On the aggregation of cells during growth and its effect on the isolation of auxotrophic mutants of Bacillus brevis ATCC 10068.

We have shown that penicillin effects a five log10 kill of a growing culture of a prototrophic strain of Bacillus brevis ATCC 10068 and less than a one log10 kill of an auxotrophic mutant incubated in the absence of the auxotrophic requirement. In spite of this, when a mixed culture of auxotrophic and prototrophic strains were incubated with penicillin under conditions conducive for selection of auxotrophic strains, such strains were not recovered. During growth in both AGM and PY media, the majority of B. brevis cells exist in clumps. Although there is no obvious correlation between the numbers of cells in clumps and the stage of growth with either of the growth media used, bigger clumps consisting of hundreds of single cells were observed in the AGM medium. In experiments with a mixed population of auxotrophic mutants previously derived from the B. brevis ATCC 10068 it was demonstrated that the aggregation of the cells occurred in either the presence or the absence of their growth requirements. The importance of this phenomenon in the aggregation of cells is discussed in relation to the isolation of auxotrophic mutants of B. brevis ATCC 10068.