Molecular epidemiologic study of an outbreak of Salmonella typhimurium infection at a newborn nursery

Plasmid analysis, restriction endonuclease analysis, antimicrobial susceptibility testing, biotyping, phage typing and outer membrane protein electrophoresis were used to study an outbreak of Salmonella typhimurium infection at a newborn nursery. Seven out of the 12 neonates had positive blood cultures for S. typhimurium, and 2 of them died of severe sepsis. Thirty epidemic strains of S. typhimurium belonging to phage type 12 had the same plasmid profiles (98.0, 6.7 and 3.8 Kb) and identical restriction digest patterns (23.0, 20.4, 15.0, 9.6, 8.2, 7.4, 5.8, 4.3, 3.8, 2.0 and 1.8 Kb) which were different from those of the 2 non-epidemic strains. Laboratory data suggested that the source of the infection was the index patient's mother who had a slight diarrhea; the mode of transmission was most likely due to the transfer of organisms from infant to infant by the contaminated hands of nurses during milk feeding.