Pili in Gram-positive pathogens

Non-flagellar appendages were first observed in Gram-negative bacteria, in the early 1950s. Since then, these structures — known as pili or fimbriae — have been extensively studied. The best characterized are the type I pili, type IV pili and curli pili. All three pilus types are formed by the non-covalent association of pilin subunits into polymeric structures.Pilus-like structures were first detected in the Gram-positive species Corynebacterium renale, in the late 1960s. Subsequently, pili were detected on the surface of other Gram-positive bacteria, including Actinomyces naeslundii, Corynebacterium diphtheriae, Streptococcus parasanguis, Streptococcus salivarius and Streptococcus sanguis. Most recently, in the past year, pili have also been characterized in all three of the principal streptococcal pathogens that cause invasive disease in humans — group A Streptococcus (GAS), group B Streptococcus (GBS) and Streptococcus pneumoniae.A general feature of the Gram-positive pili identified to date is that they comprise three protein subunits, each of which contains an LPXTG amino-acid motif (or a variant of this motif), which is the target of sortase enzymes. Several reports have shown that the pilin subunits are connected by non-disulphide covalent linkages.Pilus assembly in Gram-positives has been most extensively studied in C. diphtheriae. The first step involves the Sec-dependent secretion of the three pilus components, which remain anchored to the cell membrane. The second step is a sortase-dependent reaction in which the cell-anchored proteins are cleaved at the LPXTG motif, forming acyl-enzyme intermediates containing a covalent thioester bond between the sortase enzyme and the pilus subunit. The third and fourth steps involve the oligomerization of the pilus protein subunits and the anchoring of this oligomerized structure to the cell wall. According to this model, pilus growth occurs by the addition of subunits to the base of the pilus. It is worth noting that an alternative model can be envisaged in which pilus growth occurs by the addition of subunits to the top of the growing pilus, although this model would require energy to promote pilus bending, and at present, there are no clues how this energy could be provided.The three pilus components are the backbone or main pilus protein and two ancillary proteins, AP1 and AP2. Two schemes can be envisaged for the incorporation of the ancillary proteins into the pilus structure. The first hypothesis suggests that the ancillary proteins are incorporated into the pilus backbone in the same way as the main subunit is incorporated, with each ancillary protein preceded and followed by the main pilus subunit, and the second hypothesis suggests that ancillary proteins are 'branches' of the pilus backbone.In all Gram-positive species examined so far, the genes encoding the pilus subunits are clustered together in the same genetic locus and might have been acquired by horizontal transfer. The organization of this region in GAS, GBS and S. pneumoniae is discussed.In Gram-negative pathogens, pili are involved in adhesion to host cells, and it is likely that in Gram-positive pathogens pili have a similar role. The authors propose a model for the pilus-mediated adherence of streptococci to cell surfaces.Finally, the authors discuss the potential applications of Gram-positive pili as vaccine candidates.