Influence of marine sediment and divalent cations on the activity of oxytetracycline against Listonella anguillarum

Tube dilution MIC and MBC assays were used to estimate the influence of Mg⁺⁺ (54 mM) Ca⁺⁺ (10 mM) and 25% (w/v) marine sediment on the biological activity of oxytetracycline hydrochloride against Listonella anguillarum. Assays were performed at 22°C in phosphate-free tryptone media. MIC data indicated that, in the presence of Mg⁺⁺ and Ca⁺⁺, the bioactivity was 12.5 %, in the presence of sediment it was 1.5 % and when both were present it was <0.4%. Equivalent figures derived from MBC data were 6 %, < 0.4 % and < 0.4 % respectively. Concentrations in excess of 800 μg / ml oxytetracycline were required to kill the test bacteria in the presence of sediment compared to the 3 μg / ml required to have the same effect in the basic medium.