Growth of bacteria and other microorganisms is frequently modeled by straight objects that divide once they have reached a threshold length. Their volumes are mutually excluded, and they cannot penetrate each other. When colonies of microorganisms grow on surfaces, the individuals therefore push each other toward the outer rim. In this way, the colony expands steadily, as long as it remains flat.
As a consequence of this process, patches form for straight microorganisms. These patches are distinguished by different colors in the top image on the cover of the May 20 issue of Biophysical Journal. Colors code common orientations of all individuals within one patch, but different orientations between the patches. Corresponding observations have been made in both previous experimental and theoretical approaches.
Our study “Effects of curvature on growing films of microorganisms” explores the changes in this appearance that result when the individual microorganisms are not straight. Instead, bending of the body shape is introduced, as prevalent in various species of bacteria. Under these modified circumstances and notable curvature of the individuals, the patchy domain structures do not emerge anymore during the growth of the colonies. Instead, fragmented textures appear; see the bottom image on the cover.
Curved microorganisms in growing colonies were found to feature branch-like structures. Specifically, the curved individuals are spatially organized like cups piled into each other. The bottoms of these stacked cups tend to point to the outside of the colony. This tendency implies that orientations and positions within the colony show some coupling. Corresponding trends were not observed in comparative studies on straight objects. Thus, the research reveals modes of surface conquering that are different for curved microorganisms than for straight ones.
Understanding these trends is important far beyond a basic conceptual perspective. It is key to know how bacteria conquer surfaces in many medical contexts, from growing plaque on teeth to expanding biofilms on prostheses. This knowledge supports an enhanced strategy in coping with undesired surface coverage.
— Yuta Kuroda, Takeshi Kawasaki, and Andreas M. Menzel