Survival of Bactericidal Antibiotic Treatment by a Persister Subpopulation of Listeria monocytogenes

Abstract

ABSTRACTListeria monocytogenescan cause the serious infection listeriosis, which despite antibiotic treatment has a high mortality. Understanding the response ofL. monocytogenesto antibiotic exposure is therefore important to ensure treatment success. Some bacteria survive antibiotic treatment by formation of persisters, which are a dormant antibiotic-tolerant subpopulation. The purpose of this study was to determine whetherL. monocytogenescan form persisters and how bacterial physiology affects the number of persisters in the population. A stationary-phase culture ofL. monocytogeneswas adjusted to 108CFU ml−1, and 103to 104CFU ml−1survived 72-h treatment with 100 μg of norfloxacin ml−1, indicating a persister subpopulation. This survival was not caused by antibiotic resistance as regrown persisters were as sensitive to norfloxacin as the parental strain. Higher numbers of persisters (105to 106) were surviving when older stationary phase or surface-associated cells were treated with 100 μg of norfloxacin ml−1. The number of persisters was similar when a ΔsigBmutant and the wild type were treated with norfloxacin, but the killing rate was higher in the ΔsigBmutant. Dormant norfloxacin persisters could be activated by the addition of fermentable carbohydrates and subsequently killed by gentamicin; however, a stable surviving subpopulation of 103CFU ml−1remained. Nitrofurantoin that has a growth-independent mode of action was effective against both growing and dormant cells, suggesting that eradication of persisters is possible. Our study addsL. monocytogenesto the list of bacterial species capable of surviving bactericidal antibiotics in a dormant stage, and this persister phenomenon should be borne in mind when developing treatment regimens.