Universal Stress Proteins Contribute Edwardsiella ictaluri Virulence in Catfish

Versatile lifestyles of Edwardsiella: Free-living, pathogen, and core bacterium of the aquatic resistome

Edwardsiella species in aquatic environments exist either as individual planktonic cells or in communal biofilms. These organisms encounter multiple stresses, include changes in salinity, pH, temperature, and nutrients. Pathogenic species such as E. piscicida, can multiply within the fish hosts. Additionally, Edwardsiella species (E. tarda), can carry antibiotic resistance genes (ARGs) on chromosomes and/or plasmids, that can be transmitted to the microbiome via horizontal gene transfer. E. tarda serves as a core in the aquatic resistome. Edwardsiela uses molecular switches (RpoS and EsrB) to control gene expression for survival in different environments. We speculate that free-living Edwardsiella can transition to host-living and vice versa, using similar molecular switches. Understanding such transitions can help us understand how other similar aquatic bacteria switch from free-living to become pathogens. This knowledge can be used to devise ways to slow down the spread of ARGs and prevent disease outbreaks in aquaculture and clinical settings.

Edwardsiella piscicida: A versatile emerging pathogen of fish

Edwardsiella piscicida is an Enterobacteriaceae that is abundant in water and causes food and waterborne infections in fish, animals, and humans. The bacterium causes Edwardsiellosis in farmed fish and can lead to severe economic losses in aquaculture worldwide. E. piscicida is an intracellular pathogen that can also cause systemic infection. Type III and type VI secretion systems are the bacterium’s most lethal weapons against host defenses. It also possesses multi-antibiotic resistant genes and is selected and enriched in the environment due to the overuse of antibiotics. Therefore, the bacterium has great potential to contribute to the evolution of the resistome. All these properties have made this bacterium a perfect model to study bacteria virulence mechanisms and the spread of antimicrobial genes in the environment. We summarize recent advance in E. piscicida biology and provide insights into future research in virulence mechanisms, vaccine development and novel therapeutics.

Universal stress proteins contribute Edwardsiella piscicida adversity resistance and pathogenicity and promote blocking host immune response

Universal stress proteins (Usps) exist ubiquitously in bacteria and other organisms. Usps play an important role in adaptation of bacteria to a variety of environmental stresses. There is increasing evidence that Usps facilitate pathogens to adapt host environment and are involved in pathogenicity. Edwardsiella piscicida (formerly included in E. tarda) is a severe fish pathogen and infects various important economic fish including tilapia (Oreochromis niloticus). In E. piscicida, a number of systems and factors that are involved in stress resistance and pathogenesis were identified. However, the function of Usps in E. piscicida is totally unknown. In this study, we examined the expressions of 13 usp genes in E. piscicida and found that most of these usp genes were up-regulated expression under high temperature, oxidative stress, acid stress, and host serum stress. Particularly, among these usp genes, usp13, exhibited dramatically high expression level upon several stress conditions. To investigate the biological role of usp13, a markerless usp13 in-frame mutant strain, TX01Δusp13, was constructed. Compared to the wild type TX01, TX01Δusp13 exhibited markedly compromised tolerance to high temperature, hydrogen peroxide, and low pH. Deletion of usp13 significantly retarded bacterial biofilm growth and decreased resistance against serum killing. Pathogenicity analysis showed that the inactivation of usp13 significantly impaired the ability of E. piscicida to invade into host cell and infect host tissue. Introduction of a trans-expressed usp13 gene restored the lost virulence of TX01Δusp13. In support of these results, host immune response induced by TX01 and TX01Δusp13 was examined, and the results showed reactive oxygen species (ROS) levels in TX01Δusp13-infected macrophages were significantly higher than those in TX01-infected cells. The expression level of several cytokines (IL-6, IL-8, IL-10, TNF-α, and CC2) in TX01Δusp13-infected fish was significantly higher than that in TX01-infected fish. These results suggested that the deletion of usp13 attenuated the ability of bacteria to overcome the host immune response to pathogen infection. Taken together, our study indicated Usp13 of E. piscicida was not only important participant in adversity resistance, but also was essential for E. piscicida pathogenicity and contributed to block host immune response to pathogen infection.