Identification of heme uptake genes in the fish pathogen Aeromonas salmonicida subsp. salmonicida

Abstract

Aeromonas salmonicida subsp. salmonicida, the causative agent of furunculosis in fish, can use heme as the sole iron source. We applied the Fur Titration Assay to isolate a cluster including six genes hutAZXBCD that showed similarity to heme uptake genes of other Gram-negative bacteria, and three genes orf123 of unknown function. The spatial organization of these nine genes, arranged in five transcriptional units, was similar to that of a homologous cluster in A. hydrophila. When a TonB system was provided, this cluster allowed Escherichia coli 101ESD (an ent mutant, unable to synthesize enterobactin) to utilize hemin and hemoglobin as iron sources. Mutation of hutB, a gene that encodes a predicted periplasmic hemin-binding protein, caused a drastic defect in the ability of A. salmonicida to grow with hemin as unique source of iron. Interestingly, a mutant for hutA gene (encoding the outer membrane hemin receptor) showed initially a reduced ability to grow with hemin as sole iron source, but after 24 h it achieved growth levels similar to parental strain. Thus mutation of hutA could not abolish the growth with hemin as iron source, suggesting that redundant outer membrane heme transport functions might be encoded in the A. salmonicida genome.

Aeromonas spp. whole genomes and virulence factors implicated in fish disease

“…Siderophores are low‐molecular‐weight iron chelators that remove iron from host binding proteins, allowing it to enter the bacterial cell through outer membrane receptor proteins (Najimi et al . ). Most Aeromonas species produce siderophores, such as enterobactin and amonabactin (Janda ), and A. salmonicida (strain A449) has been reported to be able to synthesize an anguibactin‐like siderophore similar to the one present in other Gram‐negative bacteria (Reith et al .…”

“…That can be done by siderophore‐independent systems, such as the haeme‐binding proteins (Janda ; Najimi et al . ). Ebanks et al .…”

“…In Aeromonas , as in other bacterial pathogens, the mechanism for obtaining iron can be siderophore dependent or siderophore independent (Janda ; Najimi et al . , ; Chopra et al . ).…”

“…Iron is an essential nutrient for most bacteria, and in fact, many pathogenic bacteria depend on the acquisition of iron for their survival within the host, contributing significantly to their virulence (Hirst, Hastings & Ellis ; Janda ; Najimi, Lemos & Osorio , ; Chopra et al . ).…”

The MainAeromonasPathogenic Factors

“…Subsequently, transport of heme across the cytoplasmic membrane is driven by ATP hydrolysis, and an ATP-binding cassette (ABC) transporter is involved in this transport. A. salmonicida siderophore-independent mechanisms consist of bacterial outer membrane proteins able to bind specific host iron- or heme-binding proteins without the intervention of siderophores [108, 114]. …”