recap

25.3 recap

Many prokaryotes are beneficial and even necessary to other forms of life. Most animals, including humans, depend on a complex community of prokaryotes—a microbiome—to maintain health, especially of the immune and digestive systems. Pathogenic bacteria are the direct causes of diseases.

learning outcomes

You should be able to:

  • Summarize the applications of biofilms.

  • Devise experiments to test effects of microbiomes on human health.

  • Compare and contrast nitrogen fixers, denitrifiers, and nitrifiers.

Question 1

How do biofilms form, and why are they of special interest to researchers?

Biofilms represent a complex community of microbial organisms, living together in a gel-like polysaccharide matrix. Many free-living prokaryotes that come into contact with a solid surface bind to the surface and secrete a sticky polysaccharide, which protects the cells. These cells may then secrete signal molecules that attract other microorganisms to the matrix, which develops into a complex community of organisms over time. This community may become highly resistant to attack and become very difficult to remove. Biofilms are of considerable interest to human health and industry, as they may form on any solid surface (e.g., teeth, contact lenses, artificial joints, the inside of pipes), resulting in decay. Biofilms were also critical to the evolution of many multicellular communities, such as stromatolites.

Question 2

Why would elimination of all bacteria from a human gut be problematic from a health standpoint?

Bacteria are essential for healthy digestion, and only a very few types of bacteria are pathogens. Humans use some of the metabolic products, such as vitamins B12 and K, that are produced by bacteria that live in the large intestine. Large communities of bacteria also line human intestines with a dense biofilm, which aids nutrient transfer from the gut into the human body. Thus our gut microbiome is essential to human health.

Question 3

Why is nitrogen metabolism in prokaryotes vital to other organisms?

Nitrogen-fixing prokaryotes convert nitrogen from the atmosphere into a chemical form that is usable by living organisms. In addition, denitrifying bacteria release organic nitrogen back into the atmosphere as nitrogen gas, keeping nitrogen from eventually leaching into the oceans. Thus these two groups of bacteria (nitrogen fixers and denitrifiers) allow the cycling of nitrogen and make life on land possible.

Before moving on to discuss the diversity of eukaryotic life, it is appropriate to consider another category of life that includes some pathogens: the viruses. Although they are not cellular, viruses are numerically among the most abundant forms of life on Earth. Their effects on other organisms are enormous. Where did viruses come from, and how do they fit into the tree of life? Biologists are still working to answer these questions.