A New Look at Secondary Metabolites

In syntrophic interactions, metabolic pathways are integrated over different cell types. This chapter focuses on two seemingly well‐defined groups of secondary metabolites, quorum‐sensing signals and antibiotics, to demonstrate that their described biological activities do not necessarily define their functional roles in microbial communities. The study of the biology of living organisms and associated biochemical processes has, to date, focused primarily on the structures and functions of DNA, RNA, proteins, lipids, carbohydrates, and their macromolecular complexes. Many bacteria regulate gene expression in response to accumulation of secondary metabolites, and this behavior has been collectively referred to as quorum sensing or cell‐cell communication. The generalization of quorum sensing as a density‐dependent process ignores the reality that most bacteria do not exist in well‐stirred reactors and the signaling will largely be a local event between small groups of cells. The dual role of quorum‐sensing signal and antibiotic is not exclusive to nisin, subtilin, and mercascidin peptide antibiotics. A bactericidal activity produced by a strain of Rhizobium leguminosarum that inhibited the growth of several related strains was purified and demonstrated to be a typical acyl homoserine lactone (AHL) [N‐(3‐hydroxy‐7‐cis‐tetradecenoyl)‐L‐homoserine lactone]. The streptomycin and chloramphenicol resistance determinants were later shown to catalyze chemical inactivation of the corresponding antibiotic. In addition to the widespread occurrence of antibiotic resistance mechanisms, it has become apparent in recent years that there are many naturally occurring systems that interfere with cell‐cell signaling pathways.