Loss of the Acetate Switch in Vibrio vulnificus Enhances Predation Defense against Tetrahymena pyriformis

Abstract

Vibrio vulnificus is an opportunistic human pathogen and autochthonous inhabitant of coastal marine environments, where the bacterium is under constant predation by heterotrophic protists or protozoans. As a result of this selection pressure, genetic variants with anti-predation mechanisms are selected for and persist in the environment. Such natural variants may also be pathogenic to animal or human hosts, making it important to understand these defence mechanisms. To identify anti-predator strategies, thirteen V. vulnificus strains of different genotypes isolated from diverse environments were exposed to predation by the ciliated protozoan, Tetrahymena pyriformis , and only strain ENV1 was resistant to predation. Further investigation of the cell-free supernatant showed that ENV1 acidifies the environment by the excretion of organic acids, which is toxic to T. pyriformis . As this predation resistance was dependent on the availability of iron, transcriptomes of V. vulnificus in iron-replete and iron-deplete conditions were compared. This analysis revealed that ENV1 ferments pyruvate and the resultant acetyl-CoA leads to acetate synthesis under aerobic conditions, a hallmark of overflow metabolism. The anaerobic respiration global regulator, arcA , was upregulated when iron was available. An Δ arcA deletion mutant of ENV1 accumulated less acetate and importantly, was sensitive to grazing by T. pyriformis . Based on the transcriptome response and quantification of metabolites, we conclude that ENV1 has adapted to overflow metabolism and has lost a control switch that shifts metabolism from acetate excretion to acetate assimilation, enabling it to excrete acetate continuously. We show that overflow metabolism and the acetate switch contribute to prey-predator interactions. Importance Bacteria in the environment, including Vibrio spp., interact with protozoan predators. To defend against predation, bacteria evolve anti-predator mechanisms ranging from changing morphology, biofilm formation and secretion of toxins or virulence factors. Some of these adaptations may result in strains that are pathogenic to humans. Therefore, it is important to study predator defence strategies of environmental bacteria. V. vulnificus thrives in coastal waters and infects humans. Very little is know about the defence mechanisms V. vulnificus expresses against predation. Here we show that a V. vulnificus strain (ENV1) has rewired the central carbon metabolism enabling the production of excess organic acid that is toxic to the protozoan predator, T. pyriformis . This is a previously unknown mechanism of predation defence that protects against protozoan predators.

Identification of bacteria involved in non-sulfate based hydrogen sulfide production in an aquaculture environment

“…To the best of our knowledge, this is the first time that cyuA has been identified as the key gene for H 2 S production in a Vibrio sp. Previously, some members of the Vibrio genus have been found to ferment glucose to pyruvate and subsequently transform this metabolite further to acetate (80). We found Vibrio MAG65 to carry such a complete pathway for pyruvate fermentation to acetate and genes related to this fermentation were all upregulated and highly transcribed in cysteine-fed reactors compared to sulfate-fed reactors (Fig.…”

“…The family Vibrionaceae is mainly known for its many pathogens capable of causing human and animal disease (74,75), but this diverse genus also includes other members native to a wide range of aquatic habitats (e.g., sediment, column water, deep sea, or as symbionts (66,76)). The Vibrionaceae family is known to be capable of cysteine degradation (77), and at least some members, such as Vibrio cholerae, are using the putative cystathionine B-synthase ( cbs ) for H 2 S-producing cysteine degradation (78). We did not detect any cbs genes in the metagenomes, but found the genes cyuA, malY, cysK, sseA, metC , and tnaA associated with the Vibrionaceae family.…”

Low salinity activates a virulence program in the generalist marine pathogen Photobacterium damselae subsp. damselae

“…So are genes encoding an acetate kinase and an alcohol dehydrogenase, suggesting that acetate production is enhanced at seawater salinity in Pdd . The upregulation of genes involved in acetate production has been reported in V. vulnificus as an antipredation strategy against protozoans ( 60 ).…”

Resource partitioning and amino acid assimilation in a terrestrial geothermal spring

“…This result was somewhat surprising because acetate is a well-known carbon and energy source for prokaryotes [ 89 ] that can be readily converted to the fundamental metabolite acetyl-CoA via ACK-PTA or ACS, and because organic acids have been shown to be metabolized by cultivated thermophiles [ 90 – 93 ] and in situ in terrestrial geothermal springs [ 21 , 22 ]. One possible explanation for the poor incorporation of acetate into DNA is that the acetate switch may have occurred because the initial concentration of acetate in our samples (~3.2 mM) was similar to the minimum acetate concentration known to induce the acetate switch (~1 mM) [ 94 , 95 ]. However, this acetate concentration is lower than the concentrations that support the growth of many hyperthermophiles (6.1 to 12.2 mM) [ 90 – 93 ].…”