Potential role of lysozyme in bactericidal activity of in vitro-acquired salivary pellicle against Streptococcus faecium 9790.

The adherence of Streptococcus faecium 9790 to hydroxyapatite (HA) coated with whole saliva supernatant proteins (S-HA) or parotid fluid proteins was studied. The organism was labeled with [3H]thymidine, and adherence was estimated as the radioactivity remaining associated with the variously coated HA preparations after incubation and removal of unbound microbes by washing the adherence substratum. Adherence was time dependent and saturable, characteristics typical of oral streptococci in this in vitro adherence model system. However, adherence to S-HA, but not bare HA, was decreased 20-fold at 4 degrees C compared with room temperature. Furthermore, adherence at 4 degrees C to S-HA was decreased 20-fold relative to bare HA at 4 degrees C. Adherence to HA coated with parotid fluid proteins also was reduced at 4 degrees C. The magnitude of the temperature dependence and the inhibitory effect at 4 degrees C of whole saliva or parotid fluid pellicles on HA was unexpected. Of several sugars and amino sugars tested, the chitin saccharides, chitotriose, chitobiose, and N-acetylglucosamine caused greater than 90% inhibition of adherence to S-HA. These same saccharides were previously shown to inhibit lysozyme, polylysine, or autolytic lysis of the organism (N. J. Laible and G. R. Germaine, Infect. Immun. 48:720-728, 1985). Examination of unbound and adherent microbes revealed that lysis of the organism occurred during the adherence assays. A strong association (r = 0.83) between the extent of lysis and the extent of adherence was found under a variety of conditions. Depletion of lysozyme from saliva specimens used to coat HA resulted in a greater than 90% decrease in both cell lysis and adherence. Lysis of the microbe appeared dependent upon the presence of the saliva pellicle (coating) on HA, since solutions containing proteins desorbed from HA during mock-adherence incubations possessed lytic activity that was 2- to 10-fold too low to account for the extents of lysis observed with greater than or equal to 10(8) input cells. These results demonstrate the potential antibacterial activity of acquired salivary pellicle on enamel in vivo and the likely role of lysozyme in this activity. The data also serve to caution that this widely used in vitro adherence model will not distinguish whole-cell adherence from the adsorption of radiolabeled DNA released from lysing cells. Several additional controls are suggested that will indicate whether test microbes remain intact or lyse during adherence trials.