Effects and time-kill assessment of amoxicillin used in combination with chloramphenicol against bacteria of clinical importance

With the emergence of multidrug-resistant organisms in an era when drug development faces challenges causing pharmaceutical companies to curtail or abandon research on anti-infective agents, the use of combined existing antimicrobial agents may be an alternative. This study evaluated the effects of combining amoxicillin and chloramphenicol, to which many bacteria have become resistant, in vitro against Gram positive and Gram negative bacteria by agar diffusion, checkerboard and time-kill assays. The test isolates were susceptible to amoxicillin with minimum inhibitory concentrations (MICs) ranging between 0.448 and 500 µg/ml and between 1.953 and 31.25 µg/ml for chloramphenicol. Upon combining these agents, there was a drastic reduction in their MICs indicating an increased antibacterial activity that showed synergistic interaction against all the bacteria. At the highest concentrations, the inhibition zones ranges were 20.33-38.33±0.58 µg/ml for amoxicillin, 27.67-37.67±0.58 µg/ml for chloramphenicol and 31.67-39.33±0.58 µg/ml for the combined agents. The fractional inhibitory concentration indices (FICIs) showed synergy ranging from 0.129 to 0.312 while FICIs for additive interaction were between 0.688 and 1.0. There was no antagonistic interaction. At the 1 / 2 MICs of the combined antibiotics, all the tested bacteria, except for Klebsiella pneumoniae ATCC 4352, Proteus vulgaris CSIR 0030 and Enterococcus cloacae ATCC 13047 were eliminated before 24 h. At the MICs, all the tested bacteria were eliminated except Enterococcus cloacae ATCC 13047 which was almost totally eliminated. Post-antibiotic assessment after 48 h showed that all the cultures were sterile except for that of Enterococcus cloacae ATCC 13047. The lack of antagonism between these antibacterial agents in checkerboard and time-kill assays suggested that combining amoxicillin with chloramphenicol can provide an improved therapy in comparison to the use of each antibiotic individually. The study indicates the potential beneficial value of combining amoxicillin and chloramphenicol in the treatment of microbial infections in clinical settings.