Cooperative Microbial Tolerance Behaviors in Host-Microbiota Mutualism

Abstract

Animal defense strategies against microbes are most often thought of as a function of the immune system, the primary function of which is to sense and kill microbes through the execution of resistance mechanisms. However, this antagonistic view creates complications for our understanding of beneficial host-microbe interactions. Pathogenic microbes are described as employing a few common behaviors that promote their fitness at the expense of host health and fitness. Here, a complementary framework is proposed to suggest that, in addition to pathogens, beneficial microbes have evolved behaviors to manipulate host processes in order to promote their own fitness and do so through the promotion of host health and fitness. In this Perspective, I explore the idea that patterns or behaviors traditionally ascribed to pathogenic microbes are also employed by beneficial microbes to promote host tolerance defense strategies. Such strategies would promote host health without having a negative impact on microbial fitness and would thereby yield cooperative evolutionary dynamics that are likely required to drive mutualistic co-evolution of hosts and microbes.

Figure 1. Evolutionary dynamics of host-microbe interactions

(A) Resistance traits in the host population place negative selective pressures on a microbial population leading to selection of a counter-attack strategy in the microbial population. This response places a new selective pressure on the host population, driving the selection for a “new” resistance trait and a decline in the presence of the previous resistance trait. An oscillation of antagonistic traits in both host and microbe population, or the Red Queen Effect, results. Graph adapted from (Svensson and Raberg, 2010). (B) A tolerance trait in the host population will have a neutral to positive selective pressure on a microbial population. This balance will maintain the presence of the microbe population and associated selective pressures on the host population that will drive the selection and spread of the tolerance trait in the host population, eventually leading to fixation of that trait. In addition to host-encoded tolerance mechanisms, beneficial microbes likely have evolved traits that promote tolerance of their host and are predicted to yield similar evolutionary dynamics.

Figure 2. Measuring resistance and tolerance

Resistance and tolerance can be measured in a host-microbe system by examining the relationship between a selected parameter of host health and microbial levels in target tissues. Using these parameters, and assuming the health of the host when uninfected is equivalent between different host populations (vigor), a dose response curve can be generated to determine how host health changes as microbial levels change. Changes along the diagonal indicate health is changing as microbial levels change and would identify hosts that vary in resistance defenses. Changes along the y-axis would indicate health is changing without a change in microbe levels and would identify hosts that vary in tolerance. The more tolerant a host, the shallower the slope of the dose response curve would be. This method, which assumes a linear response, can be used to determine how different factors including environmental and genetic factors can influence host defenses. These relationships however are likely more complex and further experimentation and data points, for example measuring pathogen burden over the course of the infection, would reveal how differences in host populations influence resistance and tolerance at different stages of the infection. Adapted from (Ayres and Schneider, 2008).

Figure 3. Microbial adaptation behaviors leading to pathogenicity or tolerance

(A) Microbes occupy specialized niches that can cause pathogenicity or promote tolerance defenses. Left, upon entry into the intestine S. Typhimurium invades Peyer’s Patches and infects lamina propria phagocytes, which then enter the lymphatics and bloodstream and infected distal organs including the liver. Right, during intestinal and extraintestinal infections, the microbe E. coli O21:H+ translocates to white adipose tissue deposits to induce tolerance via induction of innate immune – endocrine interactions. (B) Meeting metabolic demands of microbes to promote growth. Left, the opportunistic pathogen C. difficle forages on glycans liberated by members of the microbiota, supporting pathogen growth upon induction of infection with antibiotics. Right, under conditions in which nutrients are limited, beneficial microbes alter their foraging to utilize fucosylated host species. In the context of LPS systemic injection and oral infection with Citrobacter rodentium, fucosylation and flexible foraging of microbes promotes tolerance by preventing the microbes from becoming pathogenic (Pickard et al. 2014). In other contexts, for example during chronic malnutrition, this change in foraging behavior may become pathogenic to the host. (C) Regulation of host inflammation. Many pathogens induce an inflammatory response that can facilitate infection and pathogenicity. Left, H. hepaticus induces inflammation in the colon. Right, PSA generated by B. fragilis restores inflammatory balance in the host through the induction of an IL-10 dependent anti-inflammatory response.