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How Vibrio bacteria cause disease in fish

Research project Vibrio bacteria are part of the normal microflora of marine animals. However, they also cause deadly diseases in over 40 different fish species resulting in large economical losses in fish farming.

For example, Vibrio anguillarum causes a deadly blood poisoning in marine fish. The fish scales are covered with a mucus layer that traps the bacteria and eliminates them from the skin when the mucus is renewed. Our research group is the first to analyze how bacteria bind and colonize the mucus layer of fish skin. This is important for understanding the mechanism behind how bacterial microflora found on marine animals become pathogens.

Project overview

Project period:

2007-01-01 2009-12-31

Funding

Finansår , 2004, 2005, 2006, 2007, 2008, 2009

huvudman: Debra Milton , finansiär: Vetenskapsrådet, y2004: 405, y2005: 405, y2006: 405, y2007: , y2008: , y2009: ,

huvudman: Debra Milton, finansiär: Formas, y2004: , y2005: , y2006: , y2007: 535, y2008: 786, y2009: 786,

Participating departments and units at Umeå University

Department of Molecular Biology, Faculty of Science and Technology

Research area

Biological sciences, Molecular biology and genetics

Project description

Vibrio anguillarum constitutes part of the normal microflora of the aquatic environment and may also constitute part of the microflora of marine fish. V. anguillarum is an opportunistic pathogen causing a terminal hemorrhagic septicemia when the health or immune system of marine fish is compromised or when the mucosal surfaces of the fish are damaged. To better understand how V. anguillarum interacts with its environment and its host, my group has initiated several projects that focus on the survival and virulence mechanisms of this pathogen.

1-In vivo mechanisms of bacterial colonization of fish surface mucosal epithelial

The skin is a possible portal of entry into the fish by vibrios suggesting that these bacteria colonize the surface mucosal epithelium. Epithelial cells of the fish skin are covered in a mucosal layer that separates and protects the fish from the aquatic environment. Bacteria are trapped within the mucus and removed from the fish surface during continual renewal of the mucus layer preventing attachment of bacteria to the epithelial cells. Consequently, pathogenic and normal flora bacteria must quickly attach to the underlying epithelial layer to colonize the fish surface. In a rainbow trout animal, V. anguillarum is found in the fish outer mucus layer within 5 hours post-infection and attaches to epithelial cells within 12-24 hours post-infection forming a dense biofilm on the scales. Studies on the mechanisms of colonization of V. anguillarum are ongoing in my lab and will provide information on how commensal vibrio bacteria become pathogens.

2-Communication networks

Bacteria are believed to live in their natural environment as a population instead of as single cells. Communication within the population is required to coordinate complex activities such as biofilm formation and production/secretion of extracellular proteins. This allows bacteria to alter their morphology and physiology quickly to adapt to environmental changes. One mechanism of cell-cell communication is quorum sensing, which is used by bacteria to monitor their population density. Small diffusible signal molecules are secreted by one individual and received by a second individual of the same species in which they signal a specific action. Quorum-sensing signal molecule production as N-acylhomoserine lactones is a common feature of both pathogenic and environmental isolates of V. anguillarum. Thus, quorum sensing may affect the ecology and physiology of this bacterium as well as pathogenicity. How cell-cell communication affects the survival of V. anguillarum in seawater or in the fish host is under investigation.

3-Type VI secretion systems

An essential activity required for bacterial survival and adaptation to the environment is the activation of secretion systems that transport proteins across the inner and outer membrane to the external surroundings. A novel protein secretion system that is highly conserved in sequenced genomes of Gram-negative bacteria has recently been found in Vibrio cholerae and named the type VI secretion system (T6SS). T6SS secretes a 19-kDa protein, Hcp, which lacks an N-terminal signal sequence required for protein transport in other secretion systems. In V. cholerae, three additional proteins are secreted VgrG-1, VgrG-2, and VgrG-3, which are possible actin cross-linking proteins. The role of the secreted proteins and the T6SS system remains elusive and nothing is known about the structural basis of this secretion system. However, some evidence indicates that the T6SS modulates the virulence of bacterial pathogens. T6SS is found in V. anguillarum and we are investigating the function of this secretion system in this bacterium.