Studies of the Common Aerobic Spore-forming Bacilli

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Abstract
A new technique is descr. by which fat in bact. cells may be demonstrated in permanent dried smears. Bacteria from 24-48 hr.-old glycerol or glucose agar slants are emulsified in a saturated soln. of Sudan black B in 70% alcohol for 15-20 min., then a loopful of the suspension is spread rapidly on a glass slide by a circular motion. After drying, and without heating, the smear is counterstained with 1% aqueous safranin. Fat droplets are brilliantly revealed as bluish-black granules in a pink-stained cytoplasm. Vacuola-tions in bacilli stained by ordinary dyes which may be . mistaken for spores or other special structures are shown to be spaces occupied by fat. Application of this method to 35 stock strains and to > 100 freshly isolated strains of aerobic spore-bearing bacilli demonstrates that these bacilli may be classified into a fat-negative and a fat-positive group. Among the latter class individual species show characteristic differences in the amount of stored fat. Use of the stain as a primary differential test is consequently of material aid toward rapid identification of unknown aerobic spore-bearers.

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