A frameshift in Yersinia pestis rcsD alters canonical Rcs signalling to preserve flea-mammal plague transmission cycles

The CpxAR signaling system confers a fitness advantage for flea gut colonization by the plague bacillus

The adaptation of Yersinia pestis, the flea-borne plague agent, to fluctuating environmental conditions is essential for the successful colonization of the flea vector. A previous comparative transcriptomic analysis showed that the Cpx pathway of Y. pestis is up-regulated in infected fleas. The CpxAR two-component system is a component of the envelope stress response and is critical for maintaining the integrity of the cell. Here, a phenotypic screening revealed a survival defect of the cpxAR mutant to oxidative stress and copper. The measured copper concentration in the digestive tract contents of fed fleas increased fourfold during the digestive process. By direct analysis of phosphorylation of CpxR by a Phos-Tag gel approach, we demonstrated that biologically relevant concentrations of copper triggered the system. Then, a competitive challenge highlighted the role of the CpxAR system in bacterial fitness during flea infection. Lastly, an in vitro sequential exposure to copper and then H2O2 to mimic the flea suggests a model in which, within the insect digestive tract, the CpxAR system would be triggered by copper, establishing an oxidative stress response.

IMPORTANCE

The bacterium Yersinia pestis is the agent of flea-borne plague. Our knowledge of the mechanisms used by the plague bacillus to infect the flea vector is limited. The up-regulation of the envelope stress response under the control of the Cpx signaling pathway was previously shown in a transcriptomic study. Here, our in vivo and in vitro approaches suggest a model in which Y. pestis uses the CpxAR phosphorelay system to sense and respond to the copper present in the flea gut, thereby optimizing the flea gut colonization. In other words, the system is essential for bacterial fitness in the flea.

Novel Single Nucleotide Variations Alter Pathogenicity in Korean Isolates of Erwinia amylovora

Erwinia amylovora, the causal agent of fire blight disease, has become a serious threat to the pome fruit industry in Korea since 2015. In this study, we showed that two new isolates of E. amylovora, Ea17-2187 and Ea19-7, obtained from pear orchards in Anseong, Korea, exhibited unique pathogenicity compared with other isolates thus far. Both were nonpathogenic to immature apple fruits but occasionally caused disease on immature pear fruits at varying reduced rates. Bioinformatic analyses revealed that their genomes are highly similar to those of the type strains TS3128 and ATCC49946 but have different mutations in essential virulence regulatory genes. Ea17-2187 has a single nucleotide substitution in rcsC, which encodes the core components of the Rcs system that activates the exopolysaccharide amylovoran production. In contrast, Ea19-7 contains a single nucleotide insertion in hrpL, which encodes a master regulator of the type III secretion system. In both cases, the mutation can cause premature termination and production of truncated gene products, disrupting virulence regulation. Introduction of the nonmutated rcsC and hrpL genes into Ea17-2187 and Ea19-7, respectively, fully recovered pathogenicity, comparable with that of TS3128; hence, these mutations were responsible for the altered pathogenicity observed. Interestingly, virulence assays on immature pear fruits showed that the hrpL mutant of Ea19-7 was still pathogenic, although its virulence level was markedly reduced. Taken together, these results suggest that the two new isolates might act as opportunistic pathogens or cheaters and that some Korean isolates might have evolved to acquire alternative pathways for activating pathogenicity factors.

Characterization of an aspartate aminotransferase encoded by YPO0623 with frequent nonsense mutations in Yersinia pestis

Yersinia pestis, the causative agent of plague, is a genetically monomorphic bacterial pathogen that evolved from Yersinia pseudotuberculosis approximately 7,400 years ago. We observed unusually frequent mutations in Y. pestis YPO0623, mostly resulting in protein translation termination, which implies a strong natural selection. These mutations were found in all phylogenetic lineages of Y. pestis, and there was no apparent pattern in the spatial distribution of the mutant strains. Based on these findings, we aimed to investigate the biological function of YPO0623 and the reasons for its frequent mutation in Y. pestis. Our in vitro and in vivo assays revealed that the deletion of YPO0623 enhanced the growth of Y. pestis in nutrient-rich environments and led to increased tolerance to heat and cold shocks. With RNA-seq analysis, we also discovered that the deletion of YPO0623 resulted in the upregulation of genes associated with the type VI secretion system (T6SS) at 26°C, which probably plays a crucial role in the response of Y. pestis to environment fluctuations. Furthermore, bioinformatic analysis showed that YPO0623 has high homology with a PLP-dependent aspartate aminotransferase in Salmonella enterica, and the enzyme activity assays confirmed its aspartate aminotransferase activity. However, the enzyme activity of YPO0623 was significantly lower than that in other bacteria. These observations provide some insights into the underlying reasons for the high-frequency nonsense mutations in YPO0623, and further investigations are needed to determine the exact mechanism.