Monoclonal antibodies to cell surface antigens involved in sex pheromone induced mating in Streptococcus faecalis

In vivo induction of virulence and antibiotic resistance transfer in Enterococcus faecalis mediated by the sex pheromone-sensing system of pCF10

Enterococcus faecalis has become one of the most notable nosocomial pathogens in the last decade. Aggregation substance (AS) on the sex pheromone plasmids of E. faecalis has been implicated as a virulence factor in several model systems. We investigated the AS-encoding plasmid pCF10 for its ability to increase virulence in a rabbit endocarditis model. Cells containing pCF10 increased the virulence in the model significantly, as assessed by an increase in aortic valve vegetation size. The results confirmed in vivo induction of the normally tightly controlled AS. In addition to the expression of AS when E. faecalis cells were in contact with plasma, plasmid transfer of the tetracycline resistance-carrying plasmid was also activated in vitro and in vivo. In vivo, plasmid transfer reached remarkable frequencies of 8 x 10(-2) to 9 x 10(-2). These values are comparable to the highest frequencies ever observed in vitro. Cells harboring pCF10 had a significant survival advantage over plasmid-free cells indicated by pCF10 present in two-thirds of the recipient population. Plasma induction was dependent on the presence of the plasmid-encoded PrgZ protein, indicating the requirement of the pheromone-sensing system in the induction process. The data suggested that the mechanism of in vivo induction may involve interference of plasma with the normal function of the pheromone peptide and its inhibitor.

Antibodies to a surface-exposed, N-terminal domain of aggregation substance are not protective in the rabbit model of Enterococcus faecalis infective endocarditis

The aggregation substance (AS) surface protein from Enterococcus faecalis has been implicated as an important virulence factor for the development of infective endocarditis. To evaluate the role of antibodies specific for Asc10 (the AS protein from the conjugative plasmid pCF10) in protective immunity to infective endocarditis, an N-terminal region of Asc10 lacking the signal peptide and predicted to be surface exposed (amino acids 44 to 331; AS(44-331)) was cloned with a C-terminal histidine tag translational fusion and expressed from Escherichia coli. N-terminal amino acid sequencing of the purified protein revealed the correct sequence, and rabbit polyclonal antisera raised against AS(44-331) reacted specifically to Asc10 expressed from E. faecalis OG1SSp, but not to other proteins as judged by Western blot analysis. Using these antisera, flow cytometry analysis demonstrated that antibodies to AS(44-331) bound to a surface-exposed region of Asc10. Furthermore, antibodies specific for AS(44-331) were opsonic for E. faecalis expressing Asc10 in vitro but not for cells that did not express Asc10. New Zealand White rabbits immunized with AS(44-331) were challenged intravenously with E. faecalis cells constitutively expressing Asc10 in the rabbit model of experimental endocarditis. Highly immune animals did not show significant differences in clearance of organisms from the blood or spleen or in formation of vegetations on the aortic valve, in comparison with nonimmune animals. Although in vivo expression of Asc10 was demonstrated by immunohistochemistry, these experiments provide evidence that immunity to Asc10 does not play a role in protection from experimental infective endocarditis due to E. faecalis and may have important implications for the development of immunological approaches to combat enterococcal endocarditis.

High-resolution visualization by field emission scanning electron microscopy of Enterococcus faecalis surface proteins encoded by the pheromone-inducible conjugative plasmid pCF10

Enterococcus faecalis can acquire antibiotic resistance and virulence genes by transfer of pheromone-inducible conjugative plasmids such as pCF10, which encodes tetracycline resistance. Two pCF10-encoded cell surface proteins, Sec10 and Asc10, have been previously shown to play an important role in the transfer of this plasmid. We used high-resolution, field emission scanning electron microscopy to visualize these proteins on the surfaces of a series of isogenic strains of E. faecalis. Immunogold labeling, using both 6- and 12-nm colloidal gold, unambiguously demonstrated the expression and distribution of Sec10 and Asc10 on the surface of the E. faecalis cells. On unlabeled E. faecalis cells which expressed either Sec10 or Asc10, the former appeared to be more readily detected. Immunogold labeling of E. faecalis cells expressing both Asc10 and Sec10 clearly demonstrated the abundance and intermixing of both proteins on the cell surface except at septal regions. Sec10 was observed to be distributed over the cell surface. At regions of cell-cell contact, fine strands representing Asc10 were observed directly attaching adjacent cells to one another.

Role of the pheromone-inducible surface protein Asc10 in mating aggregate formation and conjugal transfer of the Enterococcus faecalis plasmid pCF10

The high transfer frequency of pheromone-inducible conjugative plasmids of Enterococcus faecalis in liquid culture is due in part to the formation of mating aggregates. These aggregates result from the interaction of two surface components, aggregation substance (AS), which is plasmid encoded, and the chromosomally encoded binding substance (BS). In the accompanying paper (S.-M. Kao, S. B. Olmsted, A. S. Viksnins, J.C. Gallo, G. M. Dunny, J. Bacteriol, 173:7650-7664, 1991), the sequence of the prgB gene encoding the AS molecule (Asc10) produced by pheromone-induced cells carrying plasmid pCF10 is presented. Here we report the results of genetic and immunological experiments which define the role of Asc10 in aggregation and plasmid transfer. These data indicate expression of AS on the surface of an E. faecalis cell and its binding to BS expressed on a second cell are required for the formation of a mating pair and the efficient transfer of pCF10 in liquid matings. However, the orientation of the receptors was not critical for transfer; ie., AS expressed on recipient cells could facilitate plasmid transfer via binding to BS on the donor. Our results suggest that additional (as yet unidentified) products are involved in forming the channel that ultimately serves to transfer the DNA, with AS-BS binding serving primarily to generate the initial attachment between cells. The putative prgC gene product, identified by DNA sequencing (data presented in the accompanying paper), could be involved in transfer events occurring subsequent to aggregation.

Identification of aggregation substances of Enterococcus faecalis cells after induction by sex pheromones. An immunological and ultrastructural investigation

The sex pheromone system of Enterococcus faecalis is responsible for the clumping response of a plasmid carrying donor strain with a corresponding plasmid free recipient strain due to the production of sex pheromones by the recipient strain. The clumping response is mediated by a surface material (called aggregation substance) which is synthesized upon addition of sex pheromones to the cultures. Here we show that after induction a dense layer of "hairlike" structures is formed on the cell wall of the bacteria. These hairlike structures are responsible for the cell-cell contact which leads to the aggregation of cells. Formation of these structures was specific, only occurring after the addition of homologous sex pheromone.

Cloning and expression of genes encoding pheromone-inducible antigens of Enterococcus (Streptococcus) faecalis

Fragments, generated by restriction enzyme digestion, of the 58-kilobase Enterococcus (Streptococcus) faecalis tetracycline resistance plasmid pCF10 were cloned and introduced into Escherichia coli and E. faecalis to characterize the pheromone-inducible conjugation system encoded by this plasmid. Western blot (immunoblot) analyses revealed that a 130-kilodalton (kDa) antigen, identical to the Tra130 antigen shown previously to be involved in pCF10-mediated pheromone-inducible surface exclusion, was produced by both bacterial hosts carrying the recombinant plasmid pINY1825 (cloned EcoRI C fragment). Both bacterial hosts carrying pINY1825 also produced various amounts of immunologically related 118- to 125-kDa antigens (designated pre-Tra130) that resembled antigens produced by E. faecalis cells carrying pCF10. An additional 150-kDa antigen, Tra150, probably involved in pheromone-induced cellular aggregation, was produced by Escherichia coli and E. faecalis hosts carrying pINY1801 (cloned EcoRI C and E fragments). The coding sequences for the Tra150 and Tra130 antigens were further localized in the TRA region of pCF10 by transposon insertion mutagenesis. Western blot analyses of the recombinant strains, and of strains carrying derivatives of pCF10 or various recombinant plasmids containing Tn5 or Tn917 insertions, suggested that the portion of pCF10 comprising the tra3 through -6 segments (previously defined by Tn917 insertional mutagenesis) contained several genes that are involved in regulating the synthesis of Tra130 and Tra150.

Identification of a Streptococcus agalactiae protein antigen associated with bovine mastitis isolates

Immunoblotting was used to analyze the immune response of cows to Streptococcus agalactiae. Antibody from the milk of cows immunized (via the superficial inguinal lymph node) with formalinized S. agalactiae cells or from the milk of cows with S. agalactiae mastitis reacted strongly with a group of high-molecular-weight proteinaceous antigens. The two most predominant antigenic polypeptides in this group had apparent molecular weights of 97,000 and 104,000. Because the data indicated that these two antigens, as well as several minor antigens sometimes observed in the 70- to 100-kilodalton size range, seemed to be different forms of the same protein, we refer to the entire group as Sas97/104. A monoclonal antibody that was reactive with Sas97/104 was derived and was used to purify the antigen by affinity chromatography. Whole-cell and colony blot enzyme-linked immunoassays with either the monoclonal antibody or a polyclonal serum sample raised against the affinity-purified antigen indicated that this antigen (or cross-reactive proteins with higher molecular weights) is present on the S. agalactiae strains that were freshly isolated from mastitic cows. However, the antigen was not detected in S. agalactiae of human origin, bovine strains of S. agalactiae maintained for a prolonged period in the laboratory, or other streptococci. The data are consistent with the notion that Sas97/104 is a surface antigen and does not correspond to previously described type-specific antigens of group B streptococci.