Friends or Foes—Microbial Interactions in Nature

Abstract

Microorganisms are present in nearly every niche on Earth and mainly do not exist solely but form communities of single or mixed species. Within such microbial populations and between the microbes and a eukaryotic host, various microbial interactions take place in an ever-changing environment. Those microbial interactions are crucial for a successful establishment and maintenance of a microbial population. The basic unit of interaction is the gene expression of each organism in this community in response to biotic or abiotic stimuli. Differential gene expression is responsible for producing exchangeable molecules involved in the interactions, ultimately leading to community behavior. Cooperative and competitive interactions within bacterial communities and between the associated bacteria and the host are the focus of this review, emphasizing microbial cell–cell communication (quorum sensing). Further, metagenomics is discussed as a helpful tool to analyze the complex genomic information of microbial communities and the functional role of different microbes within a community and to identify novel biomolecules for biotechnological applications.

Quorum-Sensing Inhibition by Gram-Positive Bacteria

“…This mechanism is characteristic of cross-quenching of autoinducers (signaling molecules that are produced as a reaction to changes in population density of bacteria) when each autoinducing peptide (AIP) activates its related receptor but inhibits the activation of all others by competitive binding to foreign receptors. This mechanism is typical, for example, for intraspecific competition between different groups of Staphylococcus aureus [4].…”

“…The interruption of a QS signal can occur in several ways: (1) stopping the synthesis of signaling molecules; (2) inactivation or enzymatic destruction of signaling molecules, preventing accumulation to a threshold value; (3) interference with the binding of signal receptors in a bacterial cell or competition with signal molecules-receptor analogs; and (4) blocking target genes that should have been triggered by the QS signal [3,4].…”

“…Disruption of the signaling molecules' synthesis can be achieved by inhibiting the enzymes involved in this process, such as the acyl chain (acyl carrier protein, ACP) and S-adenosylmethionine synthase [4], or, for example, by blocking the Lux system, which serves as a fundamental model for Gram-negative proteobacterial QS systems [5].…”

“…The best-studied mechanism is the degradation of AHL by lactonases which are produced by many types of microorganisms. AHL lactonases are grouped into two clusters: the AiiA cluster (produced by Bacillus) and AttM (produced by Gram-negative bacteria) [4,5].…”

“…From an evolutionary point of view, there are two reasons why bacteria would develop QQ mechanisms. First, it is the mechanism of antagonism with competitors; secondly, it is the utilization of QS signals by bacteria, both to reuse resources and to regulate and "fine-tune" the processes regulated by QS signaling [4].…”