A recent review, published by Julia Schwartzman and Edward Ruby from the Department of Medical Microbiology & Immunology at University of Wisconsin-Madison (USA), argues that chemical and physical stresses should be considered a normal attribute of the host symbiotic milieu. Their arguments are as follows.
Microorganisms appeared on the Earth as early as 3.4 billion years ago, and as humans we have evolved with them in a mutualistic relationship of coevolution, coexistence and cooperation. Their persistence is shaped by host-microbe and microbe-microbe interactions. Our gut microbiota is continuously changing in the gut environment. The composition of the vertebrate gut microbiota is influenced by diet as well as host morphology and phylogeny, and stress is an important factor that can affect host-microbiota interactions.
Stress in the host environment may arise from three sources: host tissues, microbe-induced immune responses, or interactions with other microbial communities in the host environment. According to the paper, “microbes cue, and respond to, immune-associated stress” and within the beneficial microbiota stress promotes stability (no changes in microbiota composition after being subjected to disturbance) and resilience (restoration of the initial composition after disturbance). The survival of microbes in the symbiotic host environment depends on its resistance to host-associated and microbiota-associated stress. Thus, stress contributes to the stability of the symbiotic system by shaping the interaction between host and microbe, microbe and microbe, and microbe and host. Stress has been studied in symbiotic models at the level of individual microbial taxa as well as the community level.
An interesting idea is that animal and plant symbiotic communities may sense a core set of conserved stresses, which have been reported in diverse host environments (e.g., antimicrobial peptide resistance in the host gut). The responses of microbes to these stresses can be general or highly specialized; the communication pathways between hosts and their beneficial microbes remain to be studied.
In conclusion, stress may be a major contributor to the physiology and ecology of beneficial microbes that have coevolved with a host. Thus, the microbial response to different sources of stress must be taken into consideration when studying host-microbe interactions in the symbiotic environment.
Schwartzman JA, Ruby EG. Stress as a normal cue in the symbiotic environment. Trends Microbiol. 2016; 24(5):414-24.