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Bacterial communities

Swarm cells start growing like any other bacterium on laboratory-prepared nutrient medium. (Media are liquids or solids containing gel-like agar that supply bacteria with all the nutrients needed for growth.) They metabolize for a while, split in two, and repeat this until nutrients run low. Rather than halting the colony's growth, swarm cells signal each other to change the way they reproduce. The swarmer Proteus develops a regular colony when incubated, each cell about three μm in length. After several hours, cells on the colony's outer edge elongate to 40 to 80 μm and sprout numerous flagella. Ten to 12 flagellated cells team up and then squiggle away from the main colony. By forming teams of cells lined up in parallel, 50 times more flagella power the cells forward than if one Proteus headed out on its own. Several millimeters from the main colony, the swarmers stop and again begin to reproduce normally. As generations of progeny grow, they build a ring of Proteus around the original colony, shown in Figure 1.2. At a certain cell density in the ring, Proteus repeats the swarming process until a super-colony of concentric rings covers the entire surface. When two swarming Proteus colonies meet, they do not overrun each other. The two advancing fronts stop within a few μm of each other, repelled by their respective defenses. Proteus produces an antibacterial chemical called bacteriocin. The specific bacteriocin of each swarmer colony protects its turf against invasion.

Figure 1.2

Figure 1.2 The swarming bacterium swarms outward from a single ancestor cell and forms concentric growth rings with each generation.

Other swarmer bacteria use hairlike threads called pili rather than flagella, and cast their pili ahead to act as tethers. By repeatedly contracting, the cells drag themselves forward to up to 1.5 inches per hour. Petri dishes measure only 4 inches across, but if dishes were the size of pizzas, swarm cells would cover the distance.

Communities such as biofilm grow on surfaces bathed in moisture. Biofilms cover drinking water pipes, rocks in flowing streams, plant leaves, teeth, parts of the digestive tract, food manufacturing lines, medical devices, drain pipes, toilet bowls, and ships' hulls. Unlike swarming colonies, biofilm contains hundreds of different species, but they too interact via quorum sensing. (Bacteria that merely attach to surfaces such as skin are not true biofilms because they do not coalesce into a community that functions as a single entity.) Biofilm begins with a few cells that stick to a surface by laying down a coat of a sticky polysaccharide. Other bacteria hop aboard and build the diverse biofilm colony.

Biofilms facilitate survival by capturing and storing nutrients and excreting more polysaccharide, which protects all the members against chemicals such as chlorine. Eventually fungi, protozoa, algae, and inanimate specks lodge in the conglomeration of pinnacles and channels. When the biofilm thickens, signals accumulate. But because many different species live in the biofilm, the signals differ. Some bacteria stop making polysaccharide so that no more cells can join the community. The decrease in binding substance causes large chunks to break from the biofilm, move downstream, and begin new biofilm. (This constant biofilm buildup and breakdown causes great fluctuations in the number of bacteria in tap water. Within a few hours tap water can go from a few dozen to a thousand bacteria per milliliter.) Meanwhile, other bacteria ensure their own survival by increasing polysaccharide secretion, perhaps to suffocate nearby microbes and reduce competition.

Pathogens likely use similar strategies in infection by turning off polysaccharide secretion. With less polysaccharide surrounding the bacteria, the cells can reproduce rapidly. Then when pathogen numbers reach a critical level in the infected area, polysaccharide secretion returns to quash competitors.

A second type of multispecies community, the microbial mat, functions in complete harmony. Microbial mats lie on top of still waters and are evident by their mosaic of greens, reds, oranges, and purples from pigmented bacteria. Two types of photosynthetic bacteria dominate microbial mats: blue-greenish cyanobacteria and purple sulfur-using bacteria. During the day, cyanobacteria multiply and fill the mat's upper regions with oxygen. As night falls and cyanobacteria slow their metabolism, other bacteria devour the oxygen. Purple bacteria prefer anoxic conditions, so they live deep in the mat until the oxygen has been depleted. In the night, the purple bacteria swim upward and feast on organic wastes from the cyanobacteria. The sunlight returns, and the purple bacteria descend to escape the photosynthesis about to replenish the upper mat with oxygen. As they digest their meal, these bacteria expel sulfide compounds that diffuse to the top layer. There, sulfur-requiring photosynthetic bacteria join the cyanobacteria (and some algae) in a new cycle. An undisturbed mat literally breathes: absorbing oxygen and emitting it, expelling carbon dioxide and inhaling it one breath every 24 hours. Microbial mats' diurnal cycle makes them a distinctive microbial community.

Communities are mixtures of species within an ecosystem. Ecosystems contain living communities that interact with the nonliving things around them: air, water, soil, and so on. Bacteria participate in every phase of ecosystem life, but to learn about bacteria microbiologists must remove them from the environment and study one species at a time in a laboratory. A collection of bacterial cells all of the same species is called a population, or in lab talk a pure culture.

Microbiologists learn early in their training the tricky job of keeping all other life out of a pure culture by using aseptic technique. Aseptic—loosely translated as "without contamination"—technique requires that a microbiologist manipulate cultures without letting in any unwanted bacteria. They accomplish this by briefly heating the mouth of test tubes over a Bunsen burner flame, similarly flaming metal inoculating loops, and learning to keep sterilized equipment from touching unsterilized surfaces. Surgeons follow the same principles after they scrub up for surgery.

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