Lactococcus garvieae carries a chromosomally encoded pentapeptide repeat protein that confers reduced susceptibility to quinolones in Escherichia coli producing a cytotoxic effect

Mechanisms of Resistance to the Contact-Dependent Bacteriocin CdzC/D in Caulobacter crescentus

The Cdz bacteriocin system allows the aquatic oligotrophic bacterium Caulobacter crescentus to kill closely related species in a contact-dependent manner. The toxin, which aggregates on the surfaces of producer cells, is composed of two small hydrophobic proteins, CdzC and CdzD, each bearing an extended glycine-zipper motif, that together induce inner membrane depolarization and kill target cells. To further characterize the mechanism of Cdz delivery and toxicity, we screened for mutations that render a target strain resistant to Cdz-mediated killing. These mutations mapped to four loci, including a TonB-dependent receptor, a three-gene operon (named zerRAB for zipper envelope resistance), and perA (for pentapeptide envelope resistance). Mutations in the zerRAB locus led to its overproduction and to potential changes in cell envelope composition, which may diminish the susceptibility of cells to Cdz toxins. The perA gene is also required to maintain a normal cell envelope, but our screen identified mutations that confer resistance to Cdz toxins without substantially affecting the cell envelope functions of PerA. We demonstrate that PerA, which encodes a pentapeptide repeat protein predicted to form a quadrilateral β-helix, localizes primarily to the outer membrane of cells, where it may serve as a receptor for the Cdz toxins. Collectively, these results provide new insights into the function and mechanisms of an atypical, contact-dependent bacteriocin system.IMPORTANCE Bacteriocins are commonly used by bacteria to kill neighboring cells that compete for resources. Although most bacteriocins are secreted, the aquatic, oligotrophic bacterium Caulobacter crescentus produces a two-peptide bacteriocin, CdzC/D, that remains attached to the outer membranes of cells, enabling contact-dependent killing of cells lacking the immunity protein CdzI. The receptor for CdzC/D has not previously been reported. Here, we describe a genetic screen for mutations that confer resistance to CdzC/D. One locus identified, perA, encodes a pentapeptide repeat protein that resides in the outer membrane of target cells, where it may act as the direct receptor for CdzC/D. Collectively, our results provide new insight into bacteriocin function and diversity.

Cytotoxic Effect Associated with Overexpression of QNR Proteins in Escherichia coli

OBJECTIVE

The objective was to evaluate the cytotoxic effect associated with overexpression of multiple Qnr-like plasmid-mediated quinolone resistance (PMQR) mechanisms in Escherichia coli.

METHODS

Coding regions of different PMQR genes (qnrA1, qnrB1, qnrC, qnrD1, qnrS1, and qepA2) and efsqnr were cloned into pET29a(+) vector and overexpressed in E. coli BL21. E. coli BL21 with and without an empty pET29a(+) vector were used as controls. The cytotoxic effect associated with PMQR mechanism overexpression was determined by transmission electron microscopy and viability assays.

RESULTS

Overexpressed qnr genes produced loss of bacterial viability in the range of 77-97% compared with the controls, comparable with loss of viability associated with EfsQnr overexpression (97%). No loss of viability was observed in E. coli overexpressing QepA2. In transmission electron microscopy assays, signs of cytotoxicity were observed in E. coli cells overexpressing EfsQnr and Qnr proteins (30-45% of the bacterial population showed morphological changes). Morphological changes were observed in less than 5% of bacterial populations from the control strains and E. coli overexpressing QepA2.

CONCLUSIONS

Overexpression of qnr genes produces a cytotoxic cellular and structural effect in E. coli, the magnitude of which varies depending on the family of Qnr proteins.

Lactococcus garvieae: an emerging bacterial pathogen of fish

Lactococcus garvieae is the causative agent of lactococcosis, a hyperacute, haemorrhagic septicaemia of fish. This bacterium is also considered an emerging zoonotic pathogen, as reports of human infection are increasing. Significant economic loss in aquaculture is suffered as a result of lactococcosis, as numerous freshwater and marine species of commercial interest are affected. Development of antibiotic resistance in L. garvieae to several chemotherapeutic agents complicates and restricts treatment options. Effective, sustainable treatment and prevention options are thus needed, but progress is impeded by the lack of knowledge concerning several aspects of the disease and the pathogen. This review aims to present the latest research on L. garvieae, with specific focus on pathogenesis, virulence factors, risks associated with chemotherapeutic administration and possible control options.