A Strategic Combat Against Antimicrobial Resistance: A Cross-sectional Evaluation of Levofloxacin’s Efficacy against Multidrug-resistant Bacterial Isolates in Nigeria

Background: Key mechanisms of antimicrobial resistance (AMR) include the presence of efflux pumps and the production of drug-degrading enzymes upon exposure to antimicrobial drugs. Levofloxacin, a widely prescribed fluoroquinolone, has marked antimicrobial activity against Gram-positive pathogens like Streptococcus pneumoniae and Gram-negative pathogens like Pseudomonas aeruginosa . However, additional multidrug resistance (MDR) mechanisms in microbes, such as mutations in DNA gyrase and topoisomerase IV or altered efflux pump activity (EPA), threaten its clinical efficacy. To the best of our knowledge, no study has reported the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values of levofloxacin against genetically validated MDR isolates in Nigeria. Methods: A cross-sectional study assessing levofloxacin’s potency, MIC and MBC was conducted against 20 validated bacterial isolates with resistance mechanisms such as efflux pump activity, oxacillin resistance, ESBL, and carbapenemase production, following CLSI guidelines. The American Type Culture Collection (ATCC 25922) Escherichia coli reference isolate was used as the standard drug-susceptible control and all assays were performed in triplicate to ensure reproducibility and minimise observer bias. All data were expressed as mean ± standard deviation (S.D.). Significant differences among the groups were determined by one-way analysis of variance (ANOVA), and Graphs were plotted using Microsoft Excel. Results were considered to be significant at p  ≤ 0.05. Results: Three MDR Klebsiella pneumoniae strains exhibited non-susceptibility and intermediate resistance to levofloxacin. MICs ranged from 0.0625–8.00 µg/ml for Gram-positive and 0.015625–1.00 µg/ml for Gram-negative organisms. Levofloxacin demonstrated greater inhibitory activity against Gram-negative (72.73% susceptible) than Gram-positive (55.56% susceptible) organisms. MBCs ranged from 0.125–8.00 µg/ml (Gram-positive) and 0.015625–8.00 µg/ml (Gram-negative), with Micrococcus spp. showing marked susceptibility (IZD 50 µg/ml: 37.33 mm ± 0.58; MIC: 0.125 µg/ml). Conclusion: Our study findings underscore the need for continuous surveillance, molecular characterisation of resistance mechanisms, and MIC-guided antimicrobial stewardship. While levofloxacin remains effective for many infections, emerging resistance among previously susceptible organisms highlights the urgent need for informed prescribing practices and the development of novel antimicrobials to combat MDR.