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We present the morphology diagram (morphologies as function of peptone levels and agar concentrations) observed during colonial development of Paenibacillus alvei bacteria. These bacteria are close to Paenibacillus dendritiformis and have been studied extensively in the past. Like P. dendritiformis, P. alvei produces a layer of lubricating fluid for movement on hard surfaces. Unlike P. dendritiformis which attempts to swim on hard surfaces, P. alvei main movement on hard surfaces is swarming, like the movement of P. vortex. Under some growth conditions P. alvei and P. vortex exhibit collective migration of clusters of bacteria and formation of bacterial vortices. Under different growth conditions P. alvei develop branching patterns. P. alvei also develop new class of patterns which we coin “nebula patterns”. Modeling of the new patterns is still an open challenge.

Bacteria have developed sophisticated modes of cooperative behavior to cope with unfavorable environmental conditions. Here we report the effect of antibiotic stress on the colonial development of Paenibacillus dendritiformis and P. vortex. We focus on the effect of co-trimoxazole on the colonial organization of P. dendritiformis. We find that the exposure to non-lethal concentrations of antibiotic leads to dramatic changes in the colonial growth patterns. Branching, tip-splitting patterns are affected by reduction in the colonial fractal dimension from Df=2.0 to 1.7, appearance of pronounced weak chirality and pronounced radial orientation of the growth. We combine the experimental observations with numerical studies of both discrete and continuous generic models to reveal the causes for the modifications in the patterns. We conclude that the bacteria adjust their chemotactic signaling together with variations in the bacteria length and increase in the metabolic load.

Bacterial colonies have developed sophisticated modes of cooperative behavior which enable them to respond to adverse growth conditions. It has been shown that such behavior can be manifested in formation of complex colonial patterns. Certain Bacillus species exhibit collective migration, “turbulent like” flow and emergence of whirlpools during colonial development. Here we present experimental observations of collective behavior and a generic model to explain such behavior. The model incorporates self-propelled and interacting “particles” (swarmers). We show that velocity interaction between the particles can lead to a synchronized movement. To explain vortices formation, we propose a plausible mechanism involving a special chemotactic response (rotational chemotaxis) which is based on speed modulations according to the concentration of a chemoattractant. This mechanism differs from that exhibited by swimming bacteria. We show that the chemomodulation of swarmers' speed together with the velocity interactions impose a torque on the collective motion and can lead to formation of vortices. The inclusion of both attractive and repulsive rotational chemotaxis in the model captures the salient features of the observed growth patterns.

Bacterial colonies have developed sophisticated modes of cooperative behavior which enable them to respond to adverse growth conditions. It has been shown that such behavior can be manifested in the development of complex colonial patterns. Certain bacterial species exhibit formation of branching patterns during colony development. Here we present a generic model to describe such patterning of swimming (tumbling) bacteria on agar surfaces. The model incorporates: (1) food diffusion, (2) reproduction and sporulation of the cells, (3) movement of the bacterial cells within a self-produced wetting fluid and (4) chemotactic signaling. As a plausible explanation for transitions between different branching morphologies, we propose an interplay between chemotaxis towards food, self-produced short range chemoattractant and long range chemorepellent.