Cell-surface alterations in class IIa bacteriocin-resistant Listeria monocytogenes strains

Strains of the food-borne pathogen Listeria monocytogenes, showing either intermediate or high-level resistance to class IIa bacteriocins, were investigated to determine characteristics that correlated with their sensitivity levels. Two intermediate and one highly resistant spontaneous mutant of L. monocytogenes B73, a highly resistant mutant of L. monocytogenes 412, and a highly resistant, defined (mptA) mutant of L. monocytogenes EGDe were compared with their respective wild-type strains in order to investigate the contribution of different factors to resistance. Decreased mannose-specific phosphotransferase system gene expression (mptA, EIIAB(Man) component) was implicated in all levels of resistance, confirming previous studies by the authors' group. However, a clear correlation between d-alanine content in teichoic acid (TA), in particular the alanine : phosphorus ratio, and a more positive cell surface, as determined by cytochrome c binding, were found for the highly resistant strains. Furthermore, two of the three highly resistant strains showed a significant increase in sensitivity towards d-cycloserine (DCS). However, real-time PCR of the dltA (d-alanine esterification), and dal and ddlA genes (peptidoglycan biosynthesis) showed no change in transcriptional levels. The link between DCS sensitivity and increased d-alanine esterification of TA may be that DCS competes with alanine for transport via the alanine transporter. A possible tendency towards increased lysinylation of membrane phospholipid in the highly resistant strains was also found. A previous study reported that cell membranes of all the resistant strains, including the intermediate resistant strains, contained more unsaturated phosphatidylglycerol, which is an indication of a more fluid cell membrane. The results of that study correlate with the possible lysinylation, decreased mptA expression, d-alanine esterification of TA and more positive cell surface charge found in this study for resistant strains. The authors' findings strongly indicate that all these factors could contribute to class IIa bacteriocin resistance and that the combination and contribution of each of these factors determine the level of bacteriocin resistance.