Signaling and Quorum Sensing

In 1979, McLean et al. carried out an elegantly simple experiment demonstrating that QS is more than a laboratory phenomenon.  The study employed Agrobacterium tumefaciens indicator strains unable to make their own signaling molecules but which possessed the ability to respond to exogenous autoinducers. The indicator strains had a Lac (lactose) operon down stream from the promoter normally activated by the signaling molecule in (species). These indicator strains were streaked onto plates of medium containing a lactose analog (X-gal) and nearby (about 1-2 cm) was placed a pebble isolated from a local stream.  The pebble was covered with a naturally occurring slimy biofilm.

As the indicator strain grew on the medium those portions of the streak closest to the pebble turned blue as a result of the activation of the promoter region and the down stream lac operon.  b-galactosidase a product of the induced lac operon cleaves the X-gal substrate releasing a blue colored product.   This simple demonstration was the first indicating the action of QS systems and acyl-homoserine lactones in a natural biofilm. 

Subsequently Charlton et al. 2000 demonstrated why it is that the Marine Algae Delesia pulchera, unlike most other marine algae, Delesia pulchera grows free from algal contamination.  Delesia synthesizes several members of a family of compounds called Furanones, which are naturally occurring structural analogs of the acyl-homoserine lactones responsible for most QS activity in gram-negative bacteria.

Although acyl-HSLs are present in biofilms many wondered if their presence was a primary factor, necessary for biofilm development of a secondary one, a consequence of prior biofilm formation.  A study by (Davies et al. 1998) has pointed to the first hypothesis.  Mutants of Pseudomonas aeruginosa (Las-I gene mutations) deficient in the ability to produce N-(3-oxododecanoyl)-L-homoserine were unable to form the normal three dimensional tower and mushroom-like biofilms typical of this species. Instead, the biofilms formed were denser, and only about 20% the thickness of the wild type biofilms and lacked the water filled channels usually found in Pseudomonas biofilms.  Addition of the missing LasI gene product (N-(3-oxododecanoyl)-L-homoserine) to the growth medium restored the normal architecture of the wild type strain with respect to cell density and biofilm thickness.   This experiment supports the hypothesis that QS is essential for the normal development, if not the initiation, of biofilm development.