Characterization of temperature-dependent hemin uptake receptors HupA and HvtA in Vibrio vulnificus

Pathogenic mechanism of Vibrio vulnificus infection

Vibrio vulnificus is a fatal, opportunistic human pathogen transmitted through the consumption of raw/undercooked seafood or direct contact. V. vulnificus infection progresses rapidly and has severe consequences; some cases may require amputation or result in death. Growing evidence suggests that V. vulnificus virulence factors and regulators play a large role in disease progression, involving host resistance, cellular damage, iron acquisition, virulence regulation and host immune responses. Its disease mechanism remains largely undefined. Further evaluation of pathogenic mechanisms is important for selecting appropriate measures to prevent and treat V. vulnificus infection. In this review, the possible pathogenesis of V. vulnificus infection is described to provide a reference for treatment and prevention.

Molecular Mechanism of Iron Transport Systems in Vibrio

The ability to acquire iron from the environment is often an important virulence factor for pathogenic bacteria and Vibrios are no exception to this. Vibrios are reported mainly from marine habitats and most of the species are pathogenic. Among those, the pathogenic vibrios eg. V cholerae, V. parahaemolyticus, V. vulnificus causes foodborne illnesses. Vibrios are capable of producing all different classes of siderophores like hydroxamate (aerobactin), catecholate (vibriobactin, fluvibactin), carboxylate (vibrioferrin), and amphiphilic (amphibactin). Every different species of vibrios are capable of utilizing some endogenous or xenosiderophores. Being Gram-negative bacteria, Vibrios import iron siderophore via TonB dependent transport system and unlike other Gamma proteobacteria these usually possess two or even three partially redundant TonB systems for iron siderophore transport. Other than selected few iron siderophores, most pathogenic Vibrios are known to be able to utilize heme as the sole iron source, while some species are capable of importing free iron from the environment. As per the present knowledge, the spectrum of iron compound transport and utilization in Vibrios is better understood than the siderophore biosynthetic capability of individual species.

Characterization of temperature-dependent hemin uptake receptors HupA and HvtA in Vibrio vulnificus

The Gram-negative pathogen Vibrio vulnificus produces several iron-sequestration systems including a hemin uptake system in response to iron limitation as a means to acquire this essential element. Strains of this organism are capable of causing serious septicemia in humans and eels, where hemin is abundant and an advantageous source of iron. Vibrio vulnificus hemin uptake systems consist of HupA, a well studied outer membrane protein, and a recently identified HvtA protein receptor. In this study, we confirmed that the expression of the hvtA gene is iron-regulated in a fur-dependent manner. When analyzed for virulence in a hemin-overloaded murine model system, the hupA gene was more important for establishing infection than the hvtA gene. Transcriptional profiling of these genes using strains of two different biotypes, biotype 1 (human pathogen) and biotype 2 (eel pathogen), showed that the expression of the two receptors was also regulated in response to temperature. The expression of hupA was highly induced in elevated temperatures in the human pathogenic strain when tested in iron-depleted conditions. Conversely, hvtA expression was induced significantly in the eel pathogenic strain at a lower temperature, a condition where the hupA locus was relatively repressed. Our results indicate that although both hupA and hvtA are involved for optimal hemin uptake in V. vulnificus, their expression is dually regulated by the environmental cues of iron concentration and temperature. Together, these data suggest that the virulence genes hupA and hvtA are tightly regulated and strictly induced during iron limitation combined with the physiological temperature of the host organism.