In vitro susceptibility of Escherichia coli O157 to several antimicrobial agents

We evaluated the antimicrobial susceptibility of six strains of Escherichia coli O157 (E. coli O157) isolated from patients in Yamaguchi Prefecture between June and July, 1996. Seven antimicrobial agents that were expected to retain a high concentration in the intestine were selected. The minimum inhibitory concentration (MIC) and the minimum bactericidal concentration (MBC) of ciprofloxacin, polymyxin B, cefoperazone, and kanamycin for each strain were < or = 6.25 microg/ml. However, the MIC of fosfomycin was 3.13-100 microg/ml, and its MBC was > or = 100 microg/ml. The MIC of ampicillin and tetracycline was > 100 mcirog/ml in some strains. In a time-kill study of E. coli O157 at a drug concentration of 12.5 microg/ml, about 10(4) colony forming units/ml of E. coli O157 were eradicated within 10 min by ciprofloxacin, within 30 min by polymyxin B, within 4 h by cefoperazone, and within 16 h by kanamycin. These results suggest that the new quinolones with a poor absorption rate in the intestine (such as ciprofloxacin and norfloxacin) are effective against E. coli O157. When oral administration is impossible, bile excreting cephem antibiotics (such as cefoperazone, ceftriaxone, and cefotetan) may be useful.

The salting-out test to identify virulent Yersinia pseudotuberculosis

A total of 54 strains including 8 serotypes of Yersinia pseudotuberculosis were examined to find out about their cell surface hydrophobicity and virulence plasmid-associated properties. All the strains which aggregated in 0.9% saline expressed the properties of calcium-dependent growth and autoagglutination. However, the hydrophilic strains did not possess virulence plasmid-associated properties. The above results suggest that the salting-out test with 0.9% saline could be an effective measure to differentiate the virulence plasmid-carrying strains of Y. pseudotuberculosis from those of plasmidless virulence.