The sex pheromone system of Enterococcus faecalis. More than just a plasmid-collection mechanism?

Abstract

The sex pheromone system of Enterococcus faecalis was discovered by observing a clumping reaction of E. faecalis strains during conjugative transfer of plasmids. It was found that only a special type of E. faecalis plasmids, the so-called sex pheromone plasmids, are transferred via this mechanism. Various experiments, especially by the group of D. B. Clewell, led to the formulation of a model describing how the sex pheromone system works. Small linear peptides, the so-called sex pheromones, are excreted by strains not possessing the corresponding sex pheromone plasmid. Donor strains harboring the plasmid do not produce the corresponding sex pheromone; they react to the presence of the peptide by production of a plasmid-encoded adhesin, the so-called aggregation substance. This adhesin allows contact between the non-motile mating partners; after conjugative transfer of the plasmid, the former recipient possesses and replicates the new plasmid. Thereby the population of E. faecalis strains is shifted to a high percentage of donor strains. This is especially true because a donor strain will still excrete sex pheromones corresponding to plasmids it does not harbor; therefore, such a strain can also function as recipient for other sex pheromone plasmids it does not possess. Various aspects of this unique plasmid collection mechanism have been studied during the last few years. The data indicate that, with the exception of pAM373, all sex pheromone plasmids possess one DNA region which is highly similar to and codes for the adhesin. It is also becoming more and more clear that regulatory functions/proteins are not conserved between different sex pheromone plasmids. Induction of adhesin synthesis needs the action of a regulatory cascade composed of unique features; at the moment we are just beginning to understand this cascade. By sequencing the first structural gene for one of those adhesins, we realized that the aggregation substance might act also as an adhesin for eucaryotic cells, probably by interaction with integrins. At least in the case of the in vitro cultured pig kidney tubulus cell line LLC-PK1 this idea could be verified. An interesting aspect of the sex pheromone system of E. faecalis is its evolution. I will discuss the idea that two different components, both of which well might contribute to virulence of the opportunistic pathogenic bacterium, were combined in the species E. faecalis to result in this unique plasmid collection system.