Abstract
One of the more conspicuous structural features that punctuate the outer cell surface of certain bacterial Gram-positive genera and species is the sortase-dependent pilus. As these adhesive and variable-length protrusions jut outward from the cell, they provide a physically expedient and useful means for the initial contact between a bacterium and its ecological milieu. The sortase-dependent pilus displays an elongated macromolecular architecture consisting of two to three types of monomeric protein subunits (pilins), each with their own specific function and location, and that are joined together covalently by the transpeptidyl activity of a pilus-specific C-type sortase enzyme. Sortase-dependent pili were first detected among the Gram-positive pathogens and subsequently categorized as an essential virulence factor for host colonization and tissue invasion by these harmful bacteria. However, the sortase-dependent pilus was rebranded as also a niche-adaptation factor after it was revealed that “friendly” Gram-positive commensals exhibit the same kind of pilus structures, which includes two contrasting gut-adapted species from the Lactobacillus genus, allochthonous Lactobacillus rhamnosus and autochthonous Lactobacillus ruminis. This review will highlight and discuss what has been learned from the latest research carried out and published on these lactobacillar pilus types.
Highlights
The extended arm and hand is a useful tool that lets us navigate certain social settings, often as a friendly feeling gesture epitomized by a welcoming handshake, but sometimes as a protective-aggressive response that comes by wielding a clenched fist
While an amino acid sequence alignment of the predicted lrpCBA-encoded proteins from human, bovine, porcine, and equine isolates of L. ruminis showed a high level of shared identity, this was unobserved with alignments of the L. rhamnosus GG spaCBA- and spaFED-encoded pilus proteins [14]
The lrpCBA operon appears to be genome-specific to only L. ruminis, as a BlastP search of the NCBI database using the LrpCBA pilin-proteins revealed no counterparts in any other Lactobacillus species [14]
Summary
The extended arm and hand is a useful tool that lets us navigate certain social settings, often as a friendly feeling gesture epitomized by a welcoming handshake, but sometimes as a protective-aggressive response that comes by wielding a clenched fist. In a mechanistic sense for certain microscopic bacteria, they operate in their surrounding conditions somewhat through the use of long proteinaceous limb-like protrusions that reach out from the periphery of the cell wall envelope These multi-subunit appendages are commonly known as pili (sing., pilus) or fimbriae (sing., fimbria) and can be detected in both a number of Gram-negative and Gram-positive genera and species, with each type having an archetypal polymeric structure held together by either non-covalent or covalent forces, respectively (for detailed review, see [1,2]). A transpeptidase enzyme called the pilus-specific C-type sortase catalyzes the head-to-tail assembly of the pilins into the final polymerized form, which in most instances has the thickness of a single subunit molecule (~2–3 nm) [7] and lacks a complex quaternary organization [8] Surface piliation such as this is generally described as being “sortase-dependent”, the occurrence of which can be found only amongst the Gram-positive bacteria. An emphasis is placed on what new information has been learned concerning the biology of sortase-dependent pili, and as well on how these macromolecular surface features can contribute to gut colonization behavior
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