Preliminaries: What is a Bacterium?
A bacterium is a prokaryotic unicellular organism. Its structure is basically conformed by a central body of microscopic size that can take many different forms (Young, 2006) and whose size can vary from 0.01 μm3 to a volume 1010 times bigger (Angert et al., 1993; Rappe et al., 2002). Many bacteria are endowed with a series of rotating flagella in its cell surface that act as propellants, allowing them to swim at a speed of 10-35 μm/s (Guzmán et al., 2010). In addition to the appropriate structure to move in an autonomous way, bacteria have potential receivers (chemoreceptors and photoreceptors) capable of detecting temporal-space changes in the environment that surrounds them. In this way, when an external perturbation is detected, bacteria use their memory to make a temporal-space comparison of the gradients found. Depending on the external conditions sensed, bacteria change their movements from a random walk to a biased walk (Guzmán et al., 2010).