Novel class of mutations of pilS mutants, encoding plasmid R64 type IV prepilin: interface of PilS-PilV interactions

Systematic investigation of recipient cell genetic requirements reveals important surface receptors for conjugative transfer of IncI2 plasmids

Bacterial conjugation is a major horizontal gene transfer mechanism. While the functions encoded by many conjugative plasmids have been intensively studied, the contribution of recipient chromosome-encoded genes remains largely unknown. Here, we analyzed the genetic requirement of recipient cells for conjugation of IncI2 plasmid TP114, which was recently shown to transfer at high rates in the gut microbiota. We performed transfer assays with ~4,000 single-gene deletion mutants of Escherichia coli. When conjugation occurs on a solid medium, we observed that recipient genes impairing transfer rates were not associated with a specific cellular function. Conversely, transfer assays performed in broth were largely dependent on the lipopolysaccharide biosynthesis pathway. We further identified specific structures in lipopolysaccharides used as recipient cell surface receptors by PilV adhesins associated with the type IVb accessory pilus of TP114. Our strategy is applicable to study other mobile genetic elements and understand important host cell factors for their dissemination.

The Type IV Pilus of Plasmid TP114 Displays Adhesins Conferring Conjugation Specificity and Is Important for DNA Transfer in the Mouse Gut Microbiota

Type IV pili (T4P) are common bacterial surface appendages involved in different biological processes such as adherence, motility, competence, pathogenesis, and conjugation. In this work, we describe the T4P of TP114, an IncI2 enterobacterial conjugative plasmid recently shown to disseminate at high rates in the mouse intestinal tract. This pilus is composed of the major PilS and minor PilV pilins that are both important for conjugation in broth and in the gut microbiota but not on a solid support. The PilV-coding sequence is part of a shufflon and can bear different C-terminal domains. The shufflon is a multiple DNA inversion system containing many DNA cassettes flanked by recombination sites that are recognized by a shufflon-specific tyrosine recombinase (shufflase) promoting the recombination between DNA segments. The different PilV variants act as adhesins that can modify the affinity for different recipient bacteria. Eight PilV variants were identified in TP114, including one that has not been described in other shufflons. All PilV variants allowed conjugative transfer with different recipient Escherichia coli strains. We conclude that the T4P carried by TP114 plays a major role in mating pair stabilization in broth as well as in the gut microbiota and that the shufflon acts as a biological switch modifying the conjugative host range specificity. IMPORTANCE Conjugative plasmids are involved in horizontal gene transfer in the gut microbiota, which constitutes an important antibiotic resistance gene reservoir. However, the molecular mechanisms used by conjugative plasmids to select recipient bacteria and transfer at high rates in the mouse gut microbiota remain poorly characterized. We studied the type IV pilus carried by TP114 and demonstrated that the minor pilin PilV acts as an adhesin that can efficiently select target cells for conjugative transfer. Moreover, the pilV gene can be rapidly modified by a shufflon, hence modulating the nature of the recipient bacteria during conjugation. Our study highlights the role of mating pair stabilization for conjugation in broth as well as in the gut microbiome and explains how the host spectrum of a plasmid can be expanded simply by remodeling the PilV adhesin.

Incompatibility Group I1 (IncI1) Plasmids: Their Genetics, Biology, and Public Health Relevance

Bacterial plasmids are extrachromosomal genetic elements that often carry antimicrobial resistance (AMR) genes and genes encoding increased virulence and can be transmissible among bacteria by conjugation. One key group of plasmids is the incompatibility group I1 (IncI1) plasmids, which have been isolated from multiple Enterobacteriaceae of food animal origin and clinically ill human patients. The IncI group of plasmids were initially characterized due to their sensitivity to the filamentous bacteriophage If1. Two prototypical IncI1 plasmids, R64 and pColIb-P9, have been extensively studied, and the plasmids consist of unique regions associated with plasmid replication, plasmid stability/maintenance, transfer machinery apparatus, single-stranded DNA transfer, and antimicrobial resistance. IncI1 plasmids are somewhat unique in that they encode two types of sex pili, a thick, rigid pilus necessary for mating and a thin, flexible pilus that helps stabilize bacteria for plasmid transfer in liquid environments. A key public health concern with IncI1 plasmids is their ability to carry antimicrobial resistance genes, including those associated with critically important antimicrobials used to treat severe cases of enteric infections, including the third-generation cephalosporins. Because of the potential importance of these plasmids, this review focuses on the distribution of the plasmids, their phenotypic characteristics associated with antimicrobial resistance and virulence, and their replication, maintenance, and transfer.

Neglected but amazingly diverse type IVb pili

This review provides an overview of current knowledge concerning type IVb pili in Gram-negative bacteria. The number of these pili identified is steadily increasing with genome sequencing and mining studies, but studies of these pili are somewhat uneven, because their expression is tightly regulated and the signals or regulators controlling expression need to be identified. However, as illustrated here, they have a number of interesting functional, assembly-related and regulatory features.