EMERGENCE, IN OXYTETRACYCLINE-FREE MARINE MESOCOSMS, OF MICROORGANISMS CAPABLE OF COLONY FORMATION ON OXYTETRACYCLINE-CONTAINING MEDIA

Tanks (39 cm X 30 cm X 27 cm) containing marine sediment overlaid with various amounts of sterilised commercial fish food pellets and seawat er were used to study the emergence of microorganisms capable of formi ng colonies on TSCA media in both the presence and absence of 25 mu g/ ml oxytetracycline. All experimental systems were free of oxytetracycl ine. Initial levels of culturable organisms in the sediments used in t hese experiments were 7.1 . 10(4) cfu/g of which 0.2% were capable of growth on the oxytetracycline-containing agar. In tanks containing no feed, the number of resistant cfu/g increased slightly, from 1.8 . 10( 4) to 5.1 . 10(4) cfu/g, over 70 days incubation at 8.5-12 degrees C. In the tank containing low levels (1-2 cm depth) of feed, the increase in resistant organisms was from 4.9 . 10(3) to 2.3 . 10(4) cfu/g. The re was no significant increase in the relative size of the resistant s ubpopulation in either tank. In the tank containing high levels (16-17 cm depth) of fish feed, the levels of oxytetracycline-resistant cfu's /g rose from below the limit of detection (< 5 . 10(1)) to 1.9 . 10(8) and their relative abundance rose to 34% by the end of the experiment (t = 70 days). The resistant flora isolated from the tanks with high levels of feed were characteristically slow-growing and resistant to h igh levels (> 512 mu g/ml) of oxytetracycline. Over 50% of these strai ns were insensitive to oxolinic acid, cotrimoxazole and furazolidone, but the frequency of sensitivity to ampicillin and chloramphenicol was high. In contrast, the flora isolated from the sediment used in all t anks exhibited more rapid growth and lower levels of resistance to oxy tetracycline. These strains were more frequently sensitive to the othe r antimicrobial agents.