Antibiotic killing of drug-induced bacteriostatic cells

Abstract

Background: There is a long-standing belief that bacteriostatic drugs are inherently antagonistic to the action of bactericidal antibiotics. This belief is primarily due to the fact that the action of most bactericidal antibiotics requires the target bacteria to be growing. Since bacteriostatic drugs stop the growth of treated bacteria, these drugs would necessarily work against one another. We have recently shown that bacteria treated with high concentrations of bacteriostatic drugs retain some metabolic activity, dividing on average once per day.

Objectives: We seek to determine if this low level of growth is sufficient to allow for bactericidal antibiotics of different classes to still kill after bacteria are treated with bacteriostatic drugs.

Methods: We first treated Escherichia coli and Staphylococcus aureus with two different bacteriostatic drugs, followed by one of three bactericidal drugs of three different classes. The density of these bacteria was tracked over six days to determine the amount of killing that occurred.

Results: Our results question this long-standing belief by demonstrating conditions where sequential treatment with a bacteriostatic then bactericidal antibiotic is as or more effective than treatment with a bactericidal drug alone.

Conclusions: These results raise the need to investigate the pharmacodynamics of the joint action of bacteriostatic and bactericidal antibiotics in vitro and in vivo.

Keywords: Antibiotic Resistance; Antibiotics; Bacteriostasis; Population Biology.

Figure 1.. Exposure to varying concentrations of bacteriostatic drugs for six days without transferring.

Density in CFU/mL of E. coli MG1655 (A and B) and S. aureus MN8 (C and D) measured every day for six days at several concentrations of either chloramphenicol (CHL) (A and C), azithromycin (AZM) (B), or tetracycline (TET) (D). Shown in black for each panel is a control culture without antibiotics.

Figure 2.. The treatment of E. coli and S. aureus with bacteriostatic followed by bactericidal antibiotics.

Shown are bacterial densities in CFU/mL, each point represents the mean and standard deviation of three biological replicates. Cultures are sampled every day for six days and are not transferred. A, B, and C correspond to E. coli experiments and D, E, and F to S. aureus experiments. Chloramphenicol is at 4× MIC, azithromycin at 4× MIC, tetracycline is at 10× MIC, and the bactericidal drugs are all at 4× MIC for E. coli and 12× MIC for S. aureus. For panels A, B, D, and E cultures were treated with either chloramphenicol, azithromycin, or tetracycline for one day before the subsequent addition of a bactericidal antibiotic. Figure C and F are a control where cultures were treated with only a bactericidal or bacteriostatic drug.