Reading Guide
Chapter 1: The Microbial World
This chapter is an introduction to microbiology with discussion of the history
of microbiology, some examples of applications of microbiology, and introduction of
the different types of microorganisms that are members of the microbial world.
Microbiology is the scientific field that focuses on the study of
microorganisms. In this class we will focus on the study of the biology of microbes
and the interaction of these microbes with other microbes, humans, and the
environment.
There are five different categories of microorganisms introduced in this
chapter. They are algae, fungi, protozoa, bacteria, and viruses. You should review the
general characteristics of each of these groups and be able to answer the questions
in the powerpoint lecture. These questions were, which of these organisms is a
prokaryote? Which is able to perform photosynthesis? Etc. In this course we will
focus on the bacteria and viruses.
Bacteria are associated most often with disease, and historically the
occurrence of microbial epidemics has impacted the turn of events. For example
many wars were “won” because of microbial infections which impacted entire
armies. If we look at the microbial epidemics that are relevant today, HIV and
Hepatitis B and Hepatitis C are leaders of viral infections. The bacteria Methicillinresistant Staphylococcus aureus (MRSA) and Mycobacterium tuberculosis are also
the cause of life threatening bacterial infections. While pathogenic bacteria always
manage to get our attention, they only comprise about 1% of the total population of
bacteria.
The rest of the bacteria (other 99%) are found to make up the normal
microbiota, are found in the environment, participate in the production of many
food products, and are resources for recombinant DNA products.
Where are the normal microbiota located? Think of the places in/on your
body where these organisms would be found and what role they play in keeping
your body free of other possible pathogens.
Bacteria in the environment are important in the recycling of elements, think
of the process of composting! The genus Rhizobium is a unique group of bacteria
that are capable of fixing nitrogen, making it more available for plants. Farmers who
plant leguminous plants have found that when this organism is present in the soil,
they have a better yield of crops.
Microbes are important in the production of many food products. Yogurt,
cheese, cottage cheese, cabbage, and fermented beverages are made with the help of
bacteria and/or fungi. The dry weight of yogurt is 60% bacteria! With the new
labeling laws it is easy to see what kinds (both genus and species) are present in
your food product so pay attention the next time you go to the grocery store.
Recombinant DNA products were first made with the help of bacterial cells.
DNA from humans was placed into E. coli and then the cells were allowed to grow
and generate the protein product. This was the way that human insulin was made
for patients that cannot make their own insulin and are diabetic. Since the
development of this type of technology, improvements have been made such that E.
coli is not the cell model used. Instead the insertion of human DNA to generate a
protein product is now done in actual animal cells, also known as tissue culture.
The “birth” of the field of microbiology is dependent on two big
accomplishments, the discovery/development of a microscope and the many
experiments which challenged the theory of spontaneous generation. The
development of the microscope is credited to the following individuals, Zacharis
Janssen, Anton van Leewenhoek, and Robert Hooke. Review in the lecture notes
from class the contributions of each of these individuals. The theory of spontaneous
generation was first challenged by Francisco Redi with his experiment with the jars
of meat in 1668. In 1749 John Needham performed an experiment where he heated
broth and poured the broth into flasks. After a period of time the flasks grew turbid,
indication that there was growth. His experiment supported spontaneous
generation theory. What was the mistake that he made? Spallanzani performed a
similar experiment where he heated broth in flasks, sealed the flasks, and noted that
they did not become turbid. His experiments were possible evidence to refute the
theory of spontaneous generation, but were not accepted since he sealed the flasks.
Finally Louis Pasteur developed a new “curved-neck” flask which allowed the broth
to be open to the air after boiling. His broths remained sterile and supported the
theory of biogenesis. Pasteur’s experiment was the convincing evidence to disprove
the theory of spontaneous generation.
John Tyndall was also important in the story of Louis Pasteur’s work. He
could not reproduce the same experiments reported by Pasteur. He found that there
were resistant forms of microbes present in the broths (hay infusions) that he was
using and that these forms were not destroyed by boiling. Tyndall had discovered
endospores!
The last few slides of the powerpoint lecture for this chapter 1 focus on the
three Domains of all living cells. With the development of the electron microscope
we were able to see the inside architecture of cells and realize that they were
different. Some had a membrane bound nucleus, and membrane bound organelles.
Others lacked these same structures. With this new discovery, new categories had to
be made to place these new cells which lacked these structures. The Kingdom
Procaryotae was established for these cells. Then the ability to sequence rRNA
identifies that all cells can be placed into one of three categories. Instead of making a
Kingdom designation, a three domains were made. These domains are the Bacteria,
Archaea, and Eukarya. Of these two domains, bacteria are found in the bacteria and
archaea. Look at the figure of the three domains and notice the kind of bacteria that
are found in the bacteria domain. What relationship is there between the bacteria
and the eukarya domain? What types of bacteria are found in the archaea domain?
Where are humans in this phylogenic tree?
The last few comments for this lecture are to ask you to review the
characteristics of the following groups of Archaea: the methanogens, the halophiles,
and the thermophiles.