Establishment of a universal size standard strain for use with the PulseNet standardized pulsed-field gel electrophoresis protocols: converting the national databases to the new size standard.

The PulseNet National Database, established by the Centers for Disease Control and Prevention in 1996, consists of pulsed-field gel electrophoresis (PFGE) patterns obtained from isolates of food-borne pathogens (currently Escherichia coli O157:H7, Salmonella, Shigella, and Listeria) and textual information about the isolates. Electronic images and accompanying text are submitted from over 60 U.S. public health and food regulatory agency laboratories. The PFGE patterns are generated according to highly standardized PFGE protocols. Normalization and accurate comparison of gel images require the use of a well-characterized size standard in at least three lanes of each gel. Originally, a well-characterized strain of each organism was chosen as the reference standard for that particular database. The increasing number of databases, difficulty in identifying an organism-specific standard for each database, the increased range of band sizes generated by the use of additional restriction endonucleases, and the maintenance of many different organism-specific strains encouraged us to search for a more versatile and universal DNA size marker. A Salmonella serotype Braenderup strain (H9812) was chosen as the universal size standard. This strain was subjected to rigorous testing in our laboratories to ensure that it met the desired criteria, including coverage of a wide range of DNA fragment sizes, even distribution of bands, and stability of the PFGE pattern. The strategy used to convert and compare data generated by the new and old reference standards is described.

An outbreak of Escherichia coli O157 infection following exposure to a contaminated building.

CONTEXT: Infection with Escherichia coli O157 causes an estimated 70 000 diarrheal illnesses per year in the United States and can result in hemolytic-uremic syndrome and death. Environmental contamination with E coli O157 may be a public health problem. OBJECTIVES: To determine risk factors for E coli O157 infection during an outbreak investigation at a county fair and to evaluate environmental contamination as a possible cause of the outbreak. DESIGN, SETTING, AND PARTICIPANTS: Case-control study of 23 patients (median age, 15 years) and 53 age-matched controls who had attended the Lorain County, Ohio, fair between August 20 and August 26, 2001. Case-patients had laboratory-confirmed E coli O157 infection, hemolytic-uremic syndrome, or bloody diarrhea within 7 days of attending the fair; controls attended the fair and did not have diarrhea. MAIN OUTCOME MEASURES: Risk factors for infection and isolates of E coli O157 from environmental specimens. RESULTS: Six (26%) case-patients were hospitalized and 2 (9%) developed hemolytic-uremic syndrome. Case-patients were more likely than controls to have visited building A (a multipurpose community facility on the fairgrounds; matched odds ratio [MOR], 21.4 [95% confidence interval [CI], 2.7-170.7]). Among visitors to building A, illness was independently associated with attending a dance in the building (MOR, 7.5; 95% CI, 1.4-41.2), handling sawdust from the floor (MOR, 4.6; 95% CI, 1.1-20.0), or eating and/or drinking in the building (MOR, 4.5; 95% CI, 1.2-16.6). Twenty-four (44%) of 54 specimens collected from building A 6 weeks after the fair grew Shiga toxin-producing E coli O157. Isolates from sawdust, the rafters, and other surfaces were identical by molecular fingerprinting to patient isolates. Sawdust specimens collected 42 weeks after the fair also grew the same E coli O157 strain. CONCLUSIONS: Absence of evidence implicating specific food or beverage sources and the recovery of E coli O157 from the rafters suggest that airborne dispersion of bacteria contributed to the contamination. Because E coli O157 can survive in the environment for more than 10 months, humans may be at risk of infection long after an environment is initially contaminated.