Identification of a new fimbrial structure in enterotoxigenic Escherichia coli (ETEC) serotype O148:H28 which adheres to human intestinal mucosa: a potentially new human ETEC colonization factor.

Three important fimbrial colonization factor antigens (CFAs) designated CFA/I, CFA/II, and E8775 were identified originally in some human enterotoxigenic Escherichia coli (ETEC) strains because of their mannose-resistant hemagglutination properties. To identify CFA, in strains lacking mannose-resistant hemagglutination properties we exploited the ability of human ETEC strains to adhere to human proximal small intestinal mucosa. ETEC strain B7A (O148:H28) was selected for study because it belongs to an epidemiologically important serotype and does not produce a known CFA, and yet it is known to be pathogenic and cause diarrheal disease in human volunteers. Results of an human enterocyte adhesion assay indicated that some bacteria in cultures of B7A produced adhesive factors. To select for such bacteria, cultured human duodenal mucosal biopsy samples were infected with B7A for up to 12 h, after which time a large percentage of the mucosal surface became colonized by bacteria. A new fimbrial structure morphologically distinct from CFA/I, CFA/II, and E8775 fimbriae and consisting of curly fibrils (approximately 3 nm in diameter) was readily identified when bacteria were subcultured from the mucosa and examined by electron microscopy. Identical fimbriae were produced by ETEC strain 1782-77 of the same serotype. Identification of these fimbriae only on bacteria subcultured from human intestinal mucosa strongly suggests that they promote mucosal adhesion of ETEC serotype O148:H28 and thus represent a potentially new human ETEC CFA.     

A new fimbrial putative colonization factor, PCFO20, in human enterotoxigenic Escherichia coli.

The ability to colonize the small intestine is essential for enterotoxigenic Escherichia coli (ETEC) to cause diarrhea. Several colonization factor antigens (CFAs) and putative colonization factors (PCFs) have been described for ETEC. However, there are still many ETEC strains isolated from patients with diarrhea which do not possess any of these antigens. To identify CFAs in ETEC lacking the above-mentioned antigens, we exploited the ability of ETEC to adhere to tissue-cultured cells from an enterocyte-like cell line, Caco-2. An ETEC strain producing heat-labile toxin and heat-stable toxin of serotype O20:K27:H- (ARG-2) that was isolated from a child with diarrhea in Argentina and bound to Caco-2 cells was studied in further detail. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analyses of this strain revealed a band of 25 kDa when bacteria were grown at 37 degrees C that was missing when the same strain was cultured at 20 degrees C. Furthermore, electron microscopy examination revealed the presence of fimbriae on the surfaces of cells of this strain when cells were grown at 37 degrees C but not at 20 degrees C. Rabbit antiserum raised against purified fimbriae reacted with the 25-kDa protein in immunoblotting and bound specifically to the fimbriae, as shown by immunoelectron microscopy. The presence of fimbriae, adhesion to Caco-2 cells, and the 25-kDa band seen in the SDS-PAGE were all simultaneously lost by single-insertion mutations. The N-terminal amino acid sequence of the protein subunit of the fimbriae showed no relation with those of the known colonization factors of ETEC. Furthermore, the fimbriae of the ARG-2 strain did not cross-react immunologically with any of the previously described adhesive factors in human ETEC when specific antisera against colonization factor antigens and putative colonization factors were used. Moreover, a specific antiserum raised against the fimbriae in ARG-2 did not react with ETEC carrying known colonization factors. We propose to name these new fimbriae PCFO20.     

A plasmid of enterohemorrhagic Escherichia coli O157:H7 is required for expression of a new fimbrial antigen and for adhesion to epithelial cells.

Of 14 strains of Escherichia coli O157:H7 isolated from patients with hemorrhagic colitis or hemolytic uremic syndrome that were examined for fimbriae, the presence of plasmids, and the ability to adhere to intestinal cells, 13 possessed a 60-megadalton plasmid and were fimbriated as assessed by electron microscopy. These strains adhered to Henle 407 intestinal cells but not to HEp-2 cells or erythrocytes. Three strains were cured of the plasmid and thereafter failed to express fimbriae and lost the ability to adhere to intestinal cells. Conversely, E. coli K-12 transformed with the 60-megadalton plasmid from each of the three strains produced fimbriae and was able to adhere to intestinal cells. A single fimbrial subunit of 16 kilodaltons was observed when purified fimbriae from the transformants and from the 60-megadalton plasmid-containing E. coli O157:H7 strains were disaggregated and subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Antisera raised against one preparation of the purified fimbriae reacted strongly with 12 of 14 O157:H7 isolates in an agglutination assay and with purified fimbrial preparations from five E. coli O157:H7 strains in an enzyme-linked immunosorbent assay.     

Adhesion of type A Pasteurella mulocida to rabbit pharyngeal cells and its possible role in rabbit respiratory tract infections.

Pasteurella multocida serotype A was found in association with the mucosal epithelium of the nasopharynges of rabbits with respiratory tract infections. The bacteria specifically attached to squamous epithelial cells of the pharyngeal mucosa both in vivo and in vitro and to some tissue culture cell lines such as HeLa. All strains with serotype A capsules were adhesive. With the exception of one serotype D strain, strains with capsular serotypes B, D, and E were at least 10-fold less adhesive. Bacterial adhesiveness was much reduced after pronase digestion, heat treatment, and homogenization, but removal of the hyaluronic acid capsule increased adhesion. Electron microscopy revealed that fimbriae were produced by an adhesive pasteurella strain, but not by two nonadherent strains. The attachment of the former strain to pharyngeal and HeLa cells was inhibited by N-acetyl-D-glucosamine. Together, these findings suggest that this amino sugar may be a component of the receptor on both animal cell surfaces and that the fimbriae may be the adhesions. It is proposed that bacterial attachment has a role in colonization and infection of rabbit upper respiratory mucosae.     

Serological response to the P fimbriae of uropathogenic Escherichia coli in pyelonephritis.

Uropathogenic Escherichia coli strains isolated from four patients with pyelonephritis were characterized by their O:K serotype, hemolysin production, mannose-resistant hemagglutination, and the serotype of the P fimbriae. These P fimbriae were serotyped with specific monoclonal antibodies. Serum samples from the patients were analyzed for the presence of specific antibodies to the P fimbriae. In all cases antifimbrial antibodies were found, strongly suggesting that these P fimbriae are expressed in vivo. However, the antibodies in the patient sera were not able to inhibit the mannose-resistant hemagglutination. This finding suggests that these antibodies react with the fimbrial components and not with the minor components which are responsible for adhesion.     

Fimbrial serotypes of Escherichia coli strains isolated from extra-intestinal infections

Strains of Escherichia coli isolated from urinary tract infections and meningitis were characterised by their O:K serotype, haemolysin production, mannose-resistant haemagglutination, and the serotype of the P-fimbriae. The P-fimbriae of 71% of the mannose-resistant haemagglutination-positive strains from urinary tract infection and meningitis could be determined with specific monoclonal antibodies. Many strains expressed multiple P-fimbriae serotypes. The serotypes of P-fimbriae found most frequently among mannose-resistant haemagglutination-positive E. coli from urinary tract infections were the F11, F7 and F8 fimbriae, and among meningitic strains, F11, F8 and F9 fimbriae. The expression of certain F-serotypes did not correlate with O:K antigens.     

Monoclonal antibodies for serotyping the P fimbriae of uropathogenic Escherichia coli.

Monoclonal antibodies (MAbs) against seven serologically different P fimbriae (F7(1), F7(2), F8, F9, F11, F12, and F13) of uropathogenic Escherichia coli were tested for their ability to detect the P fimbriae on wild-type strains. In a plate agglutination test the MABs could detect the fimbriae on strains which expressed cloned fimbriae but not on wild-type strains. In a coagglutination test and in a whole-bacterium enzyme-linked immunosorbent assay the MAbs recognized the fimbriae on strains with cloned fimbriae and on wild-type strains. However, the coagglutination test has some disadvantages: only immunoglobulin G MAbs can be used, and the results cannot be read in an objective way. From these results, we concluded that the whole-bacterium enzyme-linked immunosorbent assay is the most convenient method for the determination of P fimbriae on wild-type E. coli strains. With this fast and easy method it is possible to do epidemiological studies on the distribution of P fimbriae among clinical isolates of uropathogenic E. coli and to extend the O:K:H serotype with the F serotype.     

Three-dimensional structure of fimbriae determines specificity of immune response.

We recently described how a fraction of isolated fimbriae from a multifimbriated strain of Escherichia coli O7:K1:H6 (WF96) could be subdivided by sequential disaggregation in disrupting agents into individual subunits with different molecular weights. In this study, antibodies were raised in rabbits against these isolated fimbrial subunits and against purified intact WF96 fimbriae. These sera were tested by Western blot analysis or by enzyme-linked immunosorbent assays for reactivity against the following antigens: intact WF96 fimbriae, dissociated WF96 fimbriae, dissociated and reaggregated WF96 fimbriae, the WF96 21K fimbrial subunit, reaggregated WF96 21K subunits, the WF96 16K subunits, reaggregated WF96 16K subunits, intact fimbriae from four other E. coli strains, and deaggregated fimbriae from these strains. We found that antibody against intact WF96 fimbriae only reacted strongly with intact WF96 fimbriae, depolymerized and reaggregated WF96 fimbriae, or reaggregated fimbrial subunits; no reactions were evident with intact fimbriae from four other E. coli strains. Conversely, antisera prepared against the WF96 16K subunit and against the WF96 21K subunit did not react with intact WF96 fimbriae or with depolymerized and reaggregated WF96 fimbriae, but did react with homologous isolated subunits. One cross-reaction between fimbrial subunits was apparent: anti-WF96 16K subunit bound to a 21K subunit of deaggregated fimbriae, from another E. coli strain. Taken together, the findings indicate that the three-dimensional structure of the fimbrial preparation used to immunize animals determines the specificity of the immune response.     

New fimbrial antigenic type (E8775) that may represent a colonization factor in enterotoxigenic Escherichia coli in humans.

An enterotoxigenic strain of Escherichia coli O25:H42 (strain E8775), isolated from a patient in Bangladesh with diarrhea, caused mannose-resistant hemagglutination (MRHA) of human and bovine erythrocytes. The strain did not show slide agglutination or immunodiffusion precipitin lines with antiserum specific for the colonization factor antigen CFA/I or CFA/II. A variant E. coli strain, E8775-B, did not cause MRHA or produce enterotoxin. Electron microscopy revealed the presence of fimbriae on the surface of strain E8775 but not strain E8775-B. When strain E8775 was grown at 22 degrees C, it became MRHA negative and fimbriae were absent. An antiserum prepared against strain E8775 was absorbed with strain E8775-B to make an antiserum specific for the fimbrial antigen. Using this absorbed antiserum, we found the fimbrial antigen in 48 of 742 enterotoxigenic E. coli strains. The 48 strains belonged to serogroups O25, O115, and O167. It is suggested by analogy to the properties of previously described colonization factors that these fimbriae may play a part in the colonization of the intestinal epithelium.     

Common virulence factors and genetic relationships between O18:K1:H7 Escherichia coli isolates of human and avian origin

Extraintestinal pathogenic (ExPEC) Escherichia coli strains of serotype O18:K1:H7 are mainly responsible for neonatal meningitis and sepsis in humans and belong to a limited number of closely related clones. The same serotype is also frequently isolated from the extraintestinal lesions of colibacillosis in poultry, but it is not well known to what extent human and avian strains of this particular serotype are related. Twenty-two ExPEC isolates of human origin and 33 isolates of avian origin were compared on the basis of their virulence determinants, lethality for chicks, pulsed-field gel electrophoresis (PFGE) patterns, and classification in the main phylogenetic groups. Both avian and human isolates were lethal for chicks and harbored similar virulence genotypes. A major virulence pattern, identified in 75% of the isolates, was characterized by the presence of F1 variant fimbriae; S fimbriae; IbeA; the aerobactin system; and genomic fragments A9, A12, D1, D7, D10, and D11 and by the absence of P fimbriae, F1C fimbriae, Afa adhesin, and CNF1. All but one of the avian and human isolates also belonged to major phylogenetic group B2. However, various subclonal populations could be distinguished by PFGE in relation to animal species and geographical origin. These results demonstrate that very closely related clones can be recovered from extraintestinal infections in humans and chickens and suggest that avian pathogenic E. coli isolates of serotype O18:K1:H7 are potential human pathogens.     

Cloning, expression, and characterization of fimbrial operon F9 from enterohemorrhagic Escherichia coli O157:H7

Recent transposon mutagenesis studies with two enterohemorrhagic Escherichia coli (EHEC) strains, a sero- type O26:H- strain and a serotype O157:H7 strain, led to identification of a putative fimbrial operon that promotes colonization of young calves (1 to 2 weeks old). The distribution of the gene encoding the major fimbrial subunit present in O-island 61 of EHEC O157:H7 in a characterized set of 78 diarrheagenic E. coli strains was determined, and this gene was found in 87.2% of the strains and is therefore not an EHEC-specific region. The cluster was amplified by long-range PCR and cloned into the inducible expression vector pBAD18. Induced expression in E. coli K-12 led to production of fimbriae, as demonstrated by transmission electron microscopy and sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis. The fimbriae were purified, and sera to the purified major subunit were raised and used to demonstrate expression from wild-type E. coli O157:H7 strains. Induced expression of the fimbriae, designated F9 fimbriae, was used to characterize binding to bovine epithelial cells, bovine gastrointestinal tissue explants, and extracellular matrix components. The fimbriae promoted increases in the levels of E. coli K-12 binding only to bovine epithelial cells. In contrast, induced expression of F9 fimbriae in E. coli O157:H7 significantly reduced adherence of the bacteria to bovine gastrointestinal explant tissue. This may have been due to physical hindrance of type III secretion-dependent attachment. The main F9 subunit gene was deleted in E. coli O157:H7, and the resulting mutant was compared with the wild-type strain for colonization in weaned cattle. While the shedding levels of the mutant were reduced, the animals were still colonized at the terminal rectum, indicating that the adhesin is not responsible for the rectal tropism observed but may contribute to colonization at other sites, as demonstrated previously with very young animals.     

Comparison of Escherichia coli fimbrial antigen F7 with type 1 fimbriae.

Two Escherichia coli O6:K2:H1 strains, C1212 and C1214, isolated from urinary tract infections, were compared for their capacity to adhere to various cells. After growth on solid medium, only C1212 bacteria agglutinate human erythrocytes and attach to urinary epithelial cells. Both of these reactions are mannose resistant. In contrast, C1214 bacteria cause a mannose-sensitive agglutination of guinea pig erythrocytes, show a mannose-sensitive attachment to buccal epithelial cells, and attach to urinary mucus. Immunoelectron microscopy revealed that C1214 bacteria possess type 1 fimbriae (mannose sensitive), which are not present in C1212 bacteria when this strain is grown on solid medium. The fimbriae of C1212 (mannose resistant) were also demonstrated by immunoelectron microscopy. We call these fimbriae demonstrated in C1212 the E. coli F7 antigen. Urinary mucus, and probably mucous material elsewhere, may function as a trap for Enterobacteriaceae with type 1 fimbriae by the specific adherence of such bacteria. We consider this a nonimmune resistance mechanism against disease caused by Enterobacteriaceae.     

Analysis of the genetic determinants coding for the S-fimbrial adhesin (sfa) in different Escherichia coli strains causing meningitis or urinary tract infections.

Recently we have described the molecular cloning of the genetic determinant coding for the S-fimbrial adhesin (Sfa), a sialic acid-recognizing pilus frequently found among extraintestinal Escherichia coli isolates. Fimbriae from the resulting Sfa+ E. coli K-12 clone were isolated, and an Sfa-specific antiserum was prepared. Western blots indicate that S fimbriae isolated from different uropathogenic and meningitis-associated E. coli strains, including O83:K1 isolates, were serologically related. The Sfa-specific antibodies did not cross-react with P fimbriae, but did cross-react with F1C fimbriae. Furthermore the sfa+ recombinant DNAs and some cloned sfa-flanking regions were used as probes in Southern experiments. Chromosomal DNAs isolated from O18:K1 and O83:K1 meningitis strains with and without S fimbriae and from uropathogenic O6:K+ strains were hybridized against these sfa-specific probes. Only one copy of the sfa determinant was identified on the chromosome of these strains. No sfa-specific sequences were observed on the chromosome of E. coli K-12 strains and an O7:K1 isolate. With the exception of small alterations in the sfa-coding region the genetic determinants for S fimbriae were identical in uropathogenic O6:K+ and meningitis O18:K1 and O83:K1 strains. The sfa determinant was also detected on the chromosome of K1 isolates with an Sfa-negative phenotype, and specific cross-hybridization signals were visible after blotting against F1C-specific DNA. In addition homology among the different strains was observed in the sfa-flanking regions.     

Cloning and characterization of genes involved in production of mannose-resistant, neuraminidase-susceptible (X) fimbriae from a uropathogenic O6:K15:H31 Escherichia coli strain.

The uropathogenic Escherichia coli strain 536 (O6:K15:H31) exhibits a mannose-resistant hemagglutination phenotype (Mrh) with bovine erythrocytes and delayed Mrh with human and guinea pig erythrocytes. Neuraminidase treatment of the erythrocytes abolishes mannose resistant hemagglutination, which is typical for X fimbriae. E. coli strain 536 synthesizes two different fimbriae (Fim phenotype) protein subunits, 16.5 and 22 kilodaltons in size. In addition the strain shows mannose-sensitive hemagglutination and common type I (F1) fimbriae. The cosmid clone E. coli K-12(pANN801) and another nine independently isolated Mrh+ cosmid clones derived from a cosmid gene bank of strain 536 express the 16.5-kilodalton protein band, but not the 22-kilodalton protein, indicating an association of the Mrh+ property with the "16.5-kilodalton fimbriae." All cosmid clones were fimbriated, and they reacted with antiserum produced against Mrh+ fimbriae of the E. coli strain HB101(pANN801) and lacked mannose-sensitive hemagglutination (F1) fimbriae. From the Mrh fim cosmid DNA pANN801, several subclones coding for hemagglutination and X fimbriae were constructed. Subclones that express both hemagglutination and fimbriae and subclones that only code for the hemagglutination antigen were isolated; subclones that only produce fimbriae were not detected. By transposon Tn5 mutagenesis we demonstrated that about 6.5 kilobases of DNA is required for the Mrh+ Fim+ phenotype, and the 1.5- to 2-kilobase DNA region coding for the structural protein of the fimbriae has been mapped adjacent to the region responsible for the Mrh+ phenotype. Two different regions can thus be distinguished in the adhesion determinant, one coding for hemagglutination and the other coding for fimbria formation. Transformation of plasmid DNA from these subclones into a Mrh- Fim- mutant of E. coli 536 and into a galE (rough) strain of Salmonella typhimurium yielded transformants that expressed both hemagglutination and fimbria production.     

Coli surface antigens 1 and 3 of colonization factor antigen II-positive enterotoxigenic Escherichia coli: morphology, purification, and immune responses in humans.

Enterotoxigenic Escherichia coli (ETEC) of serotype O6:H16, biotype A, bearing colonization factor antigen II (CFA/II) possesses two distinct coli surface antigens, CS1 and CS3, whereas CFA/II-positive ETEC of serotype O8:H9 manifests only CS3. CS1 has been shown to be fimbrial in nature, but heretofore the morphology of CS3 has not been described. Accordingly, by immune electron microscopy we investigated the morphological characteristics of CS3 on bacterial cells and after purification. CS3 was found to consist of thin (2-nm), flexible, wiry, "fibrillar" fimbriae, visible both on bacteria (O6:H16, biotype A, and O8:H9 strains) and in the pure state. In contrast, CS1 exists as wider (6-nm), rigid fimbriae on the surface of O6:H16, biotype A, strains. By the use of antisera to CS1 and CS3 in immune electron microscopy, immunodiffusion in gel, and immunoblotting techniques, CS1 and CS3 were found to be immunologically as well as morphologically distinct. Six of nine volunteers who developed diarrhea after challenge with an O139:H28 ETEC strain bearing CS1 and CS3 had significant serological rises to purified CS1 and CS3 antigens, suggesting that both antigens are elaborated in vivo, play a role in pathogenesis, and stimulate an immune response.     

Hyperadhesive mutant of type 1-fimbriated Escherichia coli associated with formation of FimH organelles (fimbriosomes).

The relationships of the genes and gene-products mediating D-mannose-specific attachment of type 1 fimbriae of Escherichia coli to eucaryotic cells were investigated by deletion mutation analysis of recombinant plasmid pSH2, which carries the genetic information for the synthesis and expression of functional type 1 fimbriae. Mutant pUT2004 was derived by a deletion remote from the structural gene encoding the 17-kilodalton (kDa) subunit protein of type 1 fimbriae. Phenotypically, the mutant demonstrated an eightfold-higher mannose-specific hemagglutination titer than the parent strain. On electron microscopy, the mutant strain expressed the same number of fimbriae as the parent strain. However, numerous 10-nm-diameter rounded structures (fimbriosomes) were observed both closely associated with fimbriae and in the culture medium. Fimbriosomes isolated from the medium agglutinated guinea pig erythrocytes in a mannose-sensitive manner. Dissociation of the fimbriosomes yielded a single 29-kDa protein, as demonstrated by sodium dodecyl sulfate gel electrophoresis. Antibodies raised against fimbriosomes reacted with a 29-kDa protein on immunoelectroblots of dissociated type 1 fimbriae and also blocked the adherence of other strains of type 1 fimbriated E. coli to eucaryotic cells. These findings suggest that the enhanced adhesive properties of the mutant pUT2004 strain are associated with overproduction of the 29-kDa FimH in the form of fimbriosomes which contain the determinant of the D-mannose-sensitive adhesion of type 1 fimbriae.     

Fimbriae and determination of host species specificity of Bordetella bronchiseptica.

A monoclonal antibody, designated CF8 and prepared against fimbrial protein enrichments of Bordetella bronchiseptica 110H, was determined by immunogold electron microscopy to bind to some but not all fimbrial filaments on intact bacterial cells. Comparison of the reactivity of this antibody with that of monoclonal antibody BPF2, which is specific for Bordetella pertussis serotype 2 fimbriae, indicated that CF8 recognizes an epitope similar to that recognized by BPF2. By Western blot (immunoblot), it was determined that monoclonal antibody CF8 does not react with proteins denatured by treatment with sodium dodecyl sulfate and beta-mercaptoethanol and by boiling for 5 min but that it does recognize fimbrial proteins in their native, nondenatured state. This antibody was used to compare fimbriae between strains of B. bronchiseptica isolated from different species. Strains from pigs, dogs, guinea pigs, and four other species were compared by an enzyme immunoassay. Strains isolated from pigs were found to express significantly more CF8-reactive and B. pertussis serotype 2 cross-reactive fimbriae than strains isolated from guinea pigs. Strains from dogs were more variable in reactivity than those from pigs or guinea pigs. The reactivity with antifimbrial monoclonal antibody CF8 did not correlate with enzyme electromorphotype but did correlate with the host species, suggesting a role for fimbriae in the determination of host species specificity of B. bronchiseptica.     

Adhesion of enterotoxigenic Escherichia coli to human small intestinal enterocytes.

An improved enterocyte adhesion assay has been used to examine a collection of 44 strains of enterotoxigenic Escherichia coli (ETEC) for their ability to adhere to the brush border of isolated human duodenal enterocytes. Fourteen strains showed good adhesion; in each case the ability to adhere correlated with the production of colonization factor antigen I or II (CFA/I or CFA/II) fimbriae. CFA/II-positive producing coli surface antigens 1 and 3 (CS1 and CS3), coli surface antigens 2 and 3 (CS2 and CS3), and only coli surface antigen 3 (CS3) each showed good adhesion. CS3-mediated brush border attachment of CFA/II-positive ETEC was demonstrated by electron microscopy with monospecific antibody and an immunogold labeling technique. One CFA/I-positive ETEC strain was nonadherent in the assay, as were ETEC producing type 1 somatic fimbriae. Five animal ETEC strains producing K88, K99, F41, and 987P fimbriae were slightly more adhesive than control strains, but adhesion was significantly less than that of CFA-positive ETEC. Twenty five human ETEC strains that lacked CFA/I and CFA/II were nonadherent, suggesting either that the surface antigens responsible for adhesion to human intestinal mucosa in these strains were not being produced or that mucosal receptors for these strains are present in regions of the small intestine other than the duodenum.     

Role of Surface Fimbriae (Fibrils) in the Adsorption of Actinomyces Species to Saliva-Treated Hydroxyapatite Surfaces

We studied the adsorption, morphological, and serological characteristics of selected Actinomyces and related species. Evaluation of uranyl acetate-stained cells by electron microscopy revealed wide variations among strains in the frequency of surface fimbriae. These variations did not always correlate with the percent adsorption to saliva-treated hydroxyapatite of the various Actinomyces strains. However, two strains of Rothia dentocariosa possessing no surface fimbriae and five strains of A. israelii possessing very few surface fimbriae exhibited feeble adsorption to saliva-treated hydroxyapatite. Although the calculated number of adsorption sites on saliva-treated hydroxypatite did not vary widely among the strains tested, significant differences were observed in the affinities calculated for some species or serotypes. The mean affinities for strains of A. viscosus serotype 2 and A. naeslundii serotype 3 were similar, and these strains adsorbed well to saliva-treated hydroxyapatite. The mean adsorption and affinity for the A. naeslundii strain serotype 1 and all strains of A. israelii tested were significantly less than those determined for the A. viscosus serotype 2 or A. naeslundii serotype 3 strains. Adsorption inhibition activity of antiserum to strain T14V, previously shown to be solely related to antibodies in immune serum directed against the VA1 fimbria (fibril) antigen, was removed by preadsorption of the antiserum with most A. viscosus and A. naelundii strains, but not with A. israelii strains. This suggests some cross-reactivity among strains of A. viscosus and A. naeslundii but not A. israelii. Adsorption to saliva-treated hydroxyapatite of all A. viscosus and A. naeslundii strains tested was strongly inhibited by fimbriae isolated from A. viscosus strain T14V. Collectively, these data suggest that the adsorption of certain A. viscosus and A. naeslundii strains is mediated by surface fimbriae, many of which appear serologically cross-reactive with strain T14V fimbriae.