Renibacterium salmoninarum is the etiological agent of bacterial kidney disease of salmonid fishes. This slow-growing bacterium is a Gram-positive coccobacillus with an absolute requirement for a sulfur-containing amino acid supplement in primary isolation and culture media. One of the earliest media used to cultivate R. salmoninarum was Mueller Hinton supplemented with 0.1% I-cysteine HCl. In 1977, an improved kidney disease medium (KDM2) which incorporated serum was developed. More recently, a phenomenon was described whereby autonomously secreted metabolic products of R. salmoninarum enhanced its own growth. This phenomenon evolved into a medium supplement (metabolite) that results in much more luxuriant bacterial growth in broth and agar media.