Antibiotic Susceptibility of Listeria monocytogenes

Listeria monocytogenes is a foodborne pathogen capable of surviving and growing under aerobic or anaerobic conditions in variety of environments, including the cytoplasm of mammalian cells. This intracellular growth requires Listeria to make multiple metabolic and physiological adaptations that are different from extracellular growth. Anaerobically grown Listeria has previously exhibited a significantly compromised intracellular growth—an observation suggesting that prior anaerobic exposure altered adaptations to conditions inside a host cell. The focus of this experiment was to elucidate how intracellular adaptations, with or without prior anaerobic exposure, alter the antibiotic susceptibility of intracellular Listeria. Listeria were grown aerobically or anaerobically prior to infection and then used to infect macrophages. Infected macrophages were treated with gentamicin to remove extracellular bacteria, then lysed with sterile water after one, four, or eighteen hours of infections. Bacterial lawns were created prior to infection for a point of reference for comparison, as well as after each time point. Filter discs containing different concentrations of ampicillin were placed on the lawns to test susceptibility in a zone of inhibition assay. We observed that anaerobically grown Listeria is more susceptible to ampicillin than aerobically grown prior to infection at the three highest concentrations. No significant difference was found in susceptibility to ampicillin between anaerobically grown or aerobically grown Listeria following eighteen hours. Aerobically grown Listeria was seen to become more susceptible to the antibiotic treatment with more time inside the macrophage, while anaerobically grown Listeria showed little change in susceptibility over the varying time points. These results demonstrate intracellular adaptions alter antibiotic susceptibility and may alter dosage requirement during antibiotic treatments.https://ecommons.udayton.edu/stander_posters/3133/thumbnail.jp