Effects that are not obvious

Processes that get contaminated easily

This is only of passing interest to environmental engineers who do not take precautions against organisms that enter their processes.
Biochemical engineers have long appreciated the ease with which some microorganisms are cultivated. Yeast, for example, are rugged and grow at the lowered pH and high alcohol concentration created by their own metabolism. Some organisms that make antibiotics also help their own cause when this antibiotic inhibits growth of potential contaminants. Microorganisms with little or no self-protection are more difficult to culture, especially in large production vessels. Cell culture is the most difficult by far because the cells tend to grow very slowly compared to the microorganisms that would contaminate them, there is no self-protection, and the media tend to be very rich with vitamins and growth factors because animal cells are very fastidious in their nutrition. Antibiotics are often added to provide some defense against contamination, but engineering precautions are much more elaborate than for microbial bioconversions. Equipment that has been ideal for microbial processes can be hopeless for cell cultures.

Sequential use of nutrients

Click to see a cartoon that shows why most organisms consume glucose first. Click to come back to here because the cartoon is part of a different presentation that goes off in another direction.
  • Diauxie

    Metabolic overload

    This has the old terminology Crabtree effect or catabolite repression. It was thought that high concentrations of glucose inhibited the predominant pathways for its breakdown such that other pathways came into play. While there is still some dispute about the details, the modern viewpoint is that high concentrations of glucose simply overload the usual pathway and spill into other pathways. The most usual example is yeast that would make little or no ethanol when there is abundant oxygen but will excrete appreciable amounts of ethanol when there are both oxygen and high glucose concentrations.

    Culture run down

    This is not of direct interest to environmental engineers except it bears on why it is not effective to add special cultures to their processes.
    Production cultures may have undergone considerable mutation, selection, or genetic engineering to get them to produce high yields of a desired product. They have been switched from doing what is best for the cells to what is best for making money. There are strong selection pressures for cells that are most efficient in growing and reproducing, and these functions are almost always impaired in the production cultures. This means that over a period of time, a few cells that have lessened ability to make the product will outcompete the cells that are best for production and will eventually take over. Loss of the cells that give the best yields is called culture run down.
  • Next topic on list: energy