Motility is an important determinant of bacterial survival.Although some only glide, many bacteria can swim by rotating their flagellum, an ability that allows them to move toward a more favorable environment when they detect toxic conditions. Bacteria sense changes in the levels of surrounding nutrients and toxic chemicals, pH, temperature, light, the magnetic field of the Earth, electron transport conditions, and the like, and they respond with osmoregulation, altering internal conditions to protect against the changing external environment, or by moving to a more favorable environment, chemotaxis. Osmoregulation is a complex "two-component system" for regulating cellular response to external conditions. Chemotaxis is also a two-component sensory system, and it is controlled by more than 40 genes and sets of genes that control the flagella, transmembrane receptors, and signal trans-duction involved in bacterial responses to stress. Bacterial cell membranes are loaded with porins, proteins that allow the diffusion of important molecules in and out to maintain homeostasis and internalize nutrients.

In a two-component system a transmembrane receptor protein transduces environmental signal into metabolic changes by phosphorylating a second protein, as described in earlier general discussions of signaling processes. The rate of phos-phorylation is under the control of the receptor proteins, and in chemotaxis, for example, the second protein affects the flagella to result in a change in the direction in which the bacterium swims. Ciliated protozoa use chemical sensing to find potential mates and avoid predators. Plants and fungi also use similar two-component response systems.

Quorum Sensing

Bacteria use the information they gather from the environment for quorum sensing, the ability that allows them to determine the density of other bacteria around them and, depending on population size, form into biofilm. This then leads to the expression of genes that aid in the survival of the biofilm, the colonization of higher organisms, such as the roots of legumes, or finding the sites of adhesion or invasion of higher organisms to initiate infection (catheters are a common site). The environmental sensing is done by transmembrane sensory proteins, often coupled with cyto-plasmic receptors, a system that has been well-characterized at the molecular level. Bacterial transmembrane receptors have common structural features: an extracellular binding domain for one or more ligand, a transmembrane region, a cytoplasmic linker, and methylation and signaling regions. The receptors form homodimers and cluster together on the cell membrane.

Slime Mold

Dictyostelium discoideum are amoeboid cells that live a well-characterized life cycle, many stages of which are triggered by environmental cues including depletion of the food supply (see, e.g., Bonner 2000). (See Figure 12-9.) Initially, the cells live in soil, where they feed on bacteria. While food is plentiful the cells multiply by mitotic division, but as the population grows the food supply can become depleted and the cells begin to starve. The cells are constantly monitoring population density by sensing and secreting a protein called conditioned medium factor (CMF), and when population density is large enough to threaten starvation, that is, when the concentration of CMF is high enough, cells signal other cells to aggregate. It is time to search for greener pastures.

~ 24 hours

Figure 12-9. Life cycle of the slime mold Dictyostelium discoideum.

~ 24 hours

Figure 12-9. Life cycle of the slime mold Dictyostelium discoideum.

The signaling molecule is a substance called acrasin, which is cAMP. Cells adjacent to the signaling cell receive the signal via cell surface G protein-coupled receptors (GPCRs) for cAMP. The GPCRs transmit signal to the cytoplasm that initiates the pathway to the release of cAMP, which is then received by neighboring cells that, in turn, send their own signal, and so on until a colony is formed. The patterns of colony formation that result vary and are a function of the spatiotemporal pattern of signal emission. The colony is mobile, which allows the cells to migrate to a richer environment in which to produce the next generation. Within about 24 hours, the colony forms a fruiting body that disperses spores and begins the life cycle over again.

The colony, like a bacterial biofilm, or in fact any collection of aggregated cells, becomes more than the sum of its parts. Genes are expressed at this stage that are not expressed in independent cells. The slug of aggregated cells is about 1 mm long and can stay together for up to several days. The slug is polar and mobile, with its movement mediated by waves of cAMP that travel from the tip to the tail, inducing individual cells to migrate toward the tip, thus moving the slug in the direction of cAMP concentrations. The slug is extremely sensitive to environmental clues, so that small differences in light, temperature, or the concentration of ammonia gas produced by the cell mass itself will affect the direction of the slug's movement. Presumably this optimizes the location of the final fruiting body with respect to food supply. Interestingly, formation of the fruiting body depends on programmed cell death to take the slug through the stages of culmination to formation of the stalk and then to maturation, so in a very real sense this collection of single celled organisms becomes a multicellular being, with once free-standing cells sacrificing themselves for the good of the group.

A Side-Comment on Quorum Sensing, Cooperation, and Group Behavior

Quorum sensing is an interesting phenomenon, if it is being properly understood. It is one of many instances in which animals aggregate, swarm, or give display behaviors in a way that appears to reflect their population size or density and that has been interpreted as being a mutual signal among individuals that they interpret to alter their behavior. Controversy has been particularly heated over whether this kind of behavior could lead to altruistic self-sacrifice, individuals restricting their reproduction so that the group does not overpopulate relative to available resources. In a phrase, the issue is group selection as compared to the classic individual selection described in Chapter 3. The problem is the need to explain how alleles leading to self-sacrifice could increase in frequency. Formally, this becomes a mathematical problem for population genetics theory; informally, one can see many ways in which behavior can evolve that is good for the group so long as the sacrifice of individuals initially responsible for the behavior is not too immediate and complete.

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