Microorganisms like bacteria, are one of the simplest systems in the living world. It is because of this that many studies have focused on the importance of colony formation, orientation, self-propulsion and collective behavior. Many researchers have been motivated by these natural systems to model the behaviors in artificially manufactured self- propelled catalytic particles using computer simulations or other physical techniques and related theories. Tracked particles suspended in soap film containing Bacillus subtilis or E. Coli bacteria (which moves by rotating its flagellum) revealed common performance. In vitro motility is used to study collective motion in particles (migration of bacteria in clusters). In vitro refers to “In an artificial environment outside the living organism” (http://www.biology-online.org/dictionary/In_vitro) to obtain more specified and efficient results. In many studies the results show that the probability of bacteria to merge as a cluster grows significantly as the bacterial density increases, though the cluster remaining uniform. Under some conditions, bacterial aggregates show transitions that include adoption of a pattern by each individual element in the system, meaning that the behaviors are most likely determined by the collective effects of the other particles in the system. However, the unit behaves completely different individually. Usually under a typical, favorable state the colonies do not demonstrate much organization. It is under adverse conditions (“like limited nutrient sources” Shapiro, 1988) that the units accumulate and move in a uniform direction revealing a larger average speed compared to that of a single unit. On a test conducted by Wu et al. (2009), it was reported that “members of a certain kind of bacteria (Myxococcus xanthus) regularly reverse their direction, heading back to the colony which they have just came from”. A mathematical simulation was manipulated to mimic the behavior of this particular bacterium. The outcome exposed a surprising habit. Reversing its direction actually generates a parallel movement between the molecules, reducing collisions among each other. Without this action the aggregate will most likely be disordered making it to move slower and possibly ceasing the movement completely.
In a recent study by Grossman et al., 2008, he explains that no rule is necessarily required to explain the phenomena. “Ordered motion emerging in a system of self-propelled particles looks like this: The particles are trying to maintain a given absolute velocity and the only interaction between them is a repulsive linear force ( ⃗F) within a short distance (i.e., they do not “calculate” the average of the velocity of their neighbors, and the only interaction is through a pair-wise central force)”.
As simple as it may seem, movement and particle orientation can generate countless questions. The nano-particle technology is still, for the most part, unknown and further investigation is needed to understand these complex systems. Observing naturally occurring behaviors in microorganisms is a significant approach to produce nano-particles of high quality for use in many fields.