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Abstract
Edwardsiella piscicida is a severe fish pathogen. Haem utilization systems play an important role in bacterial adversity adaptation and pathogenicity. In this study, a speculative haem utilization protein, HutZEp, was characterized in E. piscicida. hutZEp is encoded with two other genes, hutW and hutX, in an operon that is similar to the haem utilization operon hutWXZ identified in V. cholerae. However, protein activity analysis showed that HutZEp is probably not related to hemin utilization. To explore the biological role of HutZEp, a markerless hutZEp in-frame mutant strain, TX01ΔhutZ, was constructed. Deletion of hutZEp did not significantly affect bacterial growth in normal medium, in iron-deficient conditions, or in the presence of haem but significantly retarded bacterial biofilm growth. The expression of known genes related to biofilm growth was not affected by hutZEp deletion, which indicated that HutZEp was probably a novel factor promoting biofilm formation in E. piscicida. Compared to the wild-type TX01, TX01ΔhutZ exhibited markedly compromised tolerance to acid stress and host serum stress. Pathogenicity analysis showed that inactivation of hutZEp significantly impaired the ability of E. piscicida to invade and reproduce in host cells and to infect host tissue. In contrast to TX01, TX01ΔhutZ was defective in blocking host macrophage activation. The expression of hutZEp was directly regulated by the ferric uptake regulator Fur. This study is the first functional characterization of HutZ in a fish pathogen, and these findings suggested that HutZEp is essential for E. piscicida biofilm formation and contributes to host infection.
Highlights
Iron is an essential element for bacteria because it is necessary for a wide variety of physiological processes, including electron transfer, enzyme catalysis, energy transduction, and regulation of gene expression [1, 2]
This study is the first functional characterization of HutZ in a fish pathogen, and these findings suggested that HutZEp is essential for E. piscicida biofilm formation and contributes to host infection
The spatial structure of HutZEp is different from that of ChuY (Additional file 1); for example, seven α-helices exist in H utZEp, but only six α-helices exist in ChuY [20]
Summary
Iron is an essential element for bacteria because it is necessary for a wide variety of physiological processes, including electron transfer, enzyme catalysis, energy transduction, and regulation of gene expression [1, 2]. Iron is the most abundant metallic element on earth, the majority of iron is sequestered in iron- and haem-containing proteins within the host, so iron deficiency is the most common nutritional stress for bacteria [5, 6]. Haem uptake systems in gram-negative bacteria consist of outer membrane receptors that either directly bind haem and haemoproteins or bind haem-bound secreted haemophores. Haem transits the periplasm and is brought into the cell via ABC transporters in the inner membrane [9]. There are several types of mechanisms for haem uptake and utilization in gram-negative bacteria.
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