[Obtaining dicloxacillin-resistant mutants in Listeria monocytogenes]

We obtained ten Dicloxacillin resistant mutants from Listeria monocytogenes ATCC: 15313 (29-CCM-A: 454). None of the mutants could be differentiated from the parental strain (except for their increased resistance) neither using conventional biochemical assays nor by analysis of the electrophoretic pattern of their detergent-soluble proteins (Figure 1). The wild type strain and some of these mutants were tolerant for Dicloxacillin and/or Penicillin G, but no rigorous correlation with each decrease in their susceptibility was observed (Table 1). Morphological studies showed that some of the resistant strains (growing at subinhibitory concentrations of Dicloxacillin) presented differences, including the formation of helical structures (Figures 2-5).