Tiny Conspiracies

The secret, social lives of bacteria

Quorum sensing is not restricted to glow-in-the-dark marine bacteria. In the past decade, scientists have found it in many other species, with variations in the autoinducer molecules secreted, the means by which they are detected, the biochemical reactions they trigger, and the behavior they regulate. For example, quorum sensing controls the production of virulence factors (toxins and other disease-causing agents) in numerous human and plant pathogens that have a clinical or agricultural impact. Invading bacteria may improve their odds of overcoming a host's defenses by releasing their virulence factors simultaneously and only when they are present in great numbers. A premature release might tip off the host's immune system.

In natural environments, bacterial species compete with one another for nutrients, for entry into hosts, and for survival under hostile conditions. Many bacterial species produce antibiotics--chemical compounds to which they themselves are immune but that kill their competitors or impede their growth. Quorum sensing enables the bacteria to coordinate the release of these antibiotics in high doses.

Quorum sensing also enhances the ability of some bacteria to acquire DNA fragments that, because of the death of some of their fellows, are up for grabs in the environment. These DNA fragments are a useful resource for repairing mutated or damaged chromosomes. Only where there is a concentrated population of bacteria is there likely to be any substantial amount of free DNA available. In this case, quorum sensing turns on the machinery that enables cells to take in this DNA.

Bacterial mating, which creates a more diverse array of individuals and can spread advantageous genes through a species, seems to employ quorum sensing as well. The process involves donor cells and recipient cells. We know that in Agrobacterium tumefaciens, a species that causes tumors in susceptible plants, the donors communicate with one another through quorum sensing, but exactly what function this serves is not yet understood.

Often bacteria live in biofilms, or communities attached to a surface such as a rock in a pond or the lining of an intestine. A biofilm is surrounded by a polymer coating, or shield, that keeps the bacteria from drying out and that also resists antibiotics and other environmental assaults. The bacterial community is typically made up of several different species; as in a human metropolis, each member of the community—usually each species—has a specific job. One member, for example, may be responsible for producing the enzymes and molecular building blocks needed to create the polymer shield. Within the biofilm is a network of channels that allow water and nutrients to reach the resident bacteria and permit waste products to flow out. At least in some cases, proper formation of these channels has been shown to be dependent on quorum sensing, although the details of how this is controlled remain to be worked out.

Many bacteria are known to produce and detect several different autoinducers. For example, recent studies show that the free-living luminous bacterium V. harveyi, in addition to having the quorum-sensing system that enables it to "turn on" its glow, has a separate system that involves another autoinducer. This second chemical signal has been found in a variety of other bacteria as well. These and other findings have led to speculation that this widespread molecule is the basis of a common "language," a bacterial Esperanto providing communication between species.

The capacity to distinguish signals both from its own kind and—through a more universal code—from others could provide a population of a particular bacterial species with valuable information. It could learn not only the cell density of its own population but also whether or not it was sharing its habitat with other species and even whether its own kind was in a majority or a minority at any given time. By adjusting its behavior, this population could then make the most of the prevailing conditions.

Many bacteria that infect humans have now been shown to produce the interspecies signal molecule. They include Escherichia coli (food poisoning), Salmonella typhimurium (food poisoning), S. typhi (typhoid fever), Haemophilus influenzae (pneumonia, meningitis, sepsis), Helicobacter pylori (peptic ulcers, stomach cancer), Borrelia burgdorferi (Lyme disease), Neisseria meningitidis (meningitis), Yersinia pestis (bubonic plague), Campylobacter jejuni (food poisoning), Vibrio cholerae (cholera), Mycobacterium tuberculosis (tuberculosis), Enterococcus faecalis (endocarditis, urinary tract infections), Streptococcus pneumoniae (pneumonia, ear inflammations), and Staphylococcus aureus (pneumonia, endocarditis, septicemia, toxic shock syndrome, meningitis, food poisoning). While for the most part the specific function served by the autoinducer in these bacteria is not yet known, there is mounting evidence that, at least in some cases, it increases virulence.

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