In Vivo Induction of Virulence and Antibiotic Resistance Transfer in Enterococcus faecalis Mediated by the Sex Pheromone-Sensing System of pCF10

Abstract

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 ؋ 10 ؊2 to 9 ؋ 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 pheromonesensing 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. Aggregation substance (AS) is a 137-kDa surface protein encoded on sex pheromone plasmids of Enterococcus faecalis.This protein mediates strong binding between E. faecalis cells as manifested by the formation of visible cell aggregates preceding the conjugative transfer of a sex pheromone plasmid (14). The plasmid transfer response is initiated by 7-to 8-amino-acid-long hydrophobic peptides secreted by potential recipient cells (16,32). These peptides are part of signal sequences of chromosomally encoded lipoproteins (10) and lead to induction of AS expression.Asc10, the AS of the sex pheromone plasmid pCF10 (encoding tetracycline resistance) (17), is induced by the 7-aminoacid peptide cCF10 (32) encoded by the ccfA gene. The donor cell binds the peptide (37), employing a specific plasmid-encoded receptor, PrgZ, an OppA homologue (Fig. 1). The peptide is then transported into the cell via the oligopeptide permease system (29), leading to the expression of AS and subsequent plasmid transfer. Since expression of AS and the other plasmid transfer machinery is presumably a very energyconsuming process, it is not surprising that expression of AS is tightly regulated. This ensures that the nonmotile enterococcal cells expressing transfer functions are in close proximity for cell contact to occur between mating partners. pCF10 donor cells are faced with the dilemma that they secrete unaltered amounts of cCF10 (33), necessitating a complex scheme to prevent autoinduction by their own pheromones. A schematic overview of the events controlling AS expression is given in Fig. 1. Secreted cCF10 may accumulate to approximately 8 pg/ml (33). To...

Enterococcus faecalis strains show culture heterogeneity in cell surface charge

“…Six out of twelve E. faecalis isolated from clogged biliary stents were Agg-positive (Donelli et al, 2004). Expression of Agg enhanced the adhesion of E. faecalis strains to bile drain materials through positive cooperativity (Waar et al, 2002c) and it has been shown that Agg is expressed in vivo (Hirt et al, 2002). However, in the present study, only one of the six isolates reacted to pheromones indicating the presence of an Agg, and no correlation between the presence or absence of Agg and heterogeneity in zeta potential was present.…”

Broad host range plasmids can invade an unexpectedly diverse fraction of a soil bacterial community

“…and thus sharing a high genomic similarity. The reasons behind this are uncertain, but certain strains might have distinct abilities to achieve efficient cell-to-cell contact with a specific recipient, for example, through specific mating mediating pheromones (Hirt, 2002). Earlier studies have shown that plasmid exchange between two taxonomically different species can exceed intraspecies transfer frequencies (Bingle et al, 2003), proving that the regulatory interactions of donor, recipient and plasmid can influence transfer efficiency.…”

Pheromone-inducible conjugation in Enterococcus faecalis: A model for the evolution of biological complexity?

“…In addition to promoting conjugative plasmid transfer, pheromone induction also increases the virulence of the plasmid-containing bacteria. In particular, the expression of Asc10 increases adherence and internalization of mammalian host cells, resistance to phagocytosis and production of large cardiac vegetations in experimental endocarditis (Hirt et al, 2002). Highfrequency conjugation and Asc10 expression are induced through the pheromone-sensing system when E. faecalis cells carrying pCF10 are grown in an animal host, or in plasma (Hirt et al, 2002).…”

“…In particular, the expression of Asc10 increases adherence and internalization of mammalian host cells, resistance to phagocytosis and production of large cardiac vegetations in experimental endocarditis (Hirt et al, 2002). Highfrequency conjugation and Asc10 expression are induced through the pheromone-sensing system when E. faecalis cells carrying pCF10 are grown in an animal host, or in plasma (Hirt et al, 2002). Recent evidence indicates that in vivo induction is mediated by the endogenous pheromone produced by the donor cells themselves when a mammalian host component sequesters or degrades the iCF10 produced by these same cells (Chandler et al, 2005b).…”