Microbiology Terms Starting With Q
Microbiology Glossary: Q
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Quorum Inhibition
/ KWOR-um in-hih-BISH-un / · From Latin quorum, meaning of whom, and inhibere, meaning to hold back.
Quorum Inhibition is the disruption of bacterial chemical signaling systems to prevent the coordinated expression of group behaviors such as biofilm formation and virulence factor production.
This strategy targets quorum sensing pathways without killing bacteria directly, which reduces the selective pressure that drives resistance development compared with conventional antibiotics. Quorum inhibition can operate through several distinct mechanisms: enzymatic degradation of signaling molecules, competitive blocking of signal receptors, or interference with signal biosynthesis. The marine red alga Delisea pulchra produces halogenated furanones that mimic bacterial acyl-homoserine lactone signals and competitively block their receptors, preventing biofilm formation on the alga’s surface.
Clinical researchers are investigating quorum inhibitors as adjunct therapies for P. aeruginosa lung infections in cystic fibrosis patients, where chronic biofilms drive progressive respiratory decline.
Garlic (Allium sativum) contains organosulfur compounds, particularly ajoene and allicin, that inhibit quorum sensing in Pseudomonas aeruginosa at concentrations achievable in tissue, which may partly explain its traditional use as an antimicrobial remedy beyond any direct bactericidal effect.
Quorum inhibition kills bacteria the way antibiotics do. Quorum inhibitors disarm pathogens by blocking their communication circuits rather than targeting growth or cell-wall synthesis, so bacterial populations survive but lose the coordinated behaviors needed to establish infection.
Bacillus species found in soil produce AHL-lactonase enzymes that degrade the acyl-homoserine lactone signals released by neighboring gram-negative bacteria such as Pseudomonas fluorescens. In mixed-species biofilms, this enzymatic degradation can reduce P. fluorescens biofilm biomass by more than 50 percent compared with monoculture controls, demonstrating quorum inhibition as a competitive strategy in natural microbial communities.
Quorum Sensing
/ KWOR-um SEN-sing / · Latin quorum (of whom, minimum number) + Latin sentire (to feel)
Quorum Sensing is a cell-density-dependent communication system in bacteria that uses secreted chemical signals to coordinate gene expression and group behavior across an entire population once signal concentration crosses a threshold.
As bacterial numbers increase, secreted signaling molecules called autoinducers accumulate in the surrounding environment in proportion to cell density. Once autoinducer concentration exceeds a threshold, the molecules bind receptor proteins inside or on the surface of bacterial cells, triggering coordinated changes in gene expression across the population. Gram-negative bacteria typically use N-acyl homoserine lactones as autoinducers, while gram-positive bacteria use short peptide signals; some species, including Vibrio harveyi, produce both types to sense members of their own species and other species simultaneously.
Behaviors coordinated through quorum sensing include biofilm formation, virulence factor secretion, bioluminescence, sporulation, and antibiotic production, all of which are effective only when performed by large numbers of cells at once.
Bonnie Bassler at Princeton University discovered in the 1990s that many bacterial species share a universal autoinducer called AI-2, synthesized from the molecule S-adenosylmethionine, suggesting that cross-species communication through quorum sensing may be widespread in mixed microbial communities.
Bacteria cannot communicate with one another. Bacteria release and detect specific chemical signals that carry quantitative information about population density, and dozens of species alter their behavior in measurable ways in response to those signals.
Vibrio fischeri colonizes the light organ of the Hawaiian bobtail squid (Euprymna scolopes) and produces bioluminescence only after the population reaches roughly 10 to 100 million cells per milliliter. The squid uses this bacterial light during nighttime foraging to match downwelling moonlight and reduce its shadow against predators below within a measurable window of 24 to 48 hours.