Friday, October 22 3:00 p.m.

Quorum Sensing in Bacteria

Steve Stone, JD (Yale)
M.Ed. Student
George Mason University

Quorum sensing refers to the ability of bacteria to detect the presence of other bacteria. When a “quorum” of nearby bacteria is detected, genes coding for group behaviors may be up-regulated, causing the bacterium to switch lifestyles — e.g., from a free-swimming “planktonic” form, to a colonial form living as, e.g., (1) a sessile biofilm, (2) an aggressive “swarming” colony attacking an infected host, or (3) in one well-studied case, a colony of symbiotic bacteria producing light to assist in camouflaging a small squid.

The general mechanism of quorum sensing may be summarized as follows:

Each bacterium secretes chemical signals called “autoinducers” into the local aqueous environment. When detected in sufficient concentration these signaling molecules indicate the presence of a critical mass of nearby bacteria. The detection of a threshold level of autoinducer molecules triggers a conformational change in a receptor protein, opening a DNA binding domain on the receptor protein that then binds to the promoter region for an “operon” — a cluster of genes coding for a group of related proteins. The proteins coded for by the up-regulated operon are essential to the expression of group behaviors such as launching an attack against a host organism or forming a
biofilm — behaviors would be ineffective and wasteful if undertaken by a single bacterium.

Quorum sensing can involve either a particular species of bacteria, or generic signaling that is recognized and responded to across many species of bacteria. An example of the latter
is the formation of films of bacteria on our teeth each night.

Quorum sensing enables very small organisms to “band together” to adopt mutually beneficial
adaptive behaviors and use limited resources more efficiently and effectively. It also enables organisms with a small genome to effectively adopt a different phenotype (a different set of
expressed genes) when circumstances (i.e. presence of other bacteria) permit.

Computer modeling of quorum sensing is an active area of research.