Cellular coaggregation of oral Streptococcus milleri with actinomyces

Effect of saliva viscosity on the co-aggregation between oral streptococci and Actinomyces naeslundii

BACKGROUND

The co-aggregation of oral bacteria leads to their clearance from the oral cavity. Poor oral hygiene and high saliva viscosity are common amongst the elderly; thus, they frequently suffer from pneumonia caused by the aspiration of oral microorganisms.

OBJECTIVES

To examine the direct effect of saliva viscosity on the co-aggregation of oral streptococci with actinomyces.

MATERIALS AND METHODS

Fifteen oral streptococcal and a single actinomyces strain were used. Co-aggregation was assessed by a visual assay in phosphate buffer and a spectrophotometric assay in the same buffer containing 0-60% glycerol or whole saliva.

RESULTS

Nine oral streptococci co-aggregated with Actinomyces naeslundii ATCC12104 in the visual assay and were subsequently used for the spectrophotometric analysis. All tested strains displayed a decrease in co-aggregation with increasing amounts of glycerol in the buffer. The co-aggregation of Streptococcus oralis with A. naeslundii recovered to baseline level following the removal of glycerol. The per cent co-aggregation of S. oralis with A. naeslundii was significantly correlated with the viscosity in unstimulated and stimulated whole saliva samples (correlation coefficients: -0.52 and -0.48, respectively).

CONCLUSION

This study suggests that saliva viscosity affects the co-aggregation of oral streptococci with actinomyces and that bacterial co-aggregation decreases with increasing saliva viscosity.

Gene cloning and characterization of Streptococcus intermedius fimbriae involved in saliva-mediated aggregation

Streptococcus intermedius, an oral commensal and a cause of systemic pyogenic disease, expresses fimbriae. To identify the gene(s) encoding these fimbriae, we used a serum raised against purified fimbriae to screen libaries of recombinant lambda phages. The cloned gene cluster encoding S. intermedius fimbriae, (saliva-mediated aggregation and adherence-associated fimbriae), contained 4 ORFs, i.e. a putative ribonulease (Saf1), a putative adhesin (Saf2), the main pilus subunit (Saf3) and a sortase C (SrtC). Escherichia coli strains harboring recombinant phages and plasmids carrying the saf3 gene produced a 55kDa protein recognized by the antifimbriae serum. Saf3 contains an N-terminal signal sequence and a C-terminal cell-wall-anchoring motif LPXTG. Among strains of the Streptococcus anginosus group, only serotype g and untypable strains were found to contain the saf3 gene, to possess the fimbrial antigen and to exhibit saliva-mediated aggregation. Knockout mutants made by insertion of an erythromycin resistance gene into saf3 did not produce fimbrial structures or fimbrial antigens and did not participate in saliva-mediated aggregation. The adherent activity of mutants toward plastic wells coated with salivary agglutinin was about 65% that of the parental strain, and the reaction depended on calcium. There was no significant difference in adherence to hydroxyapatite beads pretreated with salivary agglutinin between the parental and mutant strains. These results demonstrated that Saf3 is associated with aggregation, and suggested that other molecule(s) are associated with adherence of S. intermedius.

Sensitivity of bacterial coaggregation to chelating agents

Coaggregation between pairs of microorganisms was found to be inhibited by chelating agents, such as acetylacetone, citrate, EDTA and carboxymethylcellulose. Assays were conducted on eight pairs of periodontopathogens and one pair consisting of Escherichia coli and Saccharomyces cerevisiae. The inhibitory effects of the chelating agents were reversible except for Actinomyces naeslundii 12104, the adhesin of which was irreversibly inactivated. Even though the bacteria possessed different kinds of adhesins, their sensitivity to chelating agents appears to be a common property. Non-toxic chelating agents, such as carboxymethylcellulose and citrate, may prove to be useful anti-adhesins.

Coaggregation of Prevotella intermedia with oral Actinomyces species

Five strains of Prevotella intermedia were examined for their ability to coaggregate with various gram-positive and gram-negative species of oral bacteria. Two of the P. intermedia strains coaggregated with selected Actinomyces species, P. intermedia 27 with Actinomyces viscosus T14V and Actinomyces naeslundii ATCC 12104, PK606, PK984, and PK947, and P. intermedia 113 with Actinomyces odontolyticus WVU 1546 and Actinomyces israelii WVU 838. Exposure of both Prevotella strains but not the Actinomyces strains to heat, trypsin, or proteinase K abolished most coaggregations. All pairs were disaggregated by the addition of sodium dodecyl sulfate, but only those coaggregations involving P. intermedia 113 were reversed by the addition of 2.0 M urea. P. intermedia 27 was sensitive to periodate oxidation, whereas the partner strains were stable to this treatment. Most coaggregations occurred in the presence of saliva; however, reactions involving P. intermedia 27 were not as strong as those of buffer-suspended cells. Treatment of both P. intermedia 113 coaggregations pairs with proteinase K and the results obtained from suspensions of these pairs in saliva suggest that different surface molecules of this P. intermedia strain may mediate each of these coaggregations. These data suggest that all of these coaggregations involve either a protein or glycoprotein on the Prevotella strain, which may interact with carbohydrates or carbohydrate-containing molecules on the surface of the Actinomyces strain.

Adherence of oral "Streptococcus milleri" cells to surfaces in broth cultures

Cells of representative strains of oral "Streptococcus milleri" firmly adhered to glass surfaces when grown in glucose broths as well as in fructose and sucrose broths. Cellular adherence occurred on saliva-coated glass surfaces as well as uncoated surfaces, but the cells of only a few (6 of 69) tested adherent strains agglutinated upon the addition of human whole-saliva supernatant. Thus, the firm adherence of growing "S. milleri" cells is independent of sucrose and salivary macromolecules. Two macroscopic forms of cell adherence were observed: one was a coarse-granule type and one was a tiny-granule type covered with a thin homogeneous membranous material. Scanning electron microscopy revealed that, in both types, a stringy substance interconnected the cells and connected the cells to glass surfaces, with amorphous flocklike materials present in the intercellular space. Of the 154 tested strains, 128 were adherent, of which 90 were firmly and extensively (+2 or +3) adherent. Most strains of the serovars f/F, g, h, i, j, g-, and gj- were extensively adherent. Generally, cells of the more strongly adherent strains were more hydrophobic and spontaneously aggregating. However, there also existed a few hydrophilic and nonaggregating but extensively adherent strains. The presence of trypsin or pronase completely inhibited the adherence of some strains (mostly nonaggregating) but did not at all inhibit that of others (mostly aggregating). The adhering groups did not significantly differ in their cell surface hydrophobicities. Therefore, at least two factors, one proteinaceous and one nonproteinaceous, appear to be principally involved in the firm surface adherence of "S. milleri."

Lactose-sensitive and -insensitive cell surface interactions of oral Streptococcus milleri strains and actinomyces

Of 158 oral Streptococcus milleri strains, 46 exhibited cellular coaggregation with the reagent strains of the actinomyces coaggregation groups A, B, and/or E. All but 1 of the 33 serotype b, e, f/F, and k/G strains belonged to streptococcus coaggregation group 2, and only 14 strains of limited seroclasses (g, i, Lancefield group F, or untypeable) appeared to be members of group 5, 3, or 4 (10, 3, and 1 strain, respectively). Thus, S. milleri infrequently exhibits lactose-inhibitable coaggregation with actinomyces.