Morphology of B. Acidophilus Grown in Soy Bean Milk

Abstract

If B. acidophilus be inoculated to cow's milk and to soy bean milk, the microscopic picture of the two resulting cultures after 2 or 3 days' incubation is not the same. The growth in the usual cow's milk medium needs no description. All bacteriologists are well acquainted with it. A smear preparation of the culture grown in the soy bean infusion reveals many of the expected long rods with Gram positive tinctorial reaction. In addition to these rods of usual morphology, there are vast numbers of small units, likewise of Gram positive reaction and they may be of such small size that they approach the limits of visibility under the microscope. Mingled with these are varying numbers of organisms which are somewhat swollen and distorted and which are smaller also than the commonly described form of this microorganism. In the soy bean infusion culture, then, the numbers of long slender rods apparently are less than when the same bacillus is grown in cow's milk but the observer will be under the impression that there are far more bacterial units in the soy than there are in the latter preparation. This series of experiments was carried out in an endeavor to determine whether the small units present in the soy bean milk culture are indeed B. acidophilus.

We used a strain of the organism which originally had been sent from the laboratories of Dr. Kopeloff. The soy bean culture was checked most carefully both by aerobic and by anaerobic technique to assure ourselves that there was no contamination in it. A careful survey was made to select a medium among those which had been suggested for culture of B. acidophilus which should be most suitable for the plating of this particular strain. We chose the formula of Kulp and Rettger 1 which is entered as No. 2060 in the compendium of Levine and Schoenlein. 2