An institutional outbreak of Salmonellosis due to lactose-fermenting Salmonella newport

Abstract

An institutional outbreak of salmonellosis predominantly due to a lactose-fermenting Salmonella newport is described. Control of the outbreak was hampered by delay in the initial recognition of the aberrant strain. On primary culture, salmonellae were detected on bismuth sulfite agar only; colonies that formed on MacConkey agar and Salmonella-Shigella agar could not be differentiated from lactose-fermenting nonpathogenic organisms. The reactions in triple sugar iron were atypical for Salmonella. The lactose-fermenting property was plasmid-mediated and was readily transferable. Phage typing suggested chicken as a possible source of the strain. The need for awareness of the occurrence of such strains of Salmonella that may not be recognized by cultural procedures in common use, the necessity of the routine use of bismuth sulfite agar in procedures for isolation of salmonellae, and the use of lysine iron agar in conjunction with the triple sugar iron agar are emphasized.