Effects of trimethoprim and co-trimoxazole on the morphology of Listeria monocytogenes in culture medium and after phagocytosis.

The purpose of this study was to compare the extra- and intracellular activity of antifolates on Listeria monocytogenes. The fortuitous discovery of elongated bacteria in response to trimethoprim revealed a novel effect on the morphology of Listeria in cell culture medium and after phagocytosis. This phenomenon permitted the quantification of trimethoprim activity, revealing comparable activity intra- and extracellularly. Subinhibitory concentrations of trimethoprim resulted in bacterial elongation, which was reversed after removal of trimethoprim. We attribute this effect of trimethoprim to an inhibition of cell wall synthesis and/or cell separation of Listeria.