Comparison of the Etest and the routine multi-disc agar diffusion susceptibility of Staphylococcus species

Abstract

Aims: The present study, tend to evaluate the validity and accuracy of Etest
as a method for performing in-vitro antimicrobial susceptibility testing of Staphylococcus with comparison to the routine multi disc agar diffusion. This is because the Etest susceptibility method is not yet known as a rapid, simple reliable technique in developing countries as it combine the functions of both dilution and diffusion technique. Materials and methods: Ninety-seven Staphylococcus aureus and eightythree Staphylococcus epidermidis isolates were obtained from wound samples and identified according to standard morphological and biochemical methods.
The antibiotics susceptibility patterns were determined both by agar disc diffusion and Etest
methods in accordance to NCCLS (1997) criteria and manufacturer (AB Biodisk Sweden)
respectively. Results: On the Etest strips, Staph aureus was 83.5% sensitive to ciprofloxacin, 52.6% to gentamicin, 48.5% to ampicillin and 8.2% to chloramphenicol while on the multi-disc agar diffusion plates 80.4% of Staph aureus were sensitive to ciprofloxacin, 49.5% to gentamicin, 39.2% to ampicillin and 12.4% to chloramphenicol.. On the Etest strips, 80.7% of Staph epidermidis were sensitive to ciprofloxacin, 34.9% to gentamicin, 25.3% to ampicillin and 15.7% to chloramphenicol while on the multi- disc agar diffusion plates 89.2% of Staph epidermidis were sensitive to ciprofloxacin, 34.9% to gentamicin, 25.3% to ampicillin and 32.5% to chloramphenicol. Conclusion: The sensitivity patterns between the two methods were essentially similar, however, the Etest method clearly demonstrated intermediate sensitivities which to an extent were absent in
routine multi-disc agar diffusion method. Most of the isolates Etest MICs clustered around the sensitive and resistance break points. Etest also demonstrated the MIC and diffusion results on the same strips.

Keywords: antibiotic resistance, antimicrobial, gram-positive, chemotherapy.

Sudan Journal of Medical Sciences Vol. 3 (2) 2008: pp.121-126