Bacteria associated with the digestive system of the slug Deroceras reticulatum are not required for protein digestion

Bacteria were isolated from the crop, digestive glands and salivary glands of the field slug Deroceras reticulatum (Müller). The digestive gland was the most densely populated organ with about ×3 and ×400 more bacterial colony forming units (CFU) mg−1 of tissue than in the crop and salivary glands, respectively. Growth on plate-count agar incorporating gelatin showed that 34, 16 and 3% of the total numbers of CFU present in the salivary gland, digestive gland and crop, respectively, secreted extracellular proteinases that degraded gelatin. Administration of antibiotics (chloramphenicol or tetracycline plus chlortetracycline) to slugs in an artificial diet reduced the numbers of total and proteolytic CFU present by 98–99%. Although proteinase-secreting bacteria were present in all organs studied, only the crop possessed significantly greater proteolytic activity (×3 at pH 8.0) when slugs were not fed on diet containing antibiotics.