The virulence for mice of Bacteroides fragilis is higher than that of Bacteroides vulgatus. This enhanced virulence is related to the higher growth rate of B. fragilis in broth and to the ability of B. fragilis strains to grow in serum. The failure of B. vulgatus to grow in serum could be overcome by addition of haemin and an unidentified growth factor, present in tryptone. Haemoglobin, lysed erythroeytes or iron in combination with protoporphyrin IX could replace haemin.

Of the five Bacteriodes species of the ‘fragilis group’ only Bacteroides fragilis was able to grow in human plasma. Therefore the capacity of several iron sources to stimulate to growth of Bacteroides species under iron restricted conditions in vitro was tested. The iron chelator bipyridyl was used for the restriction of iron in the media. Ferrous sulphate, ferric ammonium sulphate and ferric citrate stimulated the growth of all five Bacteroides species tested to the same extent. B. fragilis, and to a lesser extent B.thetaiotaomicron and B. distasonis were better able than B. vulgatus and B. ovatus to use haem‐compounds as an iron source in the presence of the iron chelator bipyridyl. All five Bacteroides species tested could use 30% iron‐saturated transferrin. There was no correlation between the ability of the strains to grow in human plasma and the ability to use either haem‐compounds of transferrin as a source of iron.

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Letters men was distended, and tenderness and guarding were noted in both iliac fossae. Sigmoidoscopy was performed, and red, haemorrhagic rectal mucosa was noted. Visibility was hampered because of the diarrhoea. Subsequent histology was compatible with an "infective aetiology." C difficile toxin was detected by tissue culture of stools on the eighth day of her