Microbial endocrinology as an instigator of bacterial fish disease

In recent years, bacterial disease outbreaks in sea-farmed Atlantic salmon in Norway have become increasingly frequent. This concerns both well-known diseases such as winter ulcer and yersiniosis, but also bacteria never or seldom associated with disease previously. The background for this situation remains unclear, although a steep rise in the use of physical treatment methods for removing salmon lice has occurred concurrently, and such treatments are often reported shortly prior to disease outbreaks.

It is broadly recognised that the development of infectious diseases relies upon an interplay between three factors, i.e. the agent (here bacteria), the host (here salmon) and their shared environment (here sea cages). Whilst there are no indications of neither the involved bacteria nor the salmon having undergone fundamental changes of relevance recently, the salmon are now increasingly exposed to potentially stressful and/or harmful stimuli, e.g. during physical delousing. Repeated and/or long-term stress may cause impairments to an animal’s immune system, but in the case of several disease-associated bacteria from terrestrial animals, host stress hormones have also been shown to directly stimulate their ability to cause disease. This phenomenon however, coined ‘microbial endocrinology’, remains poorly studied amongst bacteria from fish.

Our goal is to study whether (and to what extent) stress hormones from salmon, e.g. excreted during physical delousing, may cause selected fish-associated bacteria to become more ‘aggressive’ and thereby perhaps contribute towards explaining the observed rise in disease outbreaks. An array of complementary methods will thus be employed to map changes to the bacteria's physical traits (phenotypes) and gene regulation (transcriptomes), and particularly such of know association to disease development, while under stress hormone exposure. Specific tools will be developed/refined for measuring stress hormones.