Effects of drilling fluids on marine bacteria from a Nigerian offshore oilfield

Two marine bacterial isolates from drill mud cuttings obtained from Agbara oilfield,Staphylococcus sp. andBacillus sp., were cultured aerobically in the presence of varying concentrations (0, 25, 50, and 75 μg/ml) of drilling fluids to determine the effects of concentration of toxicants on their growth. With the exception of Clairsol, Enviromul, and Baroid mineral oil, which had little or no effect, the exponential growth ofBacillus sp. was depressed by all other test chemicals. Additionally, all test chemicals except Clairsol had no effect on lag phase of growth ofBacillus sp. WithStaphylococcus sp. the depressive effect on the exponential phase of growth was shown by almost all test chemicals. There was enhancement of both growth rate and generation times ofStaphylococcus sp. and decrease of those ofBacillus sp. with increasing concentrations of drilling fluids. These results show that while some drilling fluids may be stimulatory or depressive to bacterial growth, others may be without effect.