The myth of "simple bacterial cells": complexity in the microbial world

Bacteria are small organisms, frequently thought of as "simple" microorganisms, although they have a great capability to adapt to environment and to respond to its changes. In fact, bacteria sense a wide range of signals including temperature and pH changes, nutrient concentrations (chemotaxis), osmolarity, oxygen and light (phototaxis) and they integrate the information to generate several kinds of response. Recently chemotaxis studies, one of the most studied response, have demonstrated both in E. coli and in other bacteria that chemoreceptors form, in the membrane, two-dimensional lattice clusters. Each unit of the cluster is composed from three couple of chemoreceptors and two or more receptor coupling proteins, CheW or CheV. In E. coli chemoreceptors and CheW form a supramolecular complex with the amplification protein, CheA. The stoichiometric ratio about 1:1:1, suggests a direct interaction between signalling complex and the amplification protein. However, in other bacteria the stoichiometric ratio is not so strong, and, in addition, the receptors in the clusters can be not always of the same type. In the original model for chemotaxis a transduction chain was suggested, in which signal transduced from a single chemoreceptor was amplified from the diffusible protein, CheA, and transmitted by the diffusing protein, CheY, to the flagellar motor. The clusters, the evidences for functional interactions among the receptors within the clusters, the differences in stoichiometric ratios, led to hypothesize a signalling receptor network. Furthermore, the localization of the clusters mainly near the polar flagellum, asks some questions about the role of the diffusing protein CheY. The phototaxis model resembles the chemotaxis model. A photoreceptor, actually, consists of two proteins, one that captures the light and induces a conformational change in the other protein that, equal both in form and in function to a chemoreceptor, transduces the captured light signal. Then, for phototaxis the transduction model is similar to that for chemotaxis, with an added level of complexity because it has to take in account the properties of the specific photoreceptor. Chemotaxis and phototaxis, together or individually, are involved in Quorum Sensing, in Biofilm formation, in Symbiotic associations. These, in the last years, have sparked interest regarding, inter alia, bio-inspired devices; bio-based algorithms; microbial remediation; gut-microbiota and its symbiotic balance, so relevant to the human health. In this context, a new overall model for chemotaxis and phototaxis could help to obtain a better knowledge of the bacterial capabilities and of their possible applications.