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MICROBIOLOGIA GENERALE
Interactions among bacteria:
cooperation and competition
An example of a microbial consortium
Green: Pseudomonas putida
Red: Acinetobacter sp. ADP1
Citrate
Chlorobenzoate
The social life of
bacterial community :
from the individual
behaviors to group
behaviors
Quorum Sensing
Quorum Sensing
Quorum Sensing. Bacteria synthesize and secrete autoinducers, low MW molecules that
diffuse away into the surroundings. Bacteria in the environment sense the local density of
these autoinducers via quorum sensing receptors. Above a certain concentration threshold
(the quorum), binding of the autoinducer ligand is synchronized among bacteria. The
binding event is transduced into transcriptional and translational changes that result in
coordinated shifts in group behavior, e.g. virulence, bioluminescence, virulence biofilm
formation, competence.
Schematics of quorum sensing systems in Bacteria
Gram negative
Gram positive
Quorum Sensing in Gram negactive Bacteria
Quorum Sensing in Gram-negative bacteria
Quorum Sensing Autoinducers
AHL: Acyl Homoserine Lactone
Vibrio fischeri: a bioluminescence bacteria and quorum sensing
FMNH2+O2+RCHO
Luciferase
FMN+RCOOH+H2O+Light
The LuxR response regulator
Quorum Sensing in Gram-positive Bacteria: Streptomyces
GBL: y-ButyroLactone
The Sreptomyces griseus Afactor regulon. The diffusible Afactor (a γ-butyrolactone) binds
the intracellular receptor ArpA
and activates expression of the
transcriptional activator AdpA
which in-turn regulates multiple
phenotypes either indirectly via a
multi-step cascade, such as the
development of aerial hyphae
and sporulation, or directly, such
as the production of secondary
metabolites like streptomycin.
Quorum Sensing in Gram-positive Bacteria: Staphylococcus
aures
AgrD pheromone
Quorum Sensing in Gram-positive Bacteria: Staphylococcus
aures
The AgrD-dependent transcriptional regulation of the agr operon
Quorum sensing and antimicrobial agents: the nisin of Lactococcus lactis
Nisin is a 34 aa polycyclic antibacterial peptide
produced by L. lactis that is used as a food
preservative
Quorum Sensing and Bacterial Virulence
Quorum sensing regulation of bacterial virulence factors
E. coli O157:H7
AHL: AI-3 that induces virulence genes
(motility and secretion of the enterotoxin)
S. aureus
AIP: autoinducing peptide induces via a
TCRS a range of virulence proteins
(damage of host cells and interference
with immune system)
The QS LasR/LasI and RhlR/RhlI
systems of
Pseudomonas aeruginosa
Microbial Biofilms
Bacterial Biofilms in Humans
Biofilm Formation
Biofilm formation is often described as a multistep process in which bacteria adhere to an
abiotic or biotic surface, through surface charges and production of pili, fimbriae, and
exopolysaccharides. After initial attachment, three-dimensional development starts with the
building of microcolonies, in which different species already interact. The next step, biofilm
maturation, is dependent on matrix production, which ensures cohesion and the threedimensional structure of mature biofilms. The final step in biofilm formation is cellular
detachment, by which bacteria regain the planktonic lifestyle to colonize other surfaces.
Biofilm Formation
Biofilm formation and quorum sensing
Biofilms and Quorum Sensing
Macromolecules and bacterial cell structures
involved in biofilm formation
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LPS
Flagella
Pili
Fimbriae
EPS (ExoPolySaccharide)
Bacterial floccules:
bacterial aggregates without a solid support