Relation between antibiotic resistance and number of plasmids in enterotoxigenic and non-enterotoxigenic Escherichia coli strains.

We have investigated the relation between antibiotic resistance and the number of plasmids contained in 44 enterotoxigenic (ETEC) and 69 non-enterotoxigenic (non-ETEC) Escherichia coli strains. Both ETEC and non-ETEC strains were resistant to one or more antibiotics at a similar rate, but multiple resistance in resistant non-ETEC strains was higher than in resistant ETEC strains, showing a significant difference (P less than 0.001) when subjected to Student's t test. Among the ETEC strains, those producing both LT and STa enterotoxins were more sensitive to antibiotics than strains producing only a single toxin type. On the other hand, resistant ETEC strains possessed a higher number of plasmids per strain than the resistant non-ETEC strains (P less than 0.05 by Student's t test). Consequently, resistance to antibiotics could not be related to the possession of a higher number of plasmids. Furthermore, our results support the hypothesis that there is a generalized incompatibility process between plasmids which code for antibiotic resistance and plasmids which code for enterotoxin production in ETEC strains, at least in those strains producing both LT and STa enterotoxins.