CLASSIFICATION AND GENETIC-CHARACTERIZATION OF PATTERN-FORMING BACILLI

One of the more natural but less commonly studied forms of colonial ba cterial growth is pattern formation. This type of growth is characteri zed by bacterial populations behaving in an organized manner to genera te readily identifiable geometric and predictable morphologies on soli d and semi-solid surfaces. In our first attempt to study the molecular basis of pattern formation in Bacillus subtilis, we stumbled upon an enigma: some strains used to describe pattern formation in B. subtilis did not have the phenotypic or genotypic characteristics of B. subtil is. In this report, we show that these strains are actually not B. sub tilis, but belong to a different class of Bacilli, group I. We show fu rther that commonly used laboratory strains of B. subtilis can co-exis t as mixed cultures with group I Bacilli, and that the latter go unnot iced when grown on frequently used laboratory substrates. However, whe n B. subtilis is grown under more stringent semiarid conditions, membe rs of group I emerge in the form of complex patterns. When B. subtilis is grown under less stringent and more motile conditions, B. subtilis forms its own pattern, and members of group I remain unnoticed. These findings have led us to revise some of the mechanistic and evolutiona ry hypotheses that have been proposed to explain pattern growth in Bac illi.