Effect of lysozyme on glucose fermentation, cytoplasmic pH, and intracellular potassium concentrations in Streptococcus mutans 10449.

Several previous findings have suggested that the cationic nature of lysozyme is a major factor in its bactericidal activity. Since a number of cationic proteins or peptides have been reported to cause membrane damage in bacteria, we investigated the effect of lysozyme on glucose fermentation and intracellular pH and K+ in Streptococcus mutans under conditions in which lysis does not occur. Results showed that lysozyme and poly-D-lysine (PDL) cause inhibition of glucose fermentation at pH 5.5 in a dose-dependent manner. Human placental lysozyme and hen egg-white lysozyme exhibited similar inhibitory potency on glucose fermentation. Both lysozyme and PDL caused a marked acidification of the cytoplasm of S. mutans. However, when cytoplasmic pH was examined as a function of fermentation rate, the relationship was similar regardless of the presence or absence of lysozyme or PDL. Therefore, acidification of the cytoplasm appeared to not depend specifically on lysozyme or PDL. In contrast, the same relationship between the profound loss of intracellular K+, when fermenting cells were exposed to either lysozyme or PDL, and the fermentation rate was not exhibited in the controls. These results indicate that lysozyme and PDL specifically affected the ability of the cells to maintain intracellular K+. We concluded that lysozyme and PDL indeed perturb membrane function, perhaps in a selective manner. Furthermore, the similarity in action of lysozyme and the cationic homopolypeptide PDL supports the notion that the cationic property of lysozyme indeed plays a significant role in its antibacterial activity.