The present invention provides compositions including polypeptides having the characteristics of polypeptides expressed by a reference microbe such E coli or Salmonella. Examples of Salmonella strains that can be used include, for instance, S. enterica serovar Newport, S. enterica serovar Enteritidis, S. enterica serovar Typhimurium, and S. enterica serovar Dublin. Also provided are compositions including polypeptides having a particular molecular weight and a mass fingerprint that includes polypeptide fragments having a particular set of masses, or polypeptides having an amino acid sequence with at least about 95% identity with an amino acid sequence, wherein the polypeptide has seroreactive activity. The present invention also provides methods of making and methods of using such compositions.