Nonetheless there are substantial reasons for avoiding total reliance upon such

Nonetheless there are substantial reasons for

This preview shows page 411 - 412 out of 594 pages.

Nonetheless, there are substantial reasons for avoiding total reliance upon such compounds. For instance, the development of resistance by the pathogens to some widely used drugs is cause for serious concern (Wood et ai, 1986). Thus, it is disquieting that plasmids carrying antibiotic resistance factors (R-factors) have been isolated from Aer. salmonicida strains (Aoki et al, 1971). Indeed, a strain resistant to sulphathiazole and tetracycline was recovered as early as 1959. Moreover, Snieszko and Bullock (1957) reported the occurrence of cultures which were resistant to sulphonamides, although at the time the mechanism of resistance was not known. Now, it seems that resistance may reflect alterations in the outer membrane of Aer. salmonicida (Barnes et al, 1992). Aoki et al. (1983) examined 175 isolates, which had been isolated from cultured and wild salmonids in Japan, for susceptibiHty to a wide range of antimicrobial agents. They noted that 96% of the isolates from cultured fish were resistant to at least one, and up to six, of the drugs, particularly nalidixic acid and nitrofuran derivatives. In addition, transferable R-plasmids, coding for resist- ance to chloramphenicol, streptomycin and sulphonamides, and non-transferrable plasmids conveying resistance against tetracycline were found in several strains. It was concluded from these results that drug-resistant strains of Aer. salmonicida had increased in direct proportion to the enhanced use of antimicrobial compounds in fish culture. This was particularly evident in view of the observation that few isolates recovered from wild salmonids exhibited drug resistance. Toranzo et al. (1983a, b) characterised the plasmids, determining that the organism frequently possessed more than one plasmid. In particular, five strains possessed six plasmids of varying molec- ular weights. However, these workers did not observe any correlation between loss of plasmids, or changes in plasmid mobilities, and the loss of resistance to sulpha- diazine, in the case of one strain which was studied in detail. Nevertheless, the molecular genetic studies of Mitoma et al. (1984) identified gene sequences in R- plasmids coding for either chloramphenicol or tetracycline resistance. Hedges et al. (1985), using aeromonad isolates obtained from France, Ireland, Japan and the U.K., determined that plasmids from Aer. hydrophila and Aer. salmonicida were similar. These workers reported that some plasmids were transmissible to Esch. coli, whereas others were unstable in this recipient organism. The R-plasmids of Aer. salmonicida were considered to confer upon the pathogen the potential to withstand the onslaught of a wide variety of inhibitory agents, thus diminishing the effectiveness of chemo- therapy. It may be hoped that the development of new compounds, particularly synthetic or semi-synthetic molecules, and the strict rotation in the use of currently available drugs may assist with the problem of resistance in the pathogen. Obviously, a constant awareness of the problem must be maintained.
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