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General Microbiology II

Environmental Applications

Wastewater Treatment

• primary treatment - physical separation that removes 20-30% of "particulate" biochemical oxygen demand (BOD)
  • accomplished by filtration through grates and screens, followed by sedimentation of suspended solids using alum and other coagulants during settling
  • sludge (mixture of particulates and biomass) is thus generated
  • most pathogens are killed on the way to treatment plant, due to their susceptibility to metabolic products of the dominant microbes and their inability to compete with them for nutrients, etc.
• secondary treatment - microbial action further reduces BOD by 90-95% (and kills most of the remaining pathogens) by removing organics in a complex series of digestive and fermentative reactions depending on interspecies hydrogen transfer reactions during which organics are degraded to their components (carbon dioxide, hydrogen, methane, ammonia, nitrate, sulfate, phosphate) and producing more sludge; frequently starts with aerobic decomposition processes, then reduces the amount of the resulting sludge by anaerobic decomposition
  • aerobic decomposition processes
    • trickling filter
      • wastewater is slowly flowed over bed of rock or wood slats
      • microorganisms attach to surfaces of rocks or slats and digest organics
    • activated sludge
      • wastewater is mixed and forcefully aerated in an aeration basin
      • Zoogloea ramigera grows and forms a floc (biomass)
      • protozoa, bacteria, etc. attach to the floc and digest organics
      • floc settled, then sent to anaerobic digester
      • when operated at low oxygen levels, Sphaerotilus, Thiothrix, and other filamentous microbes predominate, generating bulking sludge, which causes problems with floc formation and settling
  • anaerobic decomposition process - used for insoluble organics, such as fiber and cellulose, or for certain concentrated industrial wastes
    • initial digestion of macromolecules by extracellular enzymes
    • soluble(ized) organics (glucose, etc.) are then fermented to form fatty acids (butyric, propionic, lactic, succinic and acetic acids), ethanol, hydrogen, and carbon dioxide by various facultative and obligate anaerobes including Clostridium, Bacteroides, Peptococcus, Peptostreptococcus, Eubacterium, and Lactobacillus
    • fatty acids and ethanol are then fermented to form acetate, carbon dioxide, hydrogen by bacteria such as Syntrophomonas, Syntrophobacter, and Acetobacterium
    • acetate, carbon dioxide and hydrogen are then converted to methane by methanogenic bacteria (Methanobrevibacter, Methanomicrobium, Methanogenium, Methanobacterium, Methanococcus, Methanospirillum)
    • after 2-4 weeks, the non-degradable material (still called sludge) is settled, then dried and burned, buried or used for fetlilizer (if it doesn't contain a lot of heavy metals or other toxic materials)
• tertiary treatment - removes environmentally-damaging substances including potential pathogens and eutrophic agents (especially inorganic nutrients generated during secondary treatment) heavy metals, and hard-to-digest organic compounds)
  • precipitation and filtration remove most of the inorganics, especially nitrate and phosphate
  • chlorination eliminates any remaining harmful microbes, but residual chlorine must be removed by aeration prior to dumping the effluent into a stream, etc.

Drinking Water Treatment

• drinking water is initially derived from rain, and stored in nature as surface or ground water
• purified by a combination of methods that include:
  • filtration or sedimentation
  • coagulation (floculation) and sedimentation
  • filtration
  • softening
  • carbon treatment
  • chlorination
  • fluoridation
• indicators of fecal contamination (problems)
  • indicators are allochthonous (introduced) organisms which are constantly being added to natural environments, but do not always survive due to maladaptation to the extremes of temperature, nutrients, competition, Bdellovibrio predation, etc.
  • Vibrio cholerae has been clearly shown to be present in the natural environment in a viable but nonculturable form - perhaps the same is true of other microbes

Impact of Metals on Microorganisms

• noble metals (Ag, Au, Pt) - ionized forms have potent antimicrobial activity
• heavy metals
  • As, Hg and Se often form stable bonds with carbon via methylation, which leads to bioaccumulation (biomagnification) in higher trophic levels of food chains
  • Cu, Zn, Co are heavy metals that are often required as trace elements; higher concentrations of ionized forms have antimicrobial activity

Biodegradation

The fate of genetically engineered microbes will be similar to those of other allochthonous organisms, but even more extreme because they may have been engineered to require nutrients, etc. not naturally present in the environments into which they may be introduced.

• natural products
  • petroleum - certain bacteria (some cyanobacteria, pseudomonads, corynebacteria, mycobacteria), green algae and fungi (several molds and yeasts) oxidize hydrocarbons at aerobic water/oil interfaces (with optimal conditions, up to 80% removal within 1 year after a spill)
  • biodegradable plastics
    • photobiodegradable - structure of polymer altered by UV light in sunlight so that it is now amenable to biodegradation
    • biochemically biodegradable starch-linked polymers - starch-digesting bacteria in soil attack the starch, releasing polymer fragments which are degraded by other microbes
• xenobiotics - chemically synthesized compounds not found in nature (pesticides, synthetic polymers, etc.) and thus would seem unlikely to be degradable by naturally existing microorganisms; these products tend to be persistent in nature, and many nations are working to ban the use of many of them; microbes that can degrade xenobiotics are rather diverse and typically include both bacteria and fungi
  • PCBs - certain Pseudomonas species have been engineered to accelerate breakdown of polychlorinated biphenyls (formerly used by electric industry as transformer insulation)
  • PAHs (poly aromatic hydrocarbons) can be difficult to degrade, but there are microbes in the environment that can accomplish this task, especially when working together
  • pesticides - herbicides, insecticides and fungicides
    • these are typically rather complex molecules
    • some xenobiotics are good carbon sources and electron donors for soil microbes, so they are more readily degraded than others
    • other xenobiotics, such as chlorinated insecticides, are recalcitrant to degradation; thus they have rather long persistence times in the environment
      • lindane - 3 years for 75-100% disappearance
      • DDT - 4 years for 75-100% disappearance
      • chlordane - 5 years for 75-100% disappearance
    • to degrade these xenobiotics, microbes may employ co-metabolism, in which an organic material other than the xenobiotic is used as the primary energy source and the xenobiotic is degraded as a secondary process

Biosynthesis of Commercially Important Products

• When appropriately altered via genetic engineering, E. coli can make biodiesel fuel (microdiesel)
  • Two genes from Zymomonas mobilis allow E. coli to produce ethanol from glucose.
  • An additional gene from Acinetobacter baylyi allows these E. coli to combine ethanol with the main constituent of olive oil to produce microdiesel.
  • Microdiesel is relatively cheap to produce and does not contain toxic by products, but does require a lot of land to grow the olive trees