A German physician named Robert Koch (winner of the Nobel Prize in 1905 for this work with tuberculosis) was the first scientist to connect a specific bacteria to a certain disease. He established the following four criteria for identifying pathogenic bacteria. These criteria are still the guidelines for medical microbiology today.

1. The pathogen must be found in an animal with the disease and not in a healthy animal.
2. The pathogen must be isolated and grown in a laboratory.
3. When the isolated pathogen is injected into a healthy animal, the animal must develop the disease.
4. The pathogen must be isolated in the second animal, grown in the laboratory and identified as the same pathogen.

In an experiment in 1796, Edward Jenner injected cowpox into healthy individuals in an attempt to cause each person to develop immunity to smallpox. These experiments led to the modern vaccination process. A vaccine is a solution containing dead or modified pathogens which is injected into a healthy individual to create immunity to a particular disease. Smallpox was the first disease to be considered eradicated worldwide by vaccination (1977).

Allergic responses are initiated when antigens (such as pollen, mold spores or dust) attach to mast cells causing them to release histamines. Histamines increase blood flow and secretion of fluids, prompting a range of symptoms. 

Autoimmune disorders occur when the body cannot distinguish pathogens from its own cells and tissues. Multiple sclerosis, rheumatoid arthritis and systemic lupus are examples of autoimmune disorders.
 
Acquired Immune Deficiency Syndrome, or AIDS, is an example of what can happen when the immune system becomes compromised. When the AIDS virus (HIV) becomes active, it causes a person to become sick from diseases that a functioning immune system would prevent.

Members of the microbial kingdom Protista originally were defined by structure (mainly unicellular eukaryotes) and by the difficulty to classify them as either plant, fungi or animal.  More recently, the concept of protists was expanded to include certain multicellular organisms such as kelp (Copeland, 1956). Thus defined, members of Protoctista range from microscopic one-celled organisms like dinoflagellates, to multicellular organisms, like seaweed. To untangle this confusing kingdom, biologists now are turning to molecular analysis. 

When following the traditional five- or six-kingdom classification, the Protist group contains all eukaryotes that are not fungi, plants or animals. There are unicellular, colonial, and multicellular forms, some of which show cell specialization.  Protists groups include both autotrophs and heterotrophs, some of which function as detrivores.

Animal-like groups are often referred to as Protozoans.  The term Protozoa dates back to when members of this group were considered "first animals."  Plant-like forms are generally called algae.

Traits such as method of motility, presence or absence of a shell, manner of obtaining nutrition, and reproducing, are used to categorize and discuss this diverse group, but it is important to remember that these traits do not necessarily reflect evolutionary history.  Recent work suggests that green and red algae are more closely allied with land plants, and that slime molds are more closely allied to animals (Baldauf, et al. 2000).

Protists form a broad base across the bottom of the food chain, and they supply approximately one-half of the world's oxygen (unicellular algae compose a large portion of the world's phytoplankton).  Protists, along with bacteria and fungi, are responsible for decomposing and recycling nutrients.

Many protist are helpful.  Euglena are used to help treat sewage because of their unique ability to switch from an autotrophic to a heterotrophic nutritional mode, helping to maintain oxygen levels in the balance.  Another helpful protist is Trichonympha which lives in the digestive system of termites and produces cellulase, an enzyme that enables termites to digest wood.

Animal-like protists are responsible for diseases such as malaria, amoebic dysentery, toxoplasmosis, African Sleeping Sickness and Giardiasis in humans.  Some protists dramatically have affected human history.  Phytopthana infestans, a water mold, destroyed potato crops throughout Ireland in the 1840s, leading to the Great Potato Famine and the eventual migration of large numbers of people into the United States. 

Some protists have medicinal and industrial uses.  Carrageenan, from algae, is used to produce a thickening agent in ice cream, pudding, and candy.  Chemicals from algae are used to manufacture waxes, plastics, paints and lubricants.  Other chemicals made from Protists are used in treatment of ulcers, high blood pressure, and arthritis.

Members of the kingdom Fungi are eukaryotic, heterotrophic, multicelled organisms (except for yeasts).  Examples of fungi are "mushrooms," puffballs, bracket fungi, molds, and mildews.  Fungi cell walls contain chitin, which is the same material found in the exoskeletons of arthropods.  Fungi are important decomposers in ecosystems as they break down organic materials such as dead organisms, leaves, old wood, and feces.

The body of the multicellular fungi consists of long, slender hyphae, some of which can specialize to hold spores, to anchor its body, to secrete enzymes, and to absorb decomposing material.  Fungi break down potential food sources by excreting strong hydrolytic enzymes (exoenzymes).  Once the food is broken down into smaller molecules, the fungi then absorb them into their bodies. 
 
Fungi are generally described and grouped according to the way they reproduce.  Reproduction can be both sexual and asexual, producing spores that disperse by wind or water.

The term "yeast" is used to described any single-celled fungus species, including Baker's yeast (Saccharomyces cerevisiae). Most fungi, such as mushrooms, molds and rusts, are multicellular and are not microbes. Members of the Fungus group are eukaryotes (their cells contain membrane-enclosed nuclei and organelles) that feed by absorption. The cell walls of fungi are composed of chitin, the material that gives hardness to the exoskeletons of insects.

Baker's yeast cells are larger than bacteria cells, usually about 10µm in diameter. Students will not be able to observe many internal details of yeast cells using a typical classroom microscope set-up. However, they may notice some dividing or budding yeast cells. Yeasts are able to reproduce asexually by simple cell division or by pinching off bud cells from a parent cell. Baker's yeast has many uses, including in the production of beer and bread, and as a model organism for the study of processes inside cells.

Viewing this presentation fulfills part of the requirements for completing the short course on The Science of Microbes, offered on BioEd Online for professional development contact hours. The Science of Microbes Teacher's Guide may be obtained in its entirety from the Center for Educational Outreach, Baylor College of Medicine (1-800-798-8244).

You can download a PDF of this lesson, including the pre-assessment, from BioEd Online or K8 Science.

The Science of Microbes and accompanying online professional development were supported, in part, by Science Education Partnership Award number 5R25RR018605 from the National Center for Research Resources of the National Institutes of Health (NIH) to Baylor College of Medicine. The unit was developed in partnership with the Baylor-UT Houston Center for AIDS Research, an NIH-funded program (AI036211). The opinions, findings, and conclusions expressed in this presentation are solely those of the authors and do not necessarily reflect the views of Baylor College of Medicine or the sponsoring agencies.