Activity of metronidazole against escherichia coli in experimental infra-abdominal sepsis

Metronidazole treatment was found to reduce both mortality rates and the incidence of abscess formation in experimental animals challenged with an intraperitoneal implant of pooled caecal contents. This was not anticipated since previous studies of this model indicated that death was due to Escherichia coli bacteraemia. This suggested that metronidazole was active against coliforms in vivo, despite in vitro resistance. Using an intraperitoneal challenge of E. coli plus Bacteroides fragilis or E. coli alone, metronidazole treatment reduced mortality rates, but only in animals which received E. coli plus B. fragilis. There was no significant decrease in mortality with animals challenged with E. coli alone, indicating that metronidazole may be active against E. coli only in the presence of other susceptible bacteria. In cliemostat cultures of E. coli metronidazole produced a reduction in the growth rate of E. coli, only in the presence of B. fragilis. Metronidazole has in vivo activity against bacteria which are highly resistant according to conventional in vitro susceptibility tests but only when a susceptible microbe is also concurrently present.