All of six reference strains of Bacteroides species, 36 laboratory isolates conforming to this group, and individual strains of Escherichia coli, Proteus mirabilis, Salmonella typhimurium and Clostridium welchii produced a dense black pigment, identified as ferrous sulphide, when grown in cooked-meat media containing cysteine and ferrous sulphate. This was an indicator effect resulting from the production of H2S by the bacteria in the presence of ferrous ions and was unrelated to the characteristic pigment produced by strains of B. melaninogenicus when grown on blood agar. A pigment was extracted by ultrasonic disintegration of washed cells of three reference strains of B. melaninogenicus grown for 1 week in horse-blood broth and on human-blood agar. It was intracellular or cell-associated, soluble in water and had the spectrophotometric characteristics of a derivative of haemoglobin. No such pigment was extracted from strains of B. fragilis or B. necrophorus by similar procedures. Pigment production is a stable characteristic of those strains of Bacteroides called B. nzelaninogenicus and it is a significant property in the classification of the Bacteroides group. However, the pigment-producing strains are not a homogeneous species, and there were considerable differences between the resuIts of biochemical tests and antibiograms obtained with the three strains of .


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