Sensing extruded chemicals:

https://www.the-scientist.com/sponsored-article/brush-up-quorum-sensing-in-bacteria-and...

Individual bacteria use quorum sensing to detect and respond to changes in cell density as a coordinated group. For example, the first reported evidence of quorum sensing was in two marine bacterial species that emit light in response to high cell density, which is responsible for bioluminescence in various marine hosts. Researchers have identified many species of bacteria that interrelate gene regulation with their neighbors to control a variety of processes, including symbiosis, virulence, antibiotic resistance, and biofilm formation.

"Quorum sensing is mediated by bacteria producing, releasing, and detecting extracellular signaling molecules called autoinducers. Typically, when individual bacteria divide, they produce and release autoinducers. When cell density is low, these molecules diffuse in the environment, and bacteria regulate gene expression independently. Autoinducer levels increase as bacteria continue to divide, and when the extracellular concentration of these signaling molecules reaches a certain threshold, individual bacteria detect that the quorum threshold has been reached. As a result, they regulate signaling pathways and gene transcription in response to the surrounding cell population density. When bacteria sense a quorum, they typically make more proteins involved in autoinducer production, which creates a positive feedback loop to maintain coordinated behaviors related to high population density. Induction of the feedback loop is why quorum signaling molecules are commonly called autoinducers.

"Both gram-negative and gram-positive bacteria use quorum sensing. Generally, gram-negative bacteria employ acylated homoserine lactones as autoinducers, which diffuse into the environment passively. In contrast, gram-positive bacteria typically use processed oligopeptides, which must be actively secreted through their cell wall.

"Intraspecies quorum sensing allows a variety of bacteria to communicate and alter their behavior in response to bacteria of the same species. Additionally, interspecies quorum sensing between different bacterial species can lead to both conflict and collaboration. For example, enteric bacteria in the human gut microbiome coordinate behavior with each other and against harmful bacteria. Unfortunately, interspecies quorum sensing is not always to the benefit of humans; some bacteria coordinate with one another to enhance each other’s virulence and form symbiotic relationships to create biofilms that protect pathogens from antibiotics.

"Bacteria regulate virulence genes and other pathogenic factors with quorum sensing. Activating virulence genes at low cell density might alert the host defence response, which prevents invading bacteria from establishing themselves. As such, opportunistic bacteria control virulence genes through quorum sensing to wait until they achieve a sufficient cell density to overpower host defence mechanisms. Once the optimum autoinducer concentration threshold is reached, they synthesize virulence factors to ensure their infection and survival.

***

"While studying quorum sensing’s role in mediating viral infection of bacteria, researchers identified a comparable communication system in temperate bacterial phages—viruses that enter either the lytic or lysogenic pathway when they infect a bacterium. Viruses in the lytic pathway kill and lyse their host bacterial cell, but in the lysogenic cycle, the phage integrates its genetic material into the bacterial genome and keeps its host alive. Researchers discovered that some phages monitor the concentration of arbitrium—the viral equivalent of an autoinducer—to decide when to switch from the lytic to lysogenic pathway. This decision is based on the density of lysed infected bacteria in their environment, which helps viruses avoid depleting all available hosts. Scientists hypothesized that arbitrium signals may also alter the activity of important bacterial genes, and that this quorum sensing-like communication system may be common among phages and possibly some human viral pathogens.

Comment: obviously both bacteria and viruses have receptors for these specific signaling molecules and built-in automatic responses to teh elvels involved..