Jun-ichi Wakita

Interface growth and pattern formation in bacterial colonies

Bacterial species Bacillus subtilis is known to exhibit various colony patterns, depending on the substrate softness and nutrient concentration. We have examined the self-affinity and roughness exponent α for growing interfaces of colonies which belong to regions B (Eden-like) and D (disk-like) in the morphological phase diagram of this species. We have obtained α≃0.78 and 0.50 in the regions B and D, respectively, and found that the difference of α values can be attributed to that of microscopic growth mechanisms of colony interfaces. We have also proposed a unified model which produces characteristic colony patterns observed in our experiments. It is a diffusion–reaction-type model for the population density of bacterial cells and the concentration of nutrient. The essential assumption is that there exist two types of bacterial cells; active and inactive. Our model is found to be able to globally reproduce all the colony patterns in the phase diagram.

Experimental Investigation on the Validity of Population Dynamics Approach to Bacterial Colony Formation

We have investigated the dynamics of a two-dimensional spreading of a bacterial population in a surface environment. After point inoculation of flagellated bacteria ( Bacillus subtilis ) on nutrient-rich semi-solid medium, the bacterial population grew up by multiplication and translocation of cells, and developed a homogeneous round colony. By comparing experimental results with those of numerical simulations of the model equation, we found that this homogeneous population growth of bacteria is an actual manifestation of growth dynamics described by the Fishers equation.

Formation of colony patterns by a bacterial cell population

Bacterial species Bacillus subtilis is known to exhibit various colony patterns, such as diffusion-limited aggregation (DLA)-like, compact Eden-like, dense branching morphology (DBM)-like, concentric ring-like and disk-like, depending on the substrate softness and nutrient concentration. We have established the morphological diagram of colony patterns, and examined and characterized both macroscopically and microscopically how they grow. For instance, we have found that there seem to be two kinds of bacterial cells; active and inactive cells, the former of which drive colony interfaces outward. The active cells are particularly distinguished from the inactive ones at the tips of growing branches of a DBM-like colony as the characteristic fingernail structure. We have also found that the concentric ring-like colony is formed as a consequence of alternate repetition of advancing and resting of the growing interface which consists of active cells. Based on our observations, we have constructed a phenomenological but unified model which produces characteristic colony patterns. It is a reaction–diffusion type model for the population density of bacterial cells and the concentration of nutrient. The essential assumption is that there exist two types of bacterial cells; active cells that move actively, grow and perform cell division, and inactive ones that do nothing at all. Our model is found to be able to reproduce globally all the colony patterns seen in the experimentally obtained morphological diagram, and is phenomenologically quite satisfactory.

Self-Affinity for the Growing Interface of Bacterial Colonies

We have investigated experimentally the self-affinity of bacterial colonies. We examined roughness exponent α for one-dimensional growing interfaces of colonies which belong to regions B and D in t...

Dynamic Aspects of the Structured Cell Population in a Swarming Colony of Proteus mirabilis

Proteus mirabilis forms a concentric-ring colony by undergoing periodic swarming. A colony in the process of such synchronized expansion was examined for its internal population structure. In alternating phases, i.e., swarming (active migration) and consolidation (growth without colony perimeter expansion), phase-specific distribution of cells differing in length, in situ mobility, and migration ability on an agar medium were recognized. In the consolidation phase, the distribution of mobile cells was restricted to the inner part of a new ring and a previous terrace. Cells composing the outer part of the ring were immobile in spite of their ordinary swimming ability in a viscous solution. A sectorial cell population having such an internal structure was replica printed on fresh agar medium. After printing, a transplant which was in the swarming phase continued its ongoing swarming while a transplanted consolidation front continued its scheduled consolidation. This shows that cessation of migration during the consolidation phase was not due to substances present in the underlying agar medium. The ongoing swarming schedule was modifiable by separative cutting of the swarming front or disruption of the ring pattern by random mixing of the pattern-forming cell population. The structured cell population seemed to play a role in characteristic colony growth. However, separation of a narrow consolidation front from a backward area did not induce disturbance in the ongoing swarming schedule. Thus, cells at the frontal part of consolidation area were independent of the internal cell population and destined to exert consolidation and swarming with the ongoing ordinary schedule.

Experimental Investigation on the Formation of Dense-Branching-Morphology-Like Colonies in Bacteria

We have investigated experimentally the pattern formation of bacterial colonies. We have especially examined DBM (dense-branching-morphology)-like colonies of bacterial species Bacillus subtilis . It was found from microscopic observations that many active bacterial cells collect and make a group on the tip of each growing branch. Branches repel each other and split sometimes as they grow outward. It was confirmed macroscopically that both averaged branch width and averaged branch gap decrease systematically when increasing the nutrient concentration C n , while their ratio remains unchanged with the approximate value of one. It was also found that the distribution of branch lengths is very close to exponential, suggesting that the tip-splitting of branches takes place at random.

Periodic Colony Formation by Bacterial Species Bacillus subtilis

We have investigated the periodic colony growth of bacterial species Bacillus subtilis . A colony grows cyclically with the interface repeating an advance (migration phase) and a rest (consolidatio...

Periodic Pattern Formation of Bacterial Colonies

We have experimentally investigated pattern formation of colonies of bacterial species Proteus mirabilis , which is famous for forming concentric-ring-like colonies. The colony grows cyclically with the interface repeating an advance and a stop alternately on a surface of a solid agar medium. We distinguish three phases (initial lag phase, the following migration and consolidation phases that appear alternately) for the colony growth. When we cut a colony just behind a migrating front shortly after the migration started, the migration ended earlier and the following consolidation lasted longer. However, the following cycles were not influenced by the cut, i.e., the phases of the migration and consolidation were not affected. Global chemical signals governing the colony formation from the center were not found to exist. We also quantitatively checked phase entrainment by letting two colonies collide with each other and found that it does not take place in macroscopic scales. All these experimental results ...

Statistical Features of Complex Systems –Toward Establishing Sociological Physics–

Complex systems have recently attracted much attention, both in natural sciences and in sociological sciences. Members constituting a complex system evolve through nonlinear interactions among each other. This means that in a complex system the multiplicative experience or, so to speak, the history of each member produces its present characteristics. If attention is paid to any statistical property in any complex system, the lognormal distribution is the most natural and appropriate among the standard or “normal” statistics to overview the whole system. In fact, the lognormality emerges rather conspicuously when we examine, as familiar and typical examples of statistical aspects in complex systems, the nursing-care period for the aged, populations of prefectures and municipalities, and our body height and weight. Many other examples are found in nature and society. On the basis of these observations, we discuss the possibility of sociological physics.

Morphological Diversity of the Colony Produced by Bacteria Proteus mirabilis

Morphological changes of colonies have been investigated for a bacterial strain of Proteus mirabilis , which is a famous species for producing concentric-ring-like colonies. It was found that colony patterns can be classified into three types, i.e., cyclic spreading, diffusion-limited growth (DLA-like) and three-dimensional growth (inside the agar medium) patterns. Cyclic spreading patterns can further be classified into three subgroups, i.e., concentric-ring, homogeneous and spatiotemporal patterns. These subgroups were classified by examining the development of colony structure after colonies spread all over petri-dishes. Comparison of the results with those of another bacterial species Bacillus subtilis is also discussed.