Commensal Bacteria and Yeast Adhesion (an In Vitro Study Using Epithelial Cells)

Candida albicans is an opportunistic pathogen of increasing importance and candidosis is one of the most common infections of the oral mucosa. One of the important factors in the successful colonization and infection of the oral mucosa by C, albicans is the ability of the yeast to adhere to host surfaces. Since the superficial cells of the oral mucosa are colonized by members of the oral commensal flora, it is likely that they play a part in the adherence and subsequent colonization of the mucosa by Candida, However, there is little information in the literature on this topic and, therefore, the present study was carried out. Sixteen strains of streptococci and three strains of lactobacilli were isolated from the oral cavity, and along with four type strains, were examined for their effect on the adherence of C. albicans MRL 3153 to HeLa cells. Firstly, the effect of the bacteria on yeast adhesion was examined using Candida cells cultured in defined media containing glucose, sucrose or galactose and bacteria grown in Todd Hewitt broth. These growth conditions were used since it is known that yeasts cultured in galactose are more adherent than yeasts grown in medium containing sucrose or glucose. In these experiments, pretreatment of the HeLa cells with each bacterial strain produced a reduction in the subsequent adhesion of the yeast within a range of 23-63% implying that both bacteria and yeasts compete for the same epithelial cell receptor. However, the type of carbon source used in the growth of C, albicans had little effect on the ability of the streptococci and lactobacilli to inhibit adherence of the yeast to HeLa cells. These results suggested that there may be more than one adhesin expressed on the yeast cell surface depending on the carbon source used in its growth. In the next series of experiments, the bacteria were cultured in a semi-defined medium containing either glucose, sucrose or galactose while the yeasts were cultured in a defined medium containing 50 mM glucoseo Generally, it was found that the carbon source used for growth of the bacteria did affect their ability to cause subsequent inhibition of yeast adhesion to HeLa cells. Most strains had a reduced inhibitory effect when grown in sucrose or galactose, while six isolates completely lost their inhibitory activity on the adherence of C. albicans to HeLa cells. These results suggested that the carbon source used in the growth of the bacteria affected the synthesis of some cell surface component which was important in adhesion. Since it was clear from these experiments that commensal oral bacteria were able to inhibit yeast adhesion to buccal cells, it was decided to investigate the possible mechanisms involved. Since lipoteichoic acid (LTA) has been shown to mediate the attachment of a number of streptococci to epithelial cells the next step was to extract this polymer from ten of the bacterial strains investigated previously, using the hot phenol/water extraction method. These crude preparations of LTA from seven streptococcal and three lactobacillus strains inhibited adherence of C. albicans to buccal epithelial cells up to a maximum of 48%. In order to see if the inhibitory effect of the crude extracts could be enhanced, LTA samples from four strains of bacteria were purified by column chromatography and examined for their effect on yeast adhesion. Purified LTA from L. casei NCTC 6375 inhibited adherence of C. albicans GDH 2346 by 16% more than the crude polymer. However, with Strep, salivarius D66 and NCTC 7366 and Strep, mitior C32, purified LTA had less of an inhibitory effect on yeast adhesion than the crude polymer. It was concluded that LTA is involved in mediating adherence of L. casei to epithelial cells. However, in the case of the streptococci, it is unlikely that LTA is the major adhesin. With these organisms it may be required to form a complex with some other cell surface component for adherence to occur. Further investigations involved treating LTA with ammonium hydroxide to discover which part of the polymer was responsible for inhibiting yeast adhesion. Since deacylated LTA had no effect on yeast adhesion it was reasonable to assume that the inhibitory effect of the polymer was due to the lipid portion of the molecule binding to receptors on epithelial cells and therefore blocking subsequent adherence by Candida. (Abstract shortened by ProQuest.)