Soil survival of Salmonella and transfer to freshwater and fresh produce

An increase in human outbreaks caused by Salmonella sp. has been recognised in many parts of the world. Outbreaks are increasingly related to consumption of fresh produce and an understanding of soil survival of Salmonella and transfer to water and to fresh produce are needed to control salmonellosis. The increase in outbreaks is most likely associated with better surveillance, greater consumer demand and a change in production and distribution. This review summarises the recent literature of the ecology of Salmonella sp. in soil environment including sources, survival, transport and crop contamination. Areas with a high density of animal production are recognised as a high risk for contaminating fresh produce, however the specific path of contamination remains unconfirmed. Suspected sources include improperly composted manure/wastewater, the use of irrigation water in the vicinity of animal production and wild animals. Once introduced to the soil the survival of Salmonella sp. has been shown to be influenced by method of introduction, temperature and predation by soil protozoa amongst others. Detection of Salmonella in environmental samples is traditionally measured by classical growth dependent methods - these methods are discussed and although the new molecular based techniques still lack sensitivity compared to the classic techniques new possibilities are constantly emerging such as mRNA of the Salmonella invA gene directly in soil. Salmonella sp. can be transferred to fresh water either through the preferential flow paths of the soil or in the surface run-off water. In addition to soil properties this transport is influenced cell properties such as size, electric charge and hydrophobicity. The contamination of fresh produce is finally shown to follow several different routes including sprinkling and water splashes during rain events, the fresh produce can be contaminated by cells attached to the surface or by internal colonisation of the plant cells.