The normal flora of laboratory mice at The Rockefeller Institute, fed on a bread and milk diet, was determined. Bacillus acidophilus and Bacillus bifidus outnumber the Bacillus coli, Bacillus acidi lactici, and Bacillus coli communior group about twenty-five to one. White and yellow cocci which may or may not liquefy gelatin are occasionally noted; spirochetal and vibrio forms and yeasts are usually seen in stained preparations. This flora does not change when mice are artificially infected per os with a strain of mouse typhoid bacilli (Bacillus pestis cavice) and is the same in the animals which resist the infection as in those which succumb.

Mice fed on a meat diet and showing a colon, Bacillus diffluens, and Bacillus welchii flora do not differ in susceptibility to mouse typhoid from the normal mice fed on bread and milk and showing the above acidophilus flora.

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