1.89, Environmental Microbiology Lecture 18 Microbial Population Interactions Definition of population: coexisting individuals of the same species. Interactions within populations Alle e-‘s principle → at any given time both + and – interactions play a role in structuring populations. µ + + Population density • - µ µ Population density Population density + interactions: Example: o exoenzymes (break down polymers) o infectious dose o local adjustment of pH or Redox Adaptation: o colony formation o quorum sensing (density triggers specific traits) • - interactions: o competition for resources o accumulation of toxic substances Interactions with other populations Commensalism: vitamins, co-factors, etc. are often required by bacteria isolated from the environment. eg: D. ethenogens need Vitamin B12 (which is produced by methanogens) “Cometabolism” = transformation of a substrate without gain of energy. Product can then be utilized by other bacteria. Synergism: both organisms benefit (eg: anaerobic environment) “cross-feeding” Compound A population 1 (utilizes A) Compound B detoxification population 2 Mineralization Symbiosis (mutualism): 2 types of organisms interacting in a mutually dependent way; they need each other to survive; symbionts are obligate Example: a) lichens (very drought resistant; primary colonizers) = algae or cyanobacteria + fungi b) protozoa (termite gut, cow stomachs): bacterial endo and ecto symbionts (Bacteria grow on/in protozoa) Types of metabolisms: photosynthetic bacteria, methanogens, S-oxidizers Competition: 2 organisms share/have the same limiting resource can lead to competitive exclusion difficult to assess in environment for bacteria, competition is studied using chemostats. In a chemostat, the rate of supply of medium (or limiting nutrients) determines the growth rate: D = f V D - dilution rate f - flow rate V - volume dB f = µ B − B = µ B − DB dt V at SS, dB = 0 and µ = D dt shows who will win the competition growth = dilution rate rate! Predation: death Rates are equally important to growth rates Predators of bacteria: viruses, protozoa Theory: Latka-Volterra Prey = P1 Density Predators = P2 dP1 = µ1 P1 − r P1 P2 dt dP2 = µ2 r P1 P2 − dP2 dt constant death rate

Lecture 18 Notes: Microbial Population Interactions

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