates using Nutrient and Tryptone Soy agars which are commonly used in Ghana but not recommended for the Kirby Bauer method. Results: The prevalence of multiple drug resistance as determined on Mueller-Hinton agar was 83.3% for Salmonella typhi and 80% for Staphylococcus aureus. For Salmonella typhi, resistance ranged from 6.7% (gentamicin and amikacin) to 83.3% (cotrimoxazole, ampicillin and chloramphenicol). In the case of Staphylococcus aureus resistance ranged from 16.7% (erythromycin and gentamicin) to 93.3% (penicillin). Overall, the discrepancy in susceptibility results between Nutrient agar and that of Mueller-Hinton agar was 8.9%, while discrepancy between Tryptone Soy agar and Mueller-Hinton agar, was 17.2%. Conclusion: The high resistance rates observed for the organisms to some of the drugs underscore the need for susceptibility testing. However, the use of Nutrient and Tryptone Soy agars for the Kirby Bauer method as practiced by some laboratories in Ghana is discouraged.