The Effect of Using Combinations of Commonly Used Antibiotics in Sudan on Resistant Gram Negative Bacterial Isolates

Abstract

Background: Multidrug-resistant Gram-negative bacteria pose a problem that infection control practitioners, hospital epidemiologists, clinicians, and hospital administrators are struggling to control. Failure of treatments (or failure of medical management of infections) with single antibiotics which are commonly used in Sudan is associated with dramatic increases in the Minimum Inhibitory Concentration (MIC). Achieving effective combinations of the commonly used antibiotics may kill resistant mutants, potentiating successful therapy. Theoretically and indeed in practice the Minimum Inhibitory Concentrations (MICs) of antibiotics can be reduced if combined with a second antibiotic acting synergistically or additively. Methods: Ten clinical drug resistant Gram negative bacterial isolates were selected for this study. They belong to four genera namely, Escherichia, Klebsiella, Pseudomonas and Enterobacter. A sensitive strain of Pasteurella multocida was selected for comparison. The eleven isolates were identified using normal biochemical reactions and were confirmed using API 20E kits. The MIC’s for the isolates were determined for nine antibiotics alone (singly), and in the presence of a second one. They were amoxicillin, ampicillin, cephalexin, ciprofloxacin, cloxacillin, erythromycin, sulphamethoxazole, tetracycline and trimethoprim. Different concentrations of MIC, 0.5 X MIC, 1 X MIC and 2 X MIC along time was used to determine the effect of concentration and time on the isolates. The combinations studied were:(i)Ampicillin with ciprofloxacin, (ii)ampicillin with amoxicillin, (iii)ampicillin with cloxacillin, (iv)sulphamethaxazole with trimethoprim, (v)cephalexin with ciprofloxacin, (vi)cephalexin with amoxicillin, (vii)tetracycline with erythromycin, (viii)tetracycline with amoxicillin, (ix)tetracycline with sulphamethoxazole, (x)tetracycline with ciprofloxacin and (xi)tetracycline with trimethoprim. Results: Eight strains were found to have high MICs. Pseudomonas aeruginosa and Pasteurella multocida were sensitive to ciprofloxacin while the latter was sensitive to cloxacillin and erythromycin as well. Using different concentrations of MIC, 0.5 X MIC, 1 X MIC and 2 X MIC along time indicated that resistance to ciprofloxacin might not be a frequent occurrence if adequate multiples of the MIC for the infecting organism were obtained in serum and tissue. However, using sub-inhibitory concentrations of ciprofloxacin against Klebsiella sp. and Pseudomonas aeruginosa may lead to emergence of resistant strains. Trimethoprim along time has a bacteriostatic effect on strains of E. coli, Klebsiella pneumoniae and Enterobacter. Tetracycline had bacteriostatic effect at low concentrations and bacteriocidal effect at high concentrations against some strains of E. coli and Pasteurella multocida, while erythromycin was always bacteriostatic against Klebsiella pneumoniae and did not change to bacteriocidal at high concentrations. For some strains, the MICs of certain antibiotics were successfully reduced in the presence of another antibiotic. Eleven combinations were studied. As expected, some antibiotics caused no reduction in the MIC, as they were not active against the organisms. The combination of erythromycin and tetracycline was synergistic and had bacteriocidal effect against some strains of E. coli, Enterobacter, Pseudomonas aeruginosa and Pasteurella multocida. A combination of tetracycline and ciprofloxacin was synergistic and has bacteriostatic effect on Klebsiella pneumoniae and Enterobacter. Using ampicillin in combination with ciprofloxacin against Klebsiella pneumoniae and Pseudomonas aeruginosa had synergistic as well as bacteriocidal effect on both isolates. Using cephalexin in combination with ciprofloxacin against Klebsiella pneumoniae had synergistic and bacteriocidal effect. Sulphamethoxazole with trimethoprim had synergistic and bacteriostatic effect on Pseudomonas aeruginosa. A combination of the two penicillins ampicillin and amoxicillin showed bacteriocidal and synergistic effects against Klebsiella pneumoniae along time. Tetracycline and trimethoprim combination was antagonistic against Klebsiella pneumoniae and Pseudomonas aeruginosa. Conclusion: It is recommended that every combination of antibiotics should be tested in vitro with regard to its action characteristics, to avoid undesirable effects of combinations. Practical clinical experiences are needed to decide on possible clinical benefits of using any two antibiotics in combination. Requests for testing combinations could, therefore, be directed at excluding antagonistic effects between two antimicrobial agents.