THE EFFECTS OF AGAR CONCENTRATION ON THE GROWTH AND MORPHOLOGY OF SUBMERGED COLONIES OF MOTILE AND NONMOTILE BACTERIA

The growth and morphology of submerged bacterial colonies was investig ated. Five separate colonial forms were recognized depending both on s pecies and on agar concentration. These were (i) branched, dendritic s tructures seen only with Bacillus cereus (ii) lenticular colonies for all other species at high agar concentrations; (iii) small lobed to sp herical colonies for non-motile organisms at low agar concentrations; (iv) and (v) large diffuse spherical colonies which can be further sub divided into 'snowball' or 'wispy' types for motile bacteria growing a t agar concentrations below about 0.65% w/v. Viable count determinatio ns suggested that agar concentration had little effect in the early st ages of growth but chat motile cells at low agar concentrations achiev ed higher cell numbers than did those in concentrations greater than 0 .65% w/v. Transmission electron microscopy indicated that bacteria in lenticular colonies were tightly packed within lens-shaped splits in t he agar whilst at low agar concentrations motile cells were well separ ated and appeared to move through the agar matrix.