Dr. Hannele Kettunen, R&D Manager
Bovine rumen is a big warm pot of feed-digesting microbes: bacteria, archaea, yeasts, and unicellular eukaryotes. Although these creatures are unicellular organisms, they are capable of forming communities with complex cooperative behavior, in order to enhance access to nutrients or specific environmental niches, or to compete for resources.
Quorum sensing is population density related talk between microbes
The density of microbial population is of importance for the behavior of microbial population. Often a threshold density exists for e.g. pathogenesis, virulence, biofilm formation, antibiotic production, antibiotic resistance, motility and sporulation. The individual microbes are able to measure the population density for the reliable detection of the threshold density.
Quorum sensing (QS) refers to communication and signaling systems which are related to microbial population density. The microbes produce, secrete and detect chemical signals to measure population density and to coordinate their behavior on a population level. A diverse group of molecules take part in this messaging, including lectins, peptides, amines, lactones, furanons, and many others. These molecules have also been called “autoinducers”.
The QS signaling molecules are typically freely diffusible across microbial cell membranes. The concentration of these QS signaling compounds increases in the environment with increasing population density. When the threshold concentration of the QS signal molecules is reached, the target genes regulating specific adaptive functions, such as biofilm production or pathogenesis, are activated. In other words, QS signaling enables microbes to sense the environmental conditions, to respond in appropriate ways to stimuli from the environment or other microbes, and to synchronize their behavior as a population in a given ecosystem. This type of signaling is also known from ruminal microbial communities.
The first known QS signaling system
The prototype of the QS signaling system is the production and detection of N-acylhomoserine lactones (AHL) autoinducers by Vibrio fischeri. This marine, bioluminescent bacterium inhabits the light organs of some marine fish and squid. It emits light at high cell-population density but not in low population density. Only in the light organs the population density of V. fischeri can reach 1011 per ml which is required for the threshold concentration of AHLs for light emission.
Vibrio fischeri has two regulatory protein systems: LuxI and LuxR. LuxI synthesizes the AHL molecules and the transcription factor LuxR detects the presence of AHLs in the environment. The LuxR-AHL –complex activates the genes that produce the bioluminescence. In the fascinating symbiontic relationship between V. fischeri and the squid Euprymna scolopes, the squid provides the bacterium with a safe and nutrient-rich environment to live in, and the bacterium protects the squid against predation by counter-illuminating it on bright clear nights, so that the squid does not cast a shadow beneath it when the light from the moon and stars penetrates the seawater.
Main types of QS in Gram-negative and Gram-positive bacteria
The LuxR/LuxI system (autoinducer-1 QS system; AI-1) of V. fischeri is actually common form of QS signaling in Gram-negative bacteria. In them, it serves other forms of communication, while bioluminescence is not involved. Interestingly, some bacterial species are able to synthesize only one of these regulatory enzymes. For example, Escherichia, Salmonella, Klebsiella, Enterobacter and Citrobacter are able to sense the AHLs secreted by other microbes but cannot secrete these molecules themselves. These types of systems may form a basis for interspecies cooperation in bacterial communities. In commensal gastrointestinal microbes, AHLs are less common QS signaling systems, although some enteric species like Yersinia enterocolitica and a ruminal species of Citrobacter uses them.
Gram-negative and Gram-positive bacteria use partly different QS signaling systems. Gram-negative bacteria typically use small molecules as signals, and two receptor types detect these signals — cytoplasmic transcription factors or transmembrane histidine sensor kinases. The QS molecules typically used by Gram-positive bacteria are oligopeptides, furanosyl borate diester or tetrahydroxy furan (autoinducer-2 QS system; AI-2). This signaling type is used several bacterial genera that are abundant in rumen, such as Butyrivibrio, Prevotella, Ruminococcus and Pseudobutyrivibrio.
Interaction with the microbiome and the host
Microbes of the gastrointestinal tract produce and secrete many of the same compounds that the host uses as tissue hormones, neurotransmitters, antioxidants, growth factors or mediators of immunological responses. Some of the low molecular weight signal molecules of microbial origin may diffuse into host tissues and affect various physiological responses of the host.
The big picture of QS in bovine rumen and other gastrointestinal microbial communities has not yet been drawn. Many interaction pathways are known, but due to the metabolic complexity of multispecies communities, a lot of research remains to be done within this area.
One example of a QS-based interaction in bovine rumen well illustrates the complexity of QS systems in nature. Human infection by enterohemorrhagic E. coli (EHEC) strain O157:H7 manifests as bloody diarrhea and forms typical lesions in the human intestine. One important infection risk is being in contact with cattle, as EHEC colonizes their recto-anal region. The lesion formation in cattle is needed for the recto-anal colonization of EHEC, which the facilitates the shedding of EHEC into the environment. To colonize a new cow via the oral route, EHEC needs to pass through the entire digestive canal, including the acidic abomasum. The pathogen can survive though abomasum only when it has turned off the genes involved with lesion formation, and simultaneously turned on specific resistance genes. When in the rumen, EHEC senses the AHLs secreted by commensal ruminal bacteria. These QS molecules act as a signal for EHEC to turn on the resistance genes before it flows – unharmed – through the acidic abomasum towards rectum.
Does Progut® affect the ruminal QS signaling?
Hankkija’s yeast hydrolysate Progut® is known to enhance the rate of ruminal fermentation by increasing the microbial density and production of short chain fatty acids. Because of the increase in microbial density, the potential role of QS signaling has been speculated.
In a simulation study, Progut® did not alter the bacterial profile of samples but rather stimulated those bacterial groups that were active at the given phase of the simulation. More specifically, during the first 6 hours of the simulation, Progut® favored lactic acid producers whereas at the 12 and 24 hour time points, cellulolytic and amylolytic microbes and lactic acid degraders were enhanced by Progut®. The fact that the density of many microbial groups were increased as a response to Progut amendment may suggest that QS signaling systems were involved. However, direct evidence on the role of QS in the action of Progut® remains to be gathered in future studies.
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