Scientists at the University of Liverpool have demonstrated how a single-celled organism, living freely in the environment, could be a source of Salmonella transmission to animals and humans.
Salmonella are microscopic living creatures that can contaminate almost any food type, causing diarrhoea, abdominal pain and fever. Scientists know that Salmonella – which can also cause typhoid fever – has evolved unique mechanisms to prevent the body's immune system from functioning effectively, but until now it was not understood how it survives so successfully in the environment.
Scientists at Liverpool, in collaboration with the Institute for Animal Health, have shown that Salmonella use a secretion system to protect themselves inside amoeba – a single-celled organism living on land and in the water. The research suggests that amoeba may be a major source of Salmonella within the environment and could play a significant role in transmission of infection to man and animals.
Salmonella uses a system, called SP12 type III, which acts as a bacterial machine inside organisms and causes disease in humans, animals and plants. The system employs a 'syringe-like' mechanism to inject bacteria into cells that would normally release compounds to rid the body of harmful substances. This system changes the structure of the cell and prevents these compounds from coming into contact with pathogens and destroying them.
Dr Paul Wigley, from the National Centre for Zoonosis Research, based at the University's Leahurst campus, explains: "Salmonella has managed to survive extremely successfully in the environment, finding its way into our food and causing illness, despite the body's best efforts to fight it off. We found that it uses a system which operates in the human immune system as well as inside amoeba living in the environment. This system essentially protects Salmonella within cellular compartments, called phagosomes, where it can survive and multiply.
"Its ability to survive in amoeba is a huge advantage to its continued development as it may be more resistant to disinfectants and water treatment. This means that we need to work to understand ways of controlling amoeba in water supplied to animals and prevent it acting as a 'Trojan Horse' for Salmonella and other pathogens."
The research, supported by the Society for Applied Microbiology, the Department for Environment, Food and Rural Affairs (DEFRA), and the Higher Education Funding Council for England (HEFCE), is published in Applied and Environmental Microbiology.
- Bleasdale et al. The Salmonella Pathogenicity Island 2-Encoded Type III Secretion System Is Essential for the Survival of Salmonella enterica Serovar Typhimurium in Free-Living Amoebae. Applied and Environmental Microbiology, 2009; 75 (6): 1793 DOI: 10.1128/AEM.02033-08
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