N-Acyl homoserine lactones (ASL’s) as general signaling components of Gram negatives bacteria When Nealson et al. (1) and Eberhard (2) first described the cell density phenomenon of bioluminescence in bacteria and later identified the signaling component N-(3-oxohexanoyl) homoserine lactone (3), nobody could foresee that the family of N-Acyl homoserine lactones (ASL) seem to be an almost universal signal factor in Gram negatives. When more bacteria producing ASLs became known it seemed that these were often pathogens to animals and men, as well as to plants. AHLs are produced e.g. by various opportunistic pathogens such as Pseudomonas species or phyto-pathogens like Erwinia species, or specific symbiotic strains as e.g. Rhizobium species (for recent reviews see 4–7). In these organisms AHLs induce the synthesis of compounds interacting with the host organism, such as toxins, antibiotics or exoenzymes. However, the observation that N-acyl homoserine lactones are also formed by certain plankton phototrophic bacteria (8) indicated that the phenomenon was of a more general nature. Furthermore, AHLs have been found recently in natural microbial habitats, e.g. biofilms (9), microbial mats and algal blooms (10). The term "quorum sensing" is now used to describe a general cell density dependent population response.
Mechanism of ASL production The AHLs known so far are the product of a luxI type gene (AHL synthase) for the induction of bacterial species specific reactions. AHLs formed by luxI bind to the product of the adjacent luxR gene; this activates the transcription of genes responsible for the species specific response, e.g. in Photo-bacterium fischeri the synthesis of the enzymes for bioluminescence. All these responses seem to be related so far to the secondary metabolism.
Outlook: the potential role of ASL’s A variety of seemingly phylogenetically unrelated bacteria produce the same AHL, other bacteria form more than one AHL. How these compounds inter-play in natural habitats is still completely unknown. Bacterial intercellular communication may be the basis for the expression of multiple virulence determinants thus interfering with signal transduction and may become a new tool to treat human, animal and plant infections (11). Signaling compounds seem to be important in biofilm formation (12) and exopolymer synthesis; interaction with the function of the AHL could open new ways in technical processes where biofilms cause damage by corrosion and biofouling.
Sigma-Aldrich’s contribution to quorum sensing research We are pleased to announce the introduction of most important N-Acyl homoserine lactones for quorum sensing research. The fact that these important tools are now easily available will support significantly your research. Additional ASL’s will be added to our product range later.
References: 1. K.H. Nealson et al., J. Bacteriol. 104, 313–322 (1970) 2. A. Eberhard, J. Bacteriol. 109, 1101–1105 (1972) 3. A. Eberhard et al., Biochemistry 20, 2444–2449 (1981) 4. S. Swift et al., Trends Microbiol. 2, 193–198 (1994) 5. S. Swift et al., Trends in Biochem. 21, 214–219 (1996) 6. P.V. Dunlap, In: Bacteria as multicellular organisms, Oxford University Press, Oxford, pp. 69–106 (1997) 7. S. Swift et al., In: Molecular Microbiology Springer, Berlin, pp. 185–207 (1998) 8. A. Puskas et al., J. Bact. 179, 7530–7537 (1997) 9. R.J.C. McLean et al., FEMS Microbiol. Lett. 154, 259–263 (1997) 10. R. Bachofen and A. Schenk, Microbiol. Res. 153, 61–63 (1998) 11. N.D. Robson et al., TibTech 15, 458–464 (1997) 12. D.G. Davies et al., Science 280, 295 –298 (1998)