Emergence, in oxytetracycline-free marine mesocosms, of microorganisms capable of colony formation on oxytetracycline-containing media

Tanks (39 cm × 30 cm × 27 cm) containing marine sediment overlaid with various amounts of sterilised commercial fish food pellets and seawater were used to study the emergence of microorganisms capable of forming colonies on TSCA media in both the presence and absence of 25 μg/ml oxytetracycline. All experimental systems were free of oxytetracycline. Initial levels of culturable organisms in the sediments used in these experiments were 7.1 · 10⁴ 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 · 10⁴ to 5.1 · 10⁴ cfu/g, over 70 days incubation at 8.5-12 °C. In the tank containing low levels (1-2 cm depth) of feed, the increase in resistant organisms was from 4.9 · 10³ to 2.3 · 10⁴ cfu/g. There was no significant increase in the relative size of the resistant sub-population 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 · 10¹) to 1.9 · 10⁸ 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 slowgrowing and resistant to high levels (> 512 μg/ml) of oxytetracycline. Over 50% of these strains 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 tanks exhibited more rapid growth and lower levels of resistance to oxytetracycline. These strains were more frequently sensitive to the other antimicrobial agents.