18 - Resistance Plasmids and the Risk of Transfer

Abstract

The continued use of chemotherapeutic agents to treat furunculosis outbreaks in cultured salmonids has led to the development of drug resistance in Aeromonas salmonicida carrying R plasmids. The acquired resistance of bacteria results from mutations in chromosomal genes, or from acquisition of plasmids and transposons. Transferable R (resistance) plasmids have been detected in almost all drug-resistant strains of fish pathogenic bacteria including A. salmonicida. The R plasmids are autonomous units that exist in the cell, and are self-replicating circular molecules of DNA that encode resistance to chemotherapeutants and heavy metal ions. The detected R plasmids encode resistance to combinations of chloramphenicol (CP), spectinomycin (SPC), streptomycin (SM), sulphonamides (SA), tetracycline (TC), and trimethoprim (TMP). Comparison of the DNA structure of R plasmids from A. salmonicida suggests that they are species specific. However, the region coding for drug resistance in R plasmids from A. salmonicida is similar to that of R plasmids from other Gram-negative bacteria. Non-transferable R plasmids encoding resistance to TC were also detected in A. salmonicida strains isolated in Japan. The chapter also discusses the risk associated with the transfer of resistance plasmids between A. salmonicida and other bacteria.