Microbes
(microorganisms) are living things that are usually too small to be seen
without magnification.
Microbes
include:
·
Bacteria
·
Protozoa
·
Fungi
·
Microscopic
algae
·
Viruses
(these are different from the rest—question whether they can e said to be
alive)
Out first thought is probably that these
cause disease. We call them
“germs.” A more scientific term is pathogenic (disease-causing)
microbes. These do get a lot of attention, but many more microbes are harmless
or even beneficial.
Food chains in various bodies of water begin with microbes
Nitrogen-fixing bacteria in soil
Algae--carry out photosynthesis and produce O2
Digestive tract—aid in digestion and produce vitamins, which we absorb
(B vitamins & K)
Bacteria are used in industrial processes to produce chemicals such as:
Acetone
Organic acids
Enzymes
Alcohols
Drugs such as antibiotics
Bacteria and other microbes are used in production of foods such as:
Sauerkraut Buttermilk
Pickles Cheese
Vinegar Yogurt
Alcoholic beverages
Bread
Soy Sauce
Bacteria are used in modern genetic
engineering to produce such products as:
Insulin
hGH
Interferon
It is not easy for us to even imagine a
world where microbes were completely unknown. Things that everyone today takes
for granted were complete mysteries.
Epidemics occurred and no one could imagine the cause
Food spoilage
No vaccines
No antibiotics--in 1937, 80% of pneumonia cases were fatal
Our system of nomenclature (naming) of
living organisms was established in 1735 by Carolus Linnaeus, a Swedish
botanist. Living organisms except
bacteria are classified into the following groups:
The
last 2 groups, genus and species, are known as the scientific name. Both parts
must be underlined or in italics. Genus
name is a Latin noun and is capitalized, species name
is a Latin adjective and is not capitalized. Examples: Staphylococcus aureus, Escherichia coli or Staphylococcus aureus
and Escherichia coli.
In classifying bacteria, some of the groups
may not be in common use, but the genus & species always will be. For minor
differences among members of the same bacterial species, the designation of
strain is used.
Linneaus’ original classifications were
based on observation and “common sense.” Modern classifications are mostly
based on study of DNA and RNA. These are more scientifically accurate, but the
great majority of the original classifications stand.
1. Bacteria:
tiny, one-celled microbes whose cellular characteristics differ from the
cells of all other living things.
Bacterial cells are prokaryotic cells, while the cells of all other
organisms are eukaryotic. Prokaryotic
cells are simpler cells. A major
difference is that they have no membrane separating the genetic material from
the rest of the cell contents. This
group is also sometimes called eubacteria--our common everyday bacteria. Their
cell walls are made of peptidoglycan, a huge molecule made of carbohydrate and
protein. Bacterial cells come in 3 basic shapes: bacillus (rod), coccus
(round), and spiral (corkscrew or curved).
2. Archaea: these are strange bacteria which share some
characteristics with the eubacteria, but also have some major differences.
Their appearance is very much the same, and their cells are also prokaryotic.
However, this group does not contain peptidoglycan in their cell walls, and
they thrive in extreme environments. Their metabolic pathways often differ
greatly from those of eubacteria. As far as we know, this group does not cause
disease in humans.
3. Fungi:
these organisms have eukaryotic cells, with a nuclear membrane
separating the genetic material from the other cell contents. Unicellular fungi are known as yeasts and are
all microscopic. Other fungi are
multicellular. Mushrooms are an example
of large multicellular fungi. Smaller
multicellular fungi are known as molds.
Molds are made of long filaments called hyphae, which branch and
intertwine and form a visible mass called a mycelium. Fungi are plant-like but do not have
chlorophyll and cannot carry on photosynthesis.
4. Protozoa:
unicellular, eukaryotic more animal-like organisms. Protozoa are placed into 4 groups based on
means of locomotion (movement). They may
be free-living or parasitic.
5. Algae:
photosynthetic eukaryocytes. Some
are multicellular and macroscopic. In
microbiology, the unicellular algae are studied. Some of these are motile by flagella. Algae
produce large amounts of oxygen as they carry on photosynthesis.
6. Viruses: tiny
entities which are acellular---no cellular structure. A virus consists of a core of nucleic acid
covered by a protein coat. Viruses cannot reproduce or carry out metabolic
reactions on their own, but they invade living cells and reproduce using the
mechanisms of the host cell.
7. Parasitic worms (helminths): these
organisms are microscopic during at least some stages of their life cycles, but
many also have macroscopic stages. The
two main groups are the flatworms and the roundworms
Linneaus placed all organisms into 1 or 2
kingdoms---plant or animal. This was the accepted standard for about 100 years.
In the middle 1800’s, a German named Haeckel proposed 3 kingdoms---animal,
plant and one called Protista to give a place for microorganisms. This stood
for around another 100 years, until advancing knowledge made a new system the
better choice. This was Whittaker’s 5 kingdom scheme. It included:
*1. Kingdom Monera---all the
bacteria (prokaryocytes)
*2. Kingdom Protista---unicellular
eukaryocytes
*3. Kingdom Fungi---the
fungi---yeasts and molds
4. Kingdom Plantae---plants and
higher algae
5. Kingdom Animalia---animals
Another scheme has been proposed, based on
the idea that all bacteria are not similar enough to be in the same kingdom.
The main person behind this plan is Dr. Carl Woese, who claims that there
should be 3 main groups called domains, which are taxa higher than kingdom:
1. Bacteria---regular everyday bacteria with peptidoglycan cell walls
2. Archaea---unusual bacteria lacking peptidoglycan cell walls
3. Eukarya---all others
The science of microbiology began
approximately 300 years ago, although most developments have occurred in about
the last 200 years. We will look at some important steps in the history of
microbiology.
I. DEVELOPMENT OF THE MICROSCOPE
Before we could study them, we had to be
able to see them!
Our first record of magnifying lenses dates
back to the 13th century. The first microscopes were developed in
the early 1600’s, by two scientists, Jansen and Galileo.
In 1665, Robert Hooke observed empty dead cells
in a piece of cork. He named them cells because they reminded him of the little
rooms in a monastery, which were also called cells. He never seemed to
understand the importance of his discovery, but this was a beginning.
In the years 1673-1723, a Dutch merchant, Antoni van
Leeuwenhoek, first recorded observations of microbes. He built his own
microscopes as a hobby and constantly worked to improve them, but his best
instrument probably had a magnification of 200 or 300 X. He had no scientific
education, but he constantly reported his findings to the British Royal Society, so his remarkable
work was recorded this way. He called the tiny creatures he observed
“animalcules,” and today we can recognize bacteria and protozoa from his
drawings. He is known as the Father of Microbiology.
Although it seems silly today, until the mid
1800’s many scientists believed some forms of life arose spontaneously. In
fact, this was a hotly debated issue for many years. The opposing theory was known as
biogenesis—life from life, or all cells come from previously existing cells.
Spontaneous generation:
· Toads and snakes-moist soil
· Mice-rags and grain
· Flies-from manure
· Maggots-from meat
An Italian, Fransisco Redi, in 1668 proved
maggots appeared only if flies could get to meat---beginning of the end of the
theory for macroscopic creatures but now believers turned to microbes.
1745-Needham-boiled meat to kill microbes
and left uncovered---soon microbes appeared---claimed this proved spontaneous generation
1765-Spallanzani (Italian) claimed microbes
come from air. He boiled meat and kept sealed-no microbes.
The arguments dragged on:
1858-Rudolf Virchow (German) officially
proposed the theory of biogenesis-cells arise only from pre-existing living
cells and this included microbes. Many scientists agreed with him, but there
were still those opposed.
In 1861 a French scientist, Louis Pasteur,
carried out the final proof of biogenesis. He devised a special goose-neck
flask which he used to overcome the last arguments.
·
Microbiology
was established as a science
·
Many
discoveries by Pasteur, Koch, and others
·
Causative
agents of many diseases were identified
·
Role of
immunity began to be understood
·
Improved
microscopes
·
Improved
lab culturing techniques
·
Vaccines
were developed
·
Gram
stain and other staining techniques were developed
First, more about Louis Pasteur. He was a French chemist who lived from 1822
- 1895. Although his education was in chemistry and he set out to be a
chemistry teacher, he made great contributions in microbiology, medicine, the
wine industry, agriculture, etc. He is recognized as the man most responsible
for making the connection between microbes and disease, which may be his most
important contribution.
In the 1850’s, Pasteur was asked to
determine why wine sometimes turned out badly and why good wine sometimes
spoiled. He proved fermentation was due
to the action of microbes . Good results were due to “good”
microbes, and if other
“bad” microbes were present, they caused spoilage. His solution was to carefully heat the grape
juice just enough to kill all microbes without changing the character of the
juice, and then add “good”
microbes. This technique came to be called pasteurization.
As far back as recorded history, great
epidemics of diseases such as bubonic plague, smallpox, and cholera have
occurred, and for most of that time man had no idea of the cause. In the 1300’s an outbreak of bubonic plague
killed about 25 million people in Europe, including about half the population
of
Before infectious diseases were understood,
illness was blamed on such things as divine punishment, “bad air,” “evil
humors,” “bad blood,” demons, etc.
Treatments included bleeding, physics, shutting out air, burning
feathers in the room, etc.
Over the years, some early scientists had
proposed that tiny living organisms were the cause of disease. This included an
Italian, Fracastoro, in 1547, and an Austrian, von Plenciz, in 1762. Even
though these men were right, their ideas were not widely accepted at the time.
In 1865, Pasteur proved that the cause of a
disease of silkworms (pebrine) was a protozoan organism. This was the beginning
of the proof that all infectious disease was caused by microorganisms. Pasteur
went on to find the causative agents for a number of animal and human diseases.
A German physician, Robert Koch, become very
interested in Pasteur’s work and continued to identify causative microbes. In
1876, Koch proved that anthrax, a disease of cattle, was caused by a bacterium,
Bacillus anthracis. Koch realized that to work with microbes, a
way to grow them in lab for study was needed. Koch and his associates did a lot
of the work on culture techniques. By trial and error they found ‘recipes’ for
solutions many microbes would grow well in. At first, all of these were
liquids, but Koch wanted a solid medium. He first used slices of potato, but
then discovered that adding a seaweed product called agar to his liquid media
would make them solidify.
An associate of Koch, Richard Petri,
invented the Petri dish.
Koch determined the criteria for proof of
causative agents. This set of rules is known as Koch’s Postulates. Remember, a
pathogen is a disease-causing microbe.
·
Pathogen
must be present in all cases of disease
·
Pathogen
must be isolated and grown in lab in pure culture
·
Pathogen
from pure cultures must cause disease when inoculated into healthy, susceptible
lab animal
·
Same
pathogen must be isolated from the diseased lab animal
Using his postulates, Koch identified
causative agents for tuberculosis, diphtheria, human cholera, typhoid, tetanus,
and other diseases. The postulates are still applied today, although there are
some limitations.
Even before day of proof of causative
agents, some doctors suspected that “germs” could be transferred from one
patient to another.
·
1840’s---Oliver
Wendell Holmes in the US and Ignaz Semmelweis in Hungary both realized that
puerperal fever (childbed fever) which we now know is a streptococcal
infection, was being transferred from patient to patient by doctors, and that
simple hand washing would greatly decrease the incidence
·
1860’s---Lister,
an English surgeon, pioneered the use of phenol (carbolic acid) for surgical
instruments, cleaning the operating room, and dressing for incisions.
·
1796---Edward
Jenner, an English physician, performed the first safe vaccinations in modern
medicine. He discovered that if people were deliberately exposed to a mild
disease, cowpox, they became immune to smallpox. He did not call it
vaccination-Pasteur later gave this name, in honor of Jenner, using the Latin
word for cow. Jenner’s process was a lucky accident, since
cross-immunization—becoming immune to one disease also provides immunity to
another—is rare.
·
1880-Pasteur
discovered a vaccine for fowl cholera. This was an accident, but he figured out
what was happening and set out to develop vaccines for human and animal
diseases. His first successful vaccine was for anthrax. He also discovered a
preventative treatment for rabies, which was used until recent years.
Since instances of cross immunization are
rare, vaccines are mostly produced by weakening but not killing virulent
microbes, using only selected parts of virulent microbes, or even genetic
engineering.
Chemotherapy---chemical treatment of
disease. Types of chemicals used against microbes:
Antibiotics---produced naturally by bacteria and fungi
Synthetic drugs---produced totally in lab
Semi-synthetic---we take the natural antibiotic and modify it
For all of these, the chemical must be more
poisonous to microbe than host.
First chemotherapy—used substances from
nature such as plant products (folk medicine)
Cinchona bark—quinine for malaria
Willow bark—aspirin
Foxglove--digitalis
Paul Ehrlich-deliberately looked for a
treatment for syphilis. In 1910 he discovered an arsenic compound, Salvarsan,
and this is regarded as the beginning of modern chemotherapy.
1930’s--sulfa drugs
Antibiotics---Fleming discovered penicillin
by accident in 1928. He did not realize importance, but about 10 years later 2
other scientists did. It began to be available in 1940’s.
As we learned more, various branches of
micro developed:
Bacteriology---study of bacteria
Mycology---study of fungi
Parasitology--- study of protozoa and parasitic worms
We have not yet discovered all pathogens.
Later in this chapter we will have a whole list of newly discovered pathogens
and diseases.
Immunology- began with Jenner 1796. Another big
development was the discovery of phagocytosis by white blood cells, for which
Elie Metchnikoff received a Nobel Prize. Study in this area has advanced
rapidly in recent years, mainly due to AIDS virus.
Virology—as study of bacteria continued, it began to
be recognized that some pathogens could pass through a filter small enough to
trap all known bacteria. The name given to this unknown agent was virus.
Scientists were only able to actually see them after the electron microscope was
developed in the 1940’s.
RECOMBINANT
DNA—GENETIC ENGINEERING
Little progress in molecular genetics and
molecular biology took place until we began studying these things in one-celled
organisms in the 1940’s. Then rapid progress was made.
·
1941---1
gene 1 enzyme
·
1944---DNA
was identified as hereditary material
·
1953---
Watson and Crick discovered structure of DNA
·
1960’s---
more about the way DNA controls protein
synthesis, discovery of mRNA, regulation of gene function, “cracked” the
genetic code
·
1970’s---beginning
of genetic engineering
Recombinant
DNA- genes can be isolated and inserted into other cells, even from one species
to another, and proteins still be made. This technique is currently used to produce
hormones, vaccines, etc. The hope is that eventually we may be able to insert
the needed gene into the cells of a person with a genetic defect---gene
therapy.
MORE
BENEFICIAL ACTIVITIES OF MICROBES:
·
Recycling
vital elements-decomposition of wastes and dead plants and animals by microbes
as well as O2 production by
photosynthetic microbes
·
Sewage
treatment-bacteria are used in the breakdown of sewage to recycle the water
·
Cleanup
of the environment-bacteria can aid in cleanup of pollutants and toxic wastes
by producing enzymes that break down these substances. Some of these bacteria are even genetically
engineered to clean up specific chemicals.
This is called bioremediation.
·
Enzymes
produced by bacteria (Bacillus) are
used in detergents are used to remove spots
·
Insect
control-due to concern about toxic insecticides in the environment, there is
great interest in developing microbes that affect only certain insect pests
·
Biotechnology---commercial use of microorganisms to produce
food, chemicals, etc.
·
The
most recent form involves insertion of specific genes into cells to allow
productions of new proteins- for example, Escherichia coli bacteria are now making human proteins such as
insulin.
·
Diseases
caused by genetic defects may one day be cured by insertion of the normal gene
into the cells of the patient (gene therapy)
·
Many
crops have been improved by genetic engineering—better yield, insect
resistance, l;onger shelf life, etc.
Although most microbes are harmless and some
are even beneficial, a small number do cause disease. These are pathogens (disease-causing)
microbes. Some of these pathogens can be traced as far back as human history. With the development of antibiotics, vaccines
and other modern treatments, there was a time that we believed infectious
disease could be eradicated. Development
of resistant bacteria and appearance of new diseases has ended that belief.
Some emerging infectious diseases newly recognized or found in locations where
previously unknown in recent years include:
·
Avian
influenza A (H5N1)—also called bird flu
·
SARS
·
West
Nile encephalitis
·
Bovine
spongiform encephalopathy (mad cow disease)
·
Escherichia coli O157:H7
·
Invasive
Group A strep (flesh-eating bacteria)
·
Ebola
fever
·
Marburg
virus
·
Hantavirus
pulmonary syndrome
·
Cryptosporidium
·
AIDS