Tropodithietic acid induces oxidative stress response, cell envelope biogenesis and iron uptake in Vibrio vulnificus.

The Roseobacter group is a widespread marine bacterial group, of which some species produce the broad-spectrum antibiotic tropodithietic acid (TDA). A mode of action for TDA has previously been proposed in Escherichia coli, but little is known about its effect on non-producing marine bacteria at in situ concentrations. The purpose of this study was to investigate how a sub-lethal level of TDA affects Vibrio vulnificus at different time points (30 and 60 min) using a transcriptomic approach. Exposure to TDA for as little as 30 min resulted in the differential expression of genes associated with cell regeneration, including the up-regulation of those involved in biogenesis of the cell envelope. Defence mechanisms including oxidative stress defence proteins and iron uptake systems were also up-regulated in response to TDA, while motility-related genes were down-regulated. Gene expression data and scanning electron microscopy imaging revealed a switch to a biofilm phenotype in the presence of TDA. Our study shows that a low concentration of this antibiotic triggers a defence response to reactive oxygen species and iron depletion in V. vulnificus, which indicates that the mode of action of TDA is likely more complex in this bacterium than what is known for E. coli.

Sharing of prey: coinfection of a bacterium by a virus and a prokaryotic predator.

UNLABELLED: Rarely, if ever, has a single bacterial cell been confirmed to simultaneously host two fundamentally different predators. Two such predators are viruses and the predatory prokaryotes known as Bdellovibrio and like organisms. Viruses or bacteriophage are particles requiring prey cells in an active metabolic state to complete their life cycle. The Bdellovibrio and like organisms, unlike viruses, are bacteria that can efficiently infect and grow in prey which are in stationary phase. In this study, electron microscopic examination revealed an unprecedented coinfection by the two agents of Vibrio vulnificus, introducing a new bacterial predation paradigm. Rather than the viruses and Bdellovibrio and like organisms competing for a single prey cell, both can survive in the same cell and successfully reproduce themselves. This is an especially valuable mechanism when the prey is in short supply, and the survival of the predators may be at stake. IMPORTANCE: This article describes the coinfection of a prokaryotic prey or host cell by both a bacteriophage (phage) and the predatory bacterium of the group Bdellovibrio and like organisms (BALOs). Such coinfection has not been previously reported and therefore introduces a new paradigm for predation of bacteria. This finding invites new studies on the interactions of BALOs, phage, and prey in predation. Predation is an important mechanism in nature for helping to keep bacterial populations in check and also plays a major role in the cycling of nutrients through the microbial loop. How dual infection by phage and BALOs imposes on these and other functions of predation is fertile ground for future studies and serves as a keystone reference on bacterial predation and mortality.

First description of nonmotile Vibrio vulnificus strains virulent for eels.

Nonmotile Vibrio vulnificus strains were isolated as pure cultures from body ulcers and internal organs of wild diseased European eels caught in a Mediterranean freshwater coastal lagoon. All 54 V. vulnificus isolates were nonmotile, indole-, ornithine decarboxilase-, mannitol- and cellobiose-positive, developed the opaque variant in culture, belonged to the O-antigenic serovar A and were highly virulent for eels by both intraperitoneal injection and immersion challenges. The nonmotile phenotype found in our V. vulnificus isolates was stable: nonmotile cells were always recovered from experimentally infected eels; no variation in the immobility of the V. vulnificus cells was observed for repeated subculture by daily passages on solid media, at different temperatures or incubation times and with or without magnesium sulfate. Many of the fla genes of Vibrio were present in the genome of the nonmotile strains (flaCDE and flaFBA for flagellins and flaH for the distal capping protein), although we observed by transmission electron microscopy that these V. vulnificus strains always lacked the polar flagellum. This is the first report on the existence of nonmotile wild-type V. vulnificus strains.

Bactericidal effects of toluidine blue-mediated photodynamic action on Vibrio vulnificus.

Vibrio vulnificus is a gram-negative, highly invasive bacterium responsible for human opportunistic infections. We studied the antibacterial effects of toluidine blue O (TBO)-mediated photodynamic therapy (PDT) for V. vulnificus wound infections in mice. Fifty-three percent (10 of 19) of mice treated with 100 microg of TBO per ml and exposed to broad-spectrum red light (150 J/cm(2) at 80 mW/cm(2)) survived, even though systemic septicemia had been established with a bacterial inoculum 100 times the 50% lethal dose. In vitro, the bacteria were killed after exposure to a lower light dose (100 J/cm(2) at 80 mW/cm(2)) in the presence of low-dose TBO (0.1 microg/ml). PDT severely damaged the cell wall and reduced cell motility and virulence. Cell-killing effects were dependent on the TBO concentration and light doses and were mediated partly through the reactive oxygen species generated during the photodynamic reaction. Our study has demonstrated that PDT can cure mice with otherwise fatal V. vulnificus wound infections. These promising results suggest the potential of this regimen as a possible alternative to antibiotics in future clinical applications.