Evaluation of Different Agar Media for the Antibiotic Susceptibility Testing of Some Selected Bacterial Pathogens

Igbinosa EO(1), Ogofure AG(2), Beshiru A(3),


(1) Applied Microbial Processes & Environmental Health Research Group, Faculty of Life Sciences, University of Benin, Private Mail Bag 1154 Ugbowo Campus, Benin City 300283, Edo State, Nigeria
(2) Applied Microbial Processes & Environmental Health Research Group, Faculty of Life Sciences, University of Benin, Private Mail Bag 1154 Ugbowo Campus, Benin City 300283, Edo State, Nigeria.
(3) Applied Microbial Processes & Environmental Health Research Group, Faculty of Life Sciences, University of Benin, Private Mail Bag 1154 Ugbowo Campus, Benin City 300283, Edo State, Nigeria. Department of Microbiology, College of Natural and Applied Sciences, Western Delta University, Private Mail Bag 10 Oghara, Delta State, Nigeria.
Corresponding Author

Abstract


Background: The validity of antimicrobial susceptibility testing (AST) depends on rigid standardization of every feature of the test, including but not limited to the composition of the medium used. This is necessary to obtain efficient results for proper evaluation of the patient’s treatment regime.

Objectives: This study was conducted to evaluate the AST of bacterial isolates on different media such as Mueller-Hinton agar (MHA), nutrient agar (NA), Columbia blood agar base (CBA), tryptone soya agar (TSA) and a combination of peptone water and bacteriological agar (PWA).

Methods: Three independent biological replicates of bacterial isolates were standardized (0.5 McFarland’s standard) and streaked on the plate before the introduction of antibiotics (Kirby-Bauer disc diffusion technique).

Results: Results revealed that for meropenem, a mean inhibition zone of 12.50±0.50 mm was observed for S. aureus on MHA, while the mean zones of inhibition for drug/pathogen combination in other media were 11.00±0.00 mm (NA), 11.00±0.00 mm (CBA), 9.50±0.50 mm (TSA) and 13.50±0.50 mm (PWA) respectively. There were obvious differences in the zones of inhibition observed for interaction of Salmonella and ciprofloxacin, as 36.00±0.00 mm zone was observed on MHA while other media had inhibition zones of 34.00±1.15 mm (NA), 34.00±1.15 mm (CBA), 34.50±0.95 mm (TSA) and 32.25±0.75 mm (PWA) respectively. All media had similar interpretation for all antibiotics, as well as similar cumulative antibiogram and MAR index in the study. It was clear that a strong positive relationship (relatedness) [0.98] was observed for the susceptibility of Salmonella enterica on NA and MHA. More so, it was obvious that 97% of the values for NA fit the regression analysis model for MHA. Therefore, this study implies that 97% of the values obtained for NA are similar to the values obtained for MHA with a standard error of 2.08. The probability value (p-value) at 95% confidence shows that there is no statistical difference in the zones of inhibition on NA and MHA for Salmonella enterica.

Conclusion: There were a strong positive relationship between other media and the gold standard considering the coefficient of determination [R2] for CBA [0.99], TSA [0.97] and PWA [0.94] with reference to MHA used as the gold standard.


Keywords


Antibiotic susceptibility testing, Mueller Hinton agar, Nutrient agar, Columbia agar blood base, tryptone soya agar.

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