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 activ ity of antifolates on Listeria monocytogenes. The fortuitous discovery of e longated bacteria in response to trimethoprim revealed a novel effect on th e morphology of Listeria in cell culture medium and after phagocytosis. Thi s phenomenon permitted the quantification of trimethoprim activity, reveali ng comparable activity intra- and extracellularly. Subinhibitory concentrat ions of trimethoprim resulted in bacterial elongation, which was reversed a fter removal of trimethoprim. We attribute this effect of trimethoprim to a n inhibition of cell wall synthesis and/or cell separation of Listeria.