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of infection. The different bacterial AIPs binds to the receptors of cell Gram-negative bacteria have a
strains produce different virulence surface and activates the two- thin peptidoglycan membrane
factors. The production of these component phosphokinase system and a outer lipid layer. The AHLs
virulence factors is regulated by the (TCS) to initiate the corresponding regulate QS by binding the enzyme
bacterial QS signals (Jiang et al., signal transduction and fi nally gene (LuxI-type synthase) to the LuxR-
2019). transcription. The two-component type receptor regulatory protein.
signal transduction system (TCSTS) AHLs are membrane permeable
HOW DOES QUORUM SENSING comprises of response regulatory molecules and diffused to the
WORK? protein AgrA and transmembrane exterior of cell membrane randomly.
The bacteria have to posses following sensor kinase AgrC. For example, After exceeding the threshold
three characteristics for cell-to-cell accessory gene regulation (AGR) in concentration, the molecules
signal transmission (Abisado et al., Staphylococcus aureus is associated diffused through the cell membrane
2018): with AIPs secretion. Another and bound to the amino terminus
possible mechanism includes, of LuxR receptor proteins in the
• To secrete a signaling molecule- AIPs may be transported into the cytoplasm to form the LuxI/LuxR
known as autoinducer cytosol, and binds directly to the protein complexes. The complexes
• Exceeding threshold transcription factor to initiate or regulate the expression of certain
concentration of signaling inhibit transcription (Zhang et al., functional genes and have a
molecules 2019) (Figure 2a). feedback regulation effect on the
• To regulate gene expression as a
response

The bacterial QS signals mainly
consist of autoinducing peptides
(AIPs), acyl-homoserine lactones
(AHLs), andautoinducer-2 (AI-2).
These molecules participate in
various physiological processes of
bacteria including bioluminescence,
biofi lm formation, antibiotic
resistance, plasmid conjugation,
motility, spore formation etc. by
which bacteria survives in the
adverse environmental conditions.
QS is shown by both Gram-positive Figure 2a: AIPs-mediated quorum sensing pathway in Gram-positive bacteria
and Gram-negative bacteria, but
their mechanism is different. The
AIPs signaling molecules are mainly
produced by Gram-positive bacteria
and AHLs signaling molecules are
produced by Gram-negative bacteria.
However, AI-2 signaling molecules
are produced by both Gram-positive
and Gram-negative bacteria (Jiang
et al., 2019).

Gram-positive bacteria bacteria
have a thick peptidoglycan
membrane but it doesn’t contain
outer lipid layer. Whenever the
concentration of corresponding
AIPs in the immediate environment

exceeds to the threshold level, the Figure 2b: LuxI/LuxR mediated quorum sensing pathway of Vibrio fischeri

Food Marketing & Technology 25 April 2021