[Enterohemorrhagic Escherichia coli O157 outbreak in Obihiro-City--study on antibiotic susceptivity and plasmid profiles].

The drug-resistance patterns and plasmid profiles of 147 isolates (patient origin 142 and food origin 5 isolates) from the outbreak of enterohemorrhagic Escherichia coli (EHEC) O157:H7 infection in Obihiro-city Hokkaido in late October, 1996, were examined. Thirty-six isolates were resistant to tetracycline (TC) (24.5%), 15 of which were resistant to both streptomycin and TC. The minimal growth inhibitory concentration (MIC) of fosfomycine (FOM) was examined, confirming that MIC changed by the cultivation conditions, that is 12.5 micrograms/ml at the aerobic condition, 1.6 micrograms/ml at the anaerobic condition and 3.2 micrograms/ml on blood agar plates. Furthermore, though E. coli O157 could not be detected once by the FOM medication, FOM sensitivity of the patient origin O157 isolates who became O157-positive again was examined. Any changes in FOM sensitivity were not observed. Plasmid profiles of all isolates were divided by 4 patterns from A to D. The most dominant pattern was type A, and plasmid profiles of food origin O157 belonged to pattern A. In 9 examples of the person-to-person infection in the family, plasmid patterns of O157 isolates were the same to each other, even though drug-resistant patterns were different. In 13 patients developing the duration of excretion of EHEC, the changes of the drug-resistance patterns were correlated with the changes of plasmid profiles. By comparing plasmid profiles and TC resistance, it was suggested that TC resistance was controlled on a plasmid. Since food origin O157 isolates were sensitive to all drugs and presenting the same plasmid profiles, demonstrating that TC resistance and plasmid are newly added to the bacterial cells while food origin O157 isolates passe inside the human body.