For this week's Journal Club meeting Helen posited "Organisms as ecosystem engineers" (Jones et al. 1994). This paper defined ecosystem engineering, which classifies ways that non-trophic interactions between a species and the resources in their environment affect other species in the environment as a whole. Jones and colleagues describe 5 cases considered ecosystem engineering, 2 autogenic (the organism's structural change affects a resource) and 3 allogenic (the organism modifies an aspect of the ecosystem that affects a resource). The classic examples being beaver dams altering waterways as 'case 4' allogenic engineers, and the formation of mussel beds affecting erosion of inland marshes as 'case 5' autogenic engineers. Though this paper brought to light considering ecosystem engineering as an important ecological effect, the 5-case classification system has not been canonized in the literature. More recent papers have strayed from the autogenic/allogenic classifications, such as Berke 2010, which instead breaks down ecosystem engineering into four categories: structural engineers, bioturbators, chemical engineers, and light engineers.
Helen is beginning work towards a review paper exploring ecosystem engineering in the human microbiome. Specifically, focusing on microbial structural engineers (encompassing both autogenic and allogenic examples) by combining the frameworks described in the above two papers. Some examples discussed were biofilms, which modify O2 and nutrient concentration, as well as contribute to pH heterogeneity. Fungal highways are also another clear example of structural engineers, providing a water envelope around the fungal hyphae through which bacteria travel, although this phenomenon has not been described in the human microbiome. Dental caries erode teeth and provide new habitats for oral microbiota, and cervical mucus during pregnancy is affected by bacterial vaginosis leading to increased porosity of the mucus plug and a higher risk of premature birth. Candida albicans is an example of a microbe that forms biofilms and hyphae, on which S. aureus can bind, making it an autogenic structural engineer.
The ecosystem engineering model, however, contains many grey areas and areas that do not easily translate from macro to micro biology, but the ubiquitous ability to apply ecosystem engineering provides an important framework through which to view interactions. One specific issue to overcome is the definition of a resource for microbes. For microbes a resource can come in many forms and is often concentration dependent. Another gray area is whether or not to consider immune response as ecosystem engineering, as it both affects the environment but could be further specified consider just inflammation as the engineer.