The Virulence of O157:H7 Isolates in Mice Depends on Shiga Toxin Type 2a (Stx2a)-Induction and High Levels of Stx2a in Stool.

In this study we compared nine Shiga toxin (Stx)-producing O157:H7 patient isolates for Stx levels, -phage insertion site(s), and pathogenicity in a streptomycin (Str)-treated mouse model. The strains encoded , and , or and . All of the strains elaborated 10-10 cytotoxic doses 50% (CD) into the supernatant after growth as measured on Vero cells, and showed variable levels of increased toxin production after growth with sub-inhibitory levels of ciprofloxacin (Cip). The ++ isolates were 90-100% lethal for Str-treated BALB/c mice, though one isolate, JH2013, had a delayed time-to-death. The + isolate was avirulent. Both an and a deletion mutant of one of the ++ strains, JH2010, exhibited at least a three-log decrease in cytotoxicity and both were avirulent in the mice. Stool from Str-treated mice infected with the highly virulent isolates were 10- to 100-fold more cytotoxic than feces from mice infected with the clinical isolate, JH2012, that made only Stx2a. Taken together these findings demonstrate that the -phage from JH2010 induces to higher levels than does the phage from JH2012. The ++ clinical isolates were avirulent and neutralization of Stx1 in stool from mice infected with those strains indicated that the toxin produced was primarily Stx1a. Treatment of mice infected with Stx1a+Stx2a+ isolates with Cip resulted in an increase in Stx2a production and lethality in the mice. Our data suggest that high levels of Stx2a in stool are predictive of virulence in mice.