Quinoline Antimalarials Increase the Antibacterial Activity of Ampicillin

Abstract

Bacterial and malaria co-infections are common in malaria endemic countries and thus necessitate co-administration of antibiotics and antimalarials. There have long been anecdotal clinical reports of interactions between penicillins and antimalarial agents, but the nature and mechanisms of these interactions remain to be investigated. In this study, we employed antimicrobial interaction testing methods to study the effect of two antimalarials on the antibacterial activity of ampicillin in vitro. Paper strip diffusion, a modified disc diffusion and checkerboard methods were used to determine the nature of interactions between ampicillin and quinoline antimalarials, chloroquine and quinine, against Gram-positive and Gram-negative bacteria. The impact of antimalarials and ampicillin-antimalarial drug combinations on cell integrity of test bacteria were determined by measuring potassium release. The tested antimalarials did not show substantial antibacterial activity but quinine was bactericidal at high concentrations. Chloroquine and quinine increased ampicillin activity, with increasing concentrations extending the antibacterial's inhibition zones by 2.7-4.4 mm and from 1.1 to over 60 mm, respectively. Observed interactions were largely additive with Fractional Inhibitory Concentration Indices of >0.5-1 for all ampicillin-antimalarial combinations. Quinine and, to a lesser extent, chloroquine increase the activity of ampicillin and potentially other β-lactams, which has implications for combined clinical use.

Keywords: ampicillin; chloroquine; drug combination; modified disc diffusion checkerboard; paper strip diffusion; penicillins; quinine.

FIGURE 1

Paper strip diffusion test showing (A,D), synergism between control antimicrobials trimethoprim (horizontally placed) and sulfamethoxazole (vertically placed); (B,E) potentiation of ampicillin strip (horizontal) by quinine (vertical); and (C,F) no interaction/slight inhibition between trimethoprim (horizontal) and ampicillin (vertical). (A–C) Show interaction against S. aureus NCTC 25922 while (D–F) shows interaction against E. coli ATCC 25922.

FIGURE 2

Plot of inhibition zone diameter against log of chloroquine and quinine concentration in combination with ampicillin (10 μg) using E. coli ATCC 25922 as test organism.

FIGURE 3

Plot of inhibition zone diameter against log of chloroquine and quinine concentration in combination with ampicillin (10 μg) using S. aureus NCTC 6571 as test organism.

FIGURE 4

Checkerboard analysis of drug combinations tested against E. coli ATCC 25922. Data presented as a heatmap indicating percent growth inhibition based on OD₅₉₅ values. Percent growth reduction values was calculated as 100% – [(OD of treated cells/OD of untreated cells) × 100%] (Ogundeji et al., 2017). Values represent mean values of three replicates.

FIGURE 5

Checkerboard analysis of drug combinations tested against S. aureus NCTC 6571. Data presented as a heatmap indicating percent growth inhibition based on OD₅₉₅ values. Percent growth reduction values was calculated as 100% – [(OD of treated cells/OD of untreated cells) × 100%] (Ogundeji et al., 2017). Values represent mean values of three replicates.

FIGURE 6

Checkerboard analysis of drug combinations tested against E. coli LLHO29E. Data presented as a heatmap indicating percent growth inhibition based on OD₅₉₅ values. Percent growth reduction values was calculated as 100% –[(OD of treated cells/OD of untreated cells) × 100%] (Ogundeji et al., 2017). Values represent mean values of three replicates.