DNA inversion on conjugative plasmid pVT745

Current and Emerging Diagnostic Approaches to Bacterial Diseases of Ruminants

The diagnostic approaches and methods to detect bacterial pathogens in ruminants are discussed, with a focus on cattle. Conventional diagnostic methods using culture, isolation, and characterization are being replaced or supplemented with new methods. These include molecular diagnostics such as real-time polymerase chain reaction and whole-genome sequencing. In addition, methods such as matrix-assisted laser desorption ionization-time-of-flight mass spectrometry enable rapid identification and enhanced pathogen characterization. These emerging diagnostic tools can greatly enhance the ability to detect and characterize pathogens, but performance and interpretation vary greatly across sample and pathogen types, disease syndromes, assay performance, and other factors.

Mapping of the nick site on conjugative plasmid pVT745 by interrupted mating

Conjugal transfer of plasmid DNA initiates and terminates at a specific non-coding site called the origin of transfer (oriT). Previous analysis of conjugative plasmid pVT745 from Aggregatibacter actinomycetemcomitans suggested that oriT was located adjacent to the operon responsible for initiation of ssDNA transfer. The location of oriT was confirmed by assaying both subclones of the region as well as a pVT745 deletion derivative for mobilization. The precise DNA nick site (nic) and polarity of DNA transfer were identified by use of interrupted mating assays, a technique originally used for the mapping of bacterial chromosomes. Nucleotide sequence analysis revealed that the pVT745-specific nick region was similar to the consensus nick sequence of the IncP family albeit the actual cleavage site differed. Functionality of nic was confirmed by point mutations.

Entry exclusion activity on conjugative plasmid pVT745

Conjugative plasmid transfer into a recipient cell containing the same or a closely related plasmid is inhibited by a mechanism called entry or surface exclusion. The function of entry exclusion is to reduce unproductive conjugation. The current study assessed the exclusion activity on conjugal plasmid pVT745 by conducting mating experiments with genetically distinguishable derivatives of this plasmid. Our results demonstrate that a single gene, magB05, that is located in a gene cluster associated with mating pore formation, is responsible for the entry exclusion phenotype of pVT745.

Mpi recombinase globally modulates the surface architecture of a human commensal bacterium

The mammalian gut represents a complex and diverse ecosystem, consisting of unique interactions between the host and microbial residents. Bacterial surfaces serve as an interface that promotes and responds to this dynamic exchange, a process essential to the biology of both symbionts. The human intestinal microorganism, Bacteroides fragilis, is able to extensively modulate its surface. Analysis of the B. fragilis genomic sequence, together with genetic conservation analyses, cross-species cloning experiments, and mutational studies, revealed that this organism utilizes an endogenous DNA inversion factor to globally modulate the expression of its surface structures. This DNA invertase is necessary for the inversion of at least 13 regions located throughout the genome, including the promoter regions for seven of the capsular polysaccharide biosynthesis loci, an accessory polysaccharide biosynthesis locus, and five other regions containing consensus promoter sequences. Bacterial DNA invertases of the serine site-specific recombinase family are typically encoded by imported elements such as phage and plasmids, and act locally on a single region of the imported element. In contrast, the conservation and unique global regulatory nature of the process in B. fragilis suggest an evolutionarily ancient mechanism for surface adaptation to the changing intestinal milieu during commensalism.