Abstract
Virulence gene expression in pathogenic bacteria is modulated by environmental parameters. A key factor in this expression is temperature. Its effect on virulence gene expression in bacteria infecting warm-blooded hosts is well documented. Transcription of virulence genes in these bacteria is induced upon a shift from low environmental to a higher host temperature (37°C). Interestingly, host temperatures usually correspond to the optimum for growth of these pathogenic bacteria. On the contrary, in ectothermic hosts such as fish, molluscs, and amphibians, infection processes generally occur at a temperature lower than that for the optimal growth of the bacteria. Therefore, regulation of virulence gene expression in response to temperature shift has to be modulated in a different way to that which is found in bacteria infecting warm-blooded hosts. The current understanding of virulence gene expression and its regulation in response to temperature in fish-pathogenic bacteria is limited, but constant extension of our knowledge base is essential to enable a rational approach to the problem of the bacterial fish diseases affecting the aquaculture industry. This is an interesting issue and progress needs to be made in order to diminish the economic losses caused by these diseases. The intention of this review is, for the first time, to compile the scattered results existing in the field in order to lay the groundwork for future research. This article is an overview of those relevant virulence genes that are expressed at temperatures lower than that for optimal bacterial growth in different fish-pathogenic bacteria as well as the principal mechanisms that could be involved in their regulation.
Highlights
Bacteria are constantly subjected to different environmental influences, mainly related to their particular niche or lifestyle
From these limited studies that form the baseline to initiate further approaches in the field, it seems that extracellular enzymes, iron sequestering systems, bacteriolysis-related proteins, as well as secretion systems are some of the virulence factors which are up-regulated at TBO in fish-pathogenic bacteria
Two temperature-dependent regulatory systems have been described in fish pathogens: a two-component regulatory system in E. tarda working in a similar way to the ones described in mammals and plant pathogenic bacteria, and the litR, a negative regulator of V. salmonicida involved in blocking virulence-related genes expression
Summary
Bacteria are constantly subjected to different environmental influences, mainly related to their particular niche or lifestyle. This system depends on the high stability at low temperatures of mRNA 5 sequences involved in sequestering RBS It seems that this mechanism would not be appropriate for regulating virulence gene expression at TBO in fish-pathogenic bacteria. At optimal bacterial growth temperatures, the RNA forms stem-loops sequestering RBS and preventing virulence gene expression, whereas at TBO, RNA conformation changes, resulting in an accessible RBS and the initiation of translation (Kortmann and Narberhaus, 2012; Steinmann and Dersch, 2013) An example of this kind of regulation system is the cspA gene of Escherichia coli involved in the cold shock response (Yamanaka et al, 1999; Giuliodori et al, 2010). Different approaches have been undertaken in order to identify up-regulated virulence genes at TBO in fish-pathogen bacteria
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