After the feeding period, one fish from each treatment was sacrificed by anaesthetic overdose in 3-aminobenzoic acid ethyl ester (MS-222, 50 mg/L) as described previously . After removing the ventral body plate, sterile ice-cold phosphate buffered saline (PBS) was added to the abdominal cavity. The intestine was separated immediately distal to the stomach, removed from the body cavity, uncoiled, and measured rapidly in cold PBS. The auxiliary lobe was separated from the intestine, which was then divided into three parts of equal length, defining foregut, midgut, and hindgut regions. Tissue samples were processed using the Qiagen (Germantown, MD, USA) DNeasy Blood and Tissue Kit with pre-treatments for Gram-positive and Gram-negative bacteria according to the manufacturer's instructions. DNA extracted from three samples of each GI tract region was pooled and processed for PCR amplification.