Impact of antibiotics on fluorescent Pseudomonas group and Bacillus cereus group isolated from soils exposed to effluent or waste from conventional and organic pig farming

Aims: To determine if antibiotics associated with conventional pig farming have a direct role in altering the populations of key soil micro-organisms isolated from piggery environments with and without exposure to antibiotics. Methods and Results: Fluorescent Pseudomonas spp. and the Bacillus cereus group from soils adjacent to four conventional piggeries (use of antibiotics) exposed to effluent (via irrigation) and two organic piggeries (non-use of antibiotics) were assessed against nine relevant antibiotics using disc-diffusion. The focus of the study was not to determine antibiotic resistance (or sensitivity) of isolates based on the manufacturer defined sensitive break point. Instead this point was used as the interpretation point to compare the populations (i.e. farm/organism combination) for the antibiotics tested. Each population was statistically analysed to determine whether the mean diameters were significantly above this selected interpretation point. Bacterial species from both environments did not show a distinct population pattern linked to the antibiotics. Conclusions: Antibiotics associated with conventional pig farming do not have a direct role in altering the environmental populations of Pseudomonas and Bacillus spp. when assessed by population shifts. Significance and impact of the study This study confirms that an understanding of the resident soil microbiota, as compared to the transient bacteria of pig origin, is important in addressing the impact of antibiotic usage on the food-chain as a consequence of effluent re-use in, and around, pig farms.