Survival of Escherichia coli in the intestine of fish

Establishment and persistence of Escherichia coli in the intestine of rainbow trout. Oncorhynchus mykiss (Walbaum), was investigated. Infection was achieved by ingestion of contaminated feed but not by bath exposure. At 15°C. E. coli was found to increase in number in the intestine offish after an initial decline, and could still be detected after 4 days. At 6°C it was detected for 2 days but the numbers declined steadily. A similar trend was observed when extracted gut content was inoculated with E. coli in vitro; after an initial decline, bacterial growth recovered. Gut content in vitro also was a suitable environment for plasmid-mediated transfer of resistance. Oxytetracycline resistance was transferred successfully from Aeromonas salmoniclda to E. coli. This could not be reproduced in vivo, perhaps because the inoculum of A. salmoniclda achieved per fish was not high enough as it did not withstand drying well on the feed pellets. Introduction by gavage was also tried but failed to establish an A. salmonicida infection. These results show that at temperatures around 15°C, the presence of E. coli in fish need not be an indicator of recent passage through polluted waters. It may be a consequence of infection established many days before, and perhaps some distance away. Furthermore, although transfer of antibiotic resistance from a fish pathogen did take place in the environment of fish intestinal content, the likelihood of this being a high-risk route for acquisition of resistance by bacteria that can affect humans is low.