Detection of Shiga-like (SL) toxins of enteropathogenic Escherichia coli (EPEC) of human, porcine, calf, and lamb origin on Vero and HeLa S3 cells: a comparative study.

One hundred and forty eight strains of human, porcine, calf and lamb origin belonging to different enteropathogenic O:H serotypes isolated in 13 countries in four continents were tested for production of Shiga, Shiga-like (SL) and other cytotoxins on Vero cells and HeLa (S3 subline) cells in tissue cultures. Altogether, 45% human strains and 89% porcine strains were defined as strong toxin producers (toxin titre greater than or equal to 1:100) on Vero or HeLa S3 cells while 31% of human and 9% porcine strains were regarded as moderate to weak toxin producers (toxin titre less than 1:100). Twenty three percent of human and 1.5% of porcine strains were negative for Shiga or SL-toxin. Polymyxin release of Shiga or SL toxins from bacterial colonies of blood agar grown cultures is recommended as it is simple and effective method facilitating the detection of even low levels of toxins in EPEC or non-EPEC strains. Of the ten strains from calves and lambs, only four were strong toxin producers when cell-free culture supernatants were tested while a polymyxin release method showed that 8 strains were strong toxin producers. One strain was negative by both methods. The high proportion of Shiga/SL toxin negative strains in all O:H serotypes of human origin (but not of porcine origin, especially O 139 serogroup) suggests that systematic studies should continue to look for new toxins in freshly isolated strains grown under in vivo like conditions, e.g. in iron depleted culture media.