Glowing bacteria taught scientists that microbes take a headcount before acting
The marine bacterium Aliivibrio fischeri only starts producing light once enough of its neighbours are around. Each cell releases a signalling molecule and senses how much others are making. This quorum sensing, first spotted in that glowing microbe, lets bacteria coordinate behaviour, across species as well as within them.
Cell signalling is how a cell communicates with itself, its neighbours and its environment, a property shared by every form of life. A typical system has three parts: a first messenger, called a ligand; a receptor that recognises it; and the resulting signal. Most signals are chemical, from ions like calcium to lipids, peptides and carbohydrates, but pressure, voltage, temperature and light can also serve.
Distance defines the type. Autocrine signals act on the cell that made them and intracrine signals on receptors inside that same cell. Juxtacrine signalling needs direct contact, paracrine signals reach nearby cells, and endocrine signals, hormones, travel through the blood to distant targets. Chemistry dictates the route in. Peptide hormones such as insulin are water-loving and cannot cross the fatty membrane, so they bind receptors on the surface. Steroid hormones slip straight through and meet receptors inside, which carry them into the nucleus to switch on particular genes.
Surface receptors come in three main families. Ion channel receptors open or close gates for specific ions and include most sensors of pressure and temperature. G-protein coupled receptors have outside, spanning and inside sections, and the inner part sets off a chain of chemical reactions. Enzyme-linked receptors carry catalytic machinery on their inner end. Many pathways run through second messengers, which amplify the message so that a handful of activated receptors can trigger a large response.
These conversations underpin development, tissue repair, immunity and the body's internal balance, and when they go wrong the result can be cancer, autoimmune disease or diabetes. Simpler organisms use signalling too. Slime mould cells crawl towards a chemical called acrasin and gather into fruiting bodies that make spores; some species use cyclic AMP as the cue, while Polysphondylium violaceum uses a dipeptide named glorin.
Source: Cell signaling
