1. Introduction

2. Surface sensing

3. Different types of signals

4. Sensors

5. Enveloped sensors

6. Signal transduction

7. Conclusion:

Unicellularity of bacteria

• Multicellular communities in the presence of bioticor abiotic surfaces

• Biofilms

• Swarming motility

• Both biofilm and swarming e.g E- coli, P. aeruginosa

Resistance to adverse

conditions

• Resistance to antibiotics and metal treatments

• High cell density protects Salmonella, Bacillus, Serratia [Butler, 2010] and E. coli [Perrin, unpublished data] against biocides

• Result is increased virulence [ James 2008 ]

Bioremedia-tion

• Knowledge of the molecular basis of the behavior of switch

• identification of the switch signals

Biofilm formation:

Free-floating bacterium collides with a surface

Biofilm development is the anchorage of the cell to a support (cellular or abiotic)

Cascade of physiological

reactions

Different species

and different experimental

conditions

repression of motility

functions, production of more adherence factors,

and production of exopolysaccharides

[Beloin 2008 ]

Numerous highly diverse

stimuli affect biofilmformation

contact-dependent signalling [Belango 2009]

Osmolarity

• differences in water activity = allowing bacteria to recognize a solid-liquid interface

• sessile bacteria=higher concentration of intracellular potassium, compared to the free-floating cells.

• ion-sensitive field effect transistor (ISFET) [Possonet2008 ]

• Curli-proficient [Leugene 2003]

Chemicals

• metabolites (glucose, indole, polyamines etc)

• inorganic molecules (iron, phosphate),

• quorum signals

• antimicrobials (host-derived molecules such as bile salt, epinephrin and norepinephrin, antibiotics, metals, detergents, H2O2 etc)[Karatan 2009]

Mechanical signals

respond to surface contact

surface stiffness= genetic

program=specific pattern of adhesion and

cytoskeletal

Organization [Discher2005]

Increase in substrate stiffness=

Staph.epidermis and E-coli [ Litcher 2008]

Signals Contd……

P. aeruginosa responds to increasing surface stiffness (produced by raising the concentrations of agar in solid media) by an increase in the production of type IV pili.

Surface attachment appears, then, to regulate not only the production butalso the polar localization of the pilus component. [Cowles 2010]

Urinary tract-associated E. coli often produce adhesive type 1 pili exhibitingthe adhesin FimH at the tip. FimH binds to glycoproteins carrying high-mannose-type oligosaccharidechains and, more generally, to mannose.

A histidine tagged version of FimH has been created to enable transgenic E.coli to attach to immobilized nickel.

The transcriptional response to attachment was then compared in modified E.coli cells (beads coated with nickel) and in parental cells (agarose beads coatedwith mannose) suggesting that a mechanical signal is transmitted from theoutside to the intracellular structures, following fimbrial adhesion [Bhomkar2010]

Extracellular appendages:

Flagella = role in adherence to host cells, clumping, and biofilm formation in many ways [Anderson 2010]

act as adhesins

cessation of flagellar motility and repression of flagellar gene transcription.

a molecular clutch [Blair 2008]

mutations in the flagellar motor : bacteria is immobilized on a surface =flagellar

motor stops, the ion flow through the motor ceases and the membrane potential is transiently increased= signal for biofilm formation

Bacterial Flagellum

Basal body Hook Filament

Swarming: coordinated translocation of a cell population across a solid or semi-solid surface driven by type IV pili or flagella Proteus mirabilis and in

Vibrio parahemolyticus [Jarell 2008]

No data available on the use of the flagellum as a mechanosensor and one

pending question is how widespread this role might be?????

Sensing mechanism allowing the cell to distinguish a

“semi-solid surface” (swarming) from a solid surface (biofilm) is still obscure

[Wang 2012]

Transporters: efficient co-sensors e.g Fumarate sensing: signalling pathway involving

DcuS/DcuR, controlling the response to extracellular fumarate. [Kleefeld2009]

Outer membrane protein OmpA in E. coli, and its ortholog OprF in P. aeruginosa, are both involved in biofilm formation, as well as having a variety of other functions

OmpA induces envelope stress Cpx signalling pathway [Ma Q 2009]

NlpE/Cpx pathway.

Via Cpx, Bae, Rcs, Psp and σE pathwaysChemical stresses

Physico-chemical stresses

mechanical stresses

response to various stresses applied the envelope either by use protein phosphorylation in response to environmental stimuli or sigma E regulated release or binding of a transcriptional

factor.

Scr pathway. In V. parahaemolyticus, the scrABC operon controls the switch between swarming and biofilm formation via the inverse regulation of lateral flagella genes (laf) and capsular polysaccharide genes (cps)

Focal adhesion. A focal adhesion is an anchoring junction of the cell to a substrate. In eukaryotic cells, it links the internal actin-myosin network to the extracellular matrix through transmembrane linkers, such as integrins.[Maureillo 2010]

Cytoplasmic sensors

H-NS. H-NS is a nucleoid-associated protein acting as a global regulator of gene expression, and it is essential for bacterial virulence and environmental adaptation [Higgins 1988]

Frz pathway. The soil bacteria M. xanthus uses [Bao 2009]

Conclusion

• sensing obviously occurs at multiple levels, extracellular appendages, the envelope, and/or cytoplasmic sensors

• surface-sensors are all proteins but the mechanisms of mechanical force sensing via proteins are still obscure.

• molecular basis by which the flagellum communicates with the transcriptional machinery is still poorly understood

• physical (temperature) or chemical (pH) parameters, protein deformation probably plays a major role in response to mechanical forces.

Modularization of signalling pathways:

On the basis of genome analysis, one-component systems appear to be more widely distributed, and more abundant, in bacteria.

experiments investigating E. coli biofilm formation identified the transfer of phosphate groups by two-component signal transduction systems as the main mechanism employed to process environmental information [Nifa 2007]

Sensor kinase:

Korchid 2006

Response Regulator:

Two domains constitute the RR, a highly conserved receiver domain (REC) and a variable output or effector domain.

Phosphorylation mediates dimerization of the receiver domains and activates transcriptional function, or enzymatic activity in the case of diguanylate cyclasecatalytic domains [Gao 2010]

Auxiliary-regulators:

Recently concept of co-sensors or auxiliary regulators modulating phosphotransferalong the basic two-component signalling pathway has emerged.

Auxiliary regulators are widespread, both in Gram-positive and Gram-negative bacteria, and can be found either in the cytoplasm (for example E. coli PII protein as the modulator of the NtrBC pathway in the nitrogen response) or in the inner membrane (for example IgaA and MzrA) [Gerken 2010]

Numerous regulatory devices ensuring signal transduction in response to surface-sensing have been identified in bacteria but the nature of the signal detected is still difficult to approach experimentally.

The role of extracellular appendages, such as flagella, in surfacesensing and, more generally, mechanisms of mechanotransduction need to be investigated.

An exciting hypothesis is that large proteic assemblies, able to both sense and regulate the response to a surface, may exist in bacteria. Moreover, although huge progress has been made in the characterization of individual regulatory pathways, we are just beginning to address the complexity of the interactions between the different elements.

Evaluating the in vivo extent of the numerous possibilities of crosstalk is a major challenge.