Three Microbial Musketeers of the Seas: Shewanella baltica, Aliivibrio fischeri and Vibrio harveyi, and Their Adaptation to Different Salinity Probed by a Proteomic Approach

Osmotic changes are common challenges for marine microorganisms. Bacteria have developed numerous ways of dealing with this stress, including reprogramming of global cellular processes. However, specific molecular adaptation mechanisms to osmotic stress have mainly been investigated in terrestrial model bacteria. In this work, we aimed to elucidate the basis of adjustment to prolonged salinity challenges at the proteome level in marine bacteria. The objects of our studies were three representatives of bacteria inhabiting various marine environments, Shewanella baltica, Vibrio harveyi and Aliivibrio fischeri. The proteomic studies were performed with bacteria cultivated in increased and decreased salinity, followed by proteolytic digestion of samples which were then subjected to liquid chromatography with tandem mass spectrometry analysis. We show that bacteria adjust at all levels of their biological processes, from DNA topology through gene expression regulation and proteasome assembly, to transport and cellular metabolism. The finding that many similar adaptation strategies were observed for both low- and high-salinity conditions is particularly striking. The results show that adaptation to salinity challenge involves the accumulation of DNA-binding proteins and increased polyamine uptake. We hypothesize that their function is to coat and protect the nucleoid to counteract adverse changes in DNA topology due to ionic shifts.

Shewanella baltica strain JD0705 isolated from the mangrove wetland soils in Thailand and characterization of its ligninolytic performance

Chantarasiri A. 2021. Shewanella baltica strain JD0705 isolated from the mangrove wetland soils in Thailand and characterization of its ligninolytic performance. Biodiversitas 22: 354-361. Lignin is a complex biopolymer and the third component by mass of lignocellulosic plant biomass. Its recalcitrant property hampers the hydrolysis and utilization of cellulose and hemicellulose in the lignocellulosic biomass. Thus, the delignification of lignocellulosic biomass is an enormous challenge for emerging bio-based applications. This study presented a potential ligninolytic bacterium for potential use in the biological delignification process under mild conditions. This bacterium was isolated from the mangrove wetland soils in Thailand, characterized and identified as psychrotrophic Shewanella baltica strain JD0705. It was determined for ligninolytic activity and showed laccase activity at 5.23 ± 0.10 U/mL. The optimum temperature and pH for the laccase activity were observed to be 25°C at a pH of 7.0 respectively with a stability range of 20-30°C temperature and pH of 7.0-8.0. S. baltica strain JD0705 was used in the biological delignification of rice husk powder and promoted the hydrolysis of rice husk powder to obtain more liberating sugar content. The findings from this study indicated that feasibility of using this ligninolytic bacterium for the production of laccase and the delignification of lignocellulosic plant biomass.

Ornithine decarboxylation (ODC) system of Shewanella baltica regulates putrescine production and acid resistance

Shewanella baltica , as one of the dominant spoilers in seafoods where they encounter acidic environments during spoilage, can synthesize putrescine from ornithine and cause food spoilage as well as health problems. Here, the ornithine decarboxylation (ODC) pathway composed of ornithine decarboxylases SpeC and SpeF and an ornithine-putrescine transporter PotE were identified in S. baltica by database searches and further by molecular biology operations, and SpeC functioned as an auxiliary adjusting component of ODC system. Ornithine and putrescine were found to promote putrescine accumulation through up-regulating the expression of speF and potE rather than speC . In addition, increased putrescine biosynthesis and alkalization of cytoplasm was detected at acidic pH especially at pH 6.0 compared to neutral pH. Particularly, the maximum up-regulation of ODC genes and the optimum decarboxylation activity of SpeF were detected at acidic pH around 6.0. It’s concluded that the ODC pathway plays dual roles in cytoplasmic acid counteraction and putrescine production of S. baltica . This study contributes to our understanding of the spoilage mechanism of spoilers in the food system, and provides a novel target for seafoods preservation.

Adaptation of the Marine Bacterium Shewanella baltica to Low Temperature Stress

Marine bacteria display significant versatility in adaptation to variations in the environment and stress conditions, including temperature shifts. Shewanella baltica plays a major role in denitrification and bioremediation in the marine environment, but is also identified to be responsible for spoilage of ice-stored seafood. We aimed to characterize transcriptional response of S. baltica to cold stress in order to achieve a better insight into mechanisms governing its adaptation. We exposed bacterial cells to 8 °C for 90 and 180 min, and assessed changes in the bacterial transcriptome with RNA sequencing validated with the RT-qPCR method. We found that S. baltica general response to cold stress is associated with massive downregulation of gene expression, which covered about 70% of differentially expressed genes. Enrichment analysis revealed upregulation of only few pathways, including aminoacyl-tRNA biosynthesis, sulfur metabolism and the flagellar assembly process. Downregulation was observed for fatty acid degradation, amino acid metabolism and a bacterial secretion system. We found that the entire type II secretion system was transcriptionally shut down at low temperatures. We also observed transcriptional reprogramming through the induction of RpoE and repression of RpoD sigma factors to mediate the cold stress response. Our study revealed how diverse and complex the cold stress response in S. baltica is.