Drug Resistant Bacteria and Their Conjugative R Plasmids Isolated from Materials Related with Fish Culturing. in U.S.A.

Abstract

We have been surveying incidences in this country of drug resistant bacteria and their conjgative R plasmids isolated from cultured fish and the culturing poud water. We found out the high incidences of drug resistant bacteria and their conjugative R plasmids in various fish culturing fields.Since high amounts of chemotherapeutics have been widely used for fish diseases in this country, it is no wonder to have high incidences of drug resistant bacteria. To evaluate the effects of environmental factors on high incidences of drug resistant bacteria, we surveyed incidences of drug resistant bacteria and their conjugative R plasmids in fish culturing fields in U. S. A.
We visited 74 fish culturing farms in eight fish culturing areas in the mid-summer of 1974 and sampled cultured fish and the culturing pond water. Average numbers of bacteria detected in fish intestines was 3.5 ×10⁶/g intestinal materials and incidences of bacteria resistant to sulfonamide, streptomycin, chloramphenicol and tetracycline were 1.7%, 1.4%, 0.31% and 0.95%, respectively.Drug resistant bacteria were isolated from Salmonide fish and catfish with relatively high frequencies. Average number of bacteria detected in culturing pond water was 4.3 ×10³/ml and incidences of bacteria resistant to sulfonamide, streptomycin, chloramphenicol and tetracycline were 2.8%, 8.5%, 3.5% and 3.8%, respectively. In general, incidences of drug resistant bacteria were higher in water than in intestines. All the results indicated that incidences of drug resistant bacteria in U.S.A. were far lower than those in this country.
High proportion of the drug resistant bacteria isolated from the intestinal materials belonged to Enterobacteriaceae. Genera of the isolated bacteria were Escherichia, Citrobacter, Pseudomonas, Hafnia, Enterobacter, Klebsiella, Proteus and other unidentified bacteria in the order of the isolation frequencies. Genera of the drug resistant bacteria isolated from culturing pond water were the same as those from the intestinal materials except Proteus. In this surveillance, we isolated no drug resistant fish-pathogens, such as Aeromonas hydrophila, Aeromonas salmonicida, Vibrio anguillarum, and Edwardsiella tarda. This is quite different from the results which we had obtained in this country, where we had been isolated many drug resistant fish pathogenic bacteria from the intestines of healthy fish and pond water as well as from diseased fish. There are many reports on fish diseases in U.S.A. caused by bacteria. However, most of them might be caused by drug sensitive bacteria, since we did not isolate in U.S.A. any drug resistant bacteria pathogenic for fish. Thus, most of the drug resistant Enterobacteriaceae isolated from cultured fish might be selected in other fields, such as in domestic animal farms and city sewerage.
Conjugative R plasmids were detected in 14 out of 532 drug resistant bacterial strains (2.6%). All the conjugative R plasmids were detected in the strains belonged to Enterobacteriaceae, and most of the R⁺ strains were isolated from materials from catfish farms. High water temperature and no water change might allow the growth of Enterobateria contaminated by the materials from human and domestic animals. Drug resistance markers of the detected conjugative R plasmids were combinations of sulfonamide-, streptomycin-, tetracycline- and kanamycin-resistances and no chloramphenicol- or ampicillin-resistance. These combination patterns of drug resistance were different from those detected in the fish pathogenic bacteria in Japan, but rather close to those detected in bacteria isolated from domestic animals in this country.