This invention provides isolated and characterized secreted molecules from probiotic bacteria from the genera Lactobacillus, Lactococcus, Streptococcus or Bifidobacterium for use in compositions and methods for the treatment and/or prevention of infection by harmful pathogenic bacteria such as Salmonella or E.coh The isolated secreted molecules can also be used in nutritional or medical food products which provide probioties to the gastrointestinal tract of a mammal.