Provided are compositions and methods based in part on the discovery that Enterococcus faecium heterologous secreted antigen A (SagA)-produced peptidoglycan fragments are protective against enteric bacterial infections. Modified bacteria that are engineered to express heterologous SagA are provided, and are included a nutraceutical, pharmaceutical, and probiotic formulations, and as components of food products, including dairy products. The modified bacteria include modified Lactobacillus bacteria that express heterologous SagA. The disclosure includes a method that involves introducing into an individual modified bacteria of that express and secrete heterologous SagA. The disclosure provides isolated peptidoglycan fragment preparations made by a exposing a composition that contains peptidoglycan to an isolated or recombinant SagA, or to modified bacteria expressing a heterologous SagA, allowing SagA to generate the peptidoglycan fragments, and separating the peptidoglycan fragments from the composition to obtain the isolated peptidoglycan fragments or a preparation that contains them.
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