An Ecological Perspective on Individual Disease

The Merriam-Webster dictionary defines health as freedom from physical disease or pain.

Human health, including disease resistance, is driven in numerous ways by an individualís microbiome - the inner ecosystem of human gut bacteria and their genes. The microbiome is a vast internal ecosystem which influences and is influenced by environmental conditions in the human digestive system. Within this ecosystem co-exist beneficial and detrimental bacteria in direct and indirect competition for resources; in a healthy organism this ecosystem is in a state of temporary equilibrium which can evolve over the lifespan of the host, and which can be upset by a number of factors including antibiotic use and enteric diseases.

The field of ecology studies the interrelationship of organisms and their environments, and a generally accepted theory within ecology is that community health is highly dependent upon species diversity - the total number and variety of species present and co-existing within the environment: "biodiversity in ecological systems results in better functioning and stable communities" [1]. The human microbiome "fosters development, aids digestion and protects host cells from pathogens - a function referred to as colonization resistance" [2]. "Under some circumstances, diet and other environmental factors, as well as factors of endogenous origin, such as chronic inflammation and aging, can force the intestinal microbiota to shift from a mutualistic configuration supporting health and homeostasis to a disease-associated profile, usually characterized by a lower level of phylogenetic and functional biodiversity" [3].

In a stable ecosystem, invasion by a new species (in this case the pathogen) is limited by the availability of resources and the ability of existing species to outcompete the invader. However, outside stressors can upset the balance and allow the pathogen to gain the upper hand. From a mathematical perspective stimuli such as inflammation or a sudden drop in microbiome diversity due to antibiotic use or other immune system changes can by simulated by one-time, periodic, or random recurring stochastic pulses which impact the mortality of native bacteria. Depending on the magnitude of the mortality pulse, the invading pathogen population will either quickly grow to sufficient numbers to become established in the microbiome, or will spike and crash due to insufficient resources. The outcome is closely tied to the resilience of the system - a function of its biodiversity. A more diverse mix of native species present in the microbiome will be more likely to resist colonization by the invading pathogen; a less diverse microbiome will be less so.

Thus from the perspective of the individual, enhancing the resilience of his or her own personal microbiome by increasing its biodiversity can improve the chances of resisting disease after exposure to a pathogen. Likewise, proactively minimizing the external pulses which weaken the native bacteria can have a similar beneficial effect. Measures such as these take advantage of the complex ecology of the microbiome to preserve health - freedom from disease - for individuals in any population.

Mathematical modeling, ecology and public health go hand-in-hand. If your organization is working to explore ecology and public health, contact MathEcology to learn more about how mathematical modeling can help!

[1] Human Food Project - Anthropology of Microbes. Probiotics or prebiotics: which would Darwin choose? Accessed 14 August 2012.

[2] Stecher B, W-D Hardt. The role of microbiota in infectious disease. Trends in Microbiology 2008, 16(3): 107-114.

[3] Candela M, E Biagi, S Maccaferri, S Turroni, P Brigidi. Intestinal microbiota is a plastic factor responding to environmental changes. Trends in Microbiology 2012, 20(8): 385-391.

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