In-vitro Evaluation of Antibiotic Combination Effect between Gentamicin and Penicillin G against Methicillin Resistant Staphylococcus aureus (MRSA) by Checkerboard Technique

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Ezeobiora C.E
Okore V.C
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Methicilin–resistant Staphylococcus aureus (MRSA) is a common cause of serious nosocomial infections with limited options for treatment due to its known resistance to many antibiotics. Knowledge guided antibiotic combination therapy can be of great benefit in fighting this superbug. The aim of this study was to evaluate the combined activity of penicillin G (A) and gentamicin (B) against clinical isolates of methicillin resistant Staphylococcus aureus. Ten clinical isolates of MRSA were labeled as S15, S31, S41, S42, S44, S46, S48, S50, S55 and S57. They were confirmed through microscopically examination and non susceptibility to oxacillin disk. The antibiotic combination effect of penicillin G and gentamicin was evaluated by checkerboard technique. The minimum inhibitory concentration (MIC) of each antibiotic was determined by broth dilution technique. The interaction between the two antibiotics in combination was evaluated algebraically by calculating their fractional inhibitory concentration (FIC) index according to the relationship: FIC index= FICA + FICB. The MIC values of penicillin G (A) and gentamicin (B) for the ten strains were at range of 5μg/ml - 40μg/ml and 1μg/ml - 8μg/ml respectively. The antibiotic combination effect showed synergism in most combination ratios except S44, S46, S48 and S57 where penicillin G: gentamicin (9:1) ratio showed additivity. More so, S42 showed additivity in three combination ratios of 9:1, 8:2 and 1:9. As the ratio of penicillin G to gentamicin increased, synergism dropped to additivity in those combinations where synergism was not recorded. In Conclusion, the combination of penicillin G and gentamicin has a clinical significance for use in antimicrobial therapy involving the disease states caused by methicillin resistant Staphylococcus aureus. The use of such combination must be with greater ratio of gentamicin than penicillin G.
This study supports the clinical use of penicillin G and gentamicin in antibiotic combination therapy against infections caused by methicillin-resistant Staphylococcus aureus. The combination may probably offer solution to the rapid emergence of resistance plaguing the use of these agents singly.
Penicillin G , Gentamicin , Methicillin resistant Staphylococcus aureus , Checkerboard technique
• Ezeobiora CE, Okore VC (2020). In-vitro Evaluation of Antibiotic combination effect between Gentamicin and Penicillin G Against Methicillin Resistant Staphylococcus aureus (MRSA) by Checker-board Technique. International Journal of Current Microbiology and Applied Sciences. 9(6):3576-3585