The Effects of Antibiotics on
Bacteria are the most common and ancient microorganisms on earth. Most
bacteria are microscopic, measuring 1 micron in length. However,
colonies of bacteria grown in a laboratory petri dish can be seen with
the unaided eye.
There are many divisions and classifications of bacteria that assist in
identifying them. The first two types of bacteria are archaebacteria and
eubacteria. Both groups have common ancestors dating to more than 3
billion years ago. Archaebacteria live in environments where, because of
the high temperature, no other life can grow. These environments include
hot springs and areas of volcanic activity. They contain lipids but lack
certain chemicals in their cell wall. Eubacteria are all other bacteria.
Most of them are phototrophic, i.e. they use the sun’s energy as food
through the process of photosynthesis.
Another classification of bacteria is according to their need of oxygen
to live. Those who do require oxygen to live are considered aerobes. The
bacteria who don’t use oxygen to live are known as anaerobes.
The shape of specific bacteria provides for the next step in the
identification process. Spherical bacteria are called cocci; the
bacteria that have a rodlike shape are known as bacilli; corkscrew
shaped bacteria are spirilla; and filamentous is the term for bacteria
with a threadlike appearance.
Hans Christian Joachim Gram, a Danish microbiologist, developed a method
for distinguishing bacteria by their different reaction to a stain. The
process of applying Gram’s stain is as follows: the bacteria are stained
with a violet dye and treated with Gram’s solution (1 part iodine, 2
parts potassium iodide, and 300 parts water). Ethyl alcohol is then
applied to the medium; the bacteria will either preserve the blue color
of the original dye or they will obtain a red hue. The blue colored
bacteria are gram-positive; the red bacteria are identified as
Bacteria contain DNA (deoxyribonucleic acid) just like all cells.
However, in bacteria the DNA is arranged in a circular fashion rather
than in strands. Bacteria also contain ribosomes which, like in
eukaryotic cells, provide for protein synthesis. In order for a
bacterium to attach itself to a surface, it requires the aid of pili, or
hairlike growths. Bacteria, just like sperm cells, have flagella which
assist in movement. But, sperm cells only have one flagellum, whereas
bacteria contain flagella at several locations throughout their body
Although most bacteria are not harmful, a small fraction of them are
responsible for many diseases. These bacterial pathogens have affected
humans throughout history. The “plague”, an infamous disease caused by
bacteria, has killed millions of people. Also, such a disease as
tuberculosis, a disease responsible for the lives of many, is caused by
bacterial pathogens ingested into the body.
Bacteria affect everyone in their daily life because they are found
nearly everywhere. They are found in the air, in food, in living things,
in non-living things, and on every imaginable surface.
Escherichia coli is a disease causing gram-negative bacillus. These
bacteria are commonly found within the intestines of humans as well as
other vertebrates. This widely spread bacteria is known to cause urinary
tract infections as well as diarrhea.
Microcococcus Luteus are gram-positive parasitic spherical bacteria
which usually grows in grapelike clusters. This species is commonly
found in milk and dairy products as well as on dust particles.
Bacillus Cereus are a spore forming type of bacteria. They are
gram-positive and contain rods. Due to the fact that this bacteria is
known to survive cooking, it is a common cause of food poisoning and
Seratia Marscens a usually anaerobic bacteria which contains
gram-negative rods. This bacteria feeds on decaying plant and animal
material. S. marscens are found in water, soil, milk, foods, and certain
In spite of the fact that bacteria are harmful to the body, certain
measures can be taken in order to inhibit their growth and reproduction.
The most common form of bacteria fighting medicines are antibiotics.
Antibiotics carry out the action which their Greek origin suggests: anti
meaning against, and bios meaning life. In the early parts of the 20th
century, a German chemist, Paul Ehrlich began experimentation using
organic compounds to combat harmful organisms without causing damage to
the host. The results of his experimentation began the study and use of
antibiotics to fight bacteria.
Antibiotics are classified in various ways. They can be arranged
according to the specific action it has on the cell. For example,
certain antibiotics attack the cell wall, others concentrate on the cell
membrane, but most obstruct protein synthesis. Another form of indexing
antibiotics is by their actual chemical structure.
Practically all antibiotics deal with the obstruction of synthesis of
the cell wall, proteins, or nucleic acids. Some antibacterials interfere
with the messenger RNA, consequently mixing up the bacterial genetic
Penicillins act by inhibiting the formation of a cell wall. This
antibiotic works most effectively against gram-positive streptococci,
staphylococci (e.g. Micrococcus Luteus) as well as certain gram-negative
bacteria. Penicillin is usually prescribed to treat syphilis, gonorrhea,
meningitis, and anthrax.
Tetracycline inhibits protein synthesis in pathogenic organism. This
antibiotic is obtained from the culture of Streptomyces.
Streptomycin an antibiotic agent which is obtained from Streptomyces
griseus. This antibiotic acts by limiting normal protein synthesis.
Streptomycin is effective against E. Coli, gram-negative bacilli, as
well as many cocci.
Neomycin an antibiotic derived from a strain of Streptomyces fradiae.
Neomycin effectively destroys a wide range of bacteria.
Kanamycin an antibiotic substance derived from Streptomyces kanamycetius.
Its antibacterial action is very similar to that of neomycin. Kanamycin
works against many aerobic gram-positive and gram-negative bacteria,
especially E. coli. Protracted use may result in auditory as well as
Erythromycin is an antibiotic produced by a strain of Streptomyces
erythreaus. This antibiotic works by inhibiting protein synthesis but
not nucleic synthesis. Erythromycin has inhibitory effects on
gram-negative cocci as well as some gram-positive bacteria.
Chloramphenicol is a clinically useful antibiotic in combating serious
infections caused by certain bacteria in place of potentially hazardous
means of solving the problem. In lab tests, it has been shown that this
medicine stopped bacterial reproduction in a wide range of both
gram-positive and gram-negative bacteria. The inhibition of cell
reproduction caused by Chloramphenicol takes place through interference
with protein synthesis.
An experiment was conducted in order to determine which antibiotics are
most effective in inhibiting bacterial growth. First, the different
bacteria were placed on agar inside petri dishes. Then, antibiotic discs
were placed into the dishes. Each bacteria was exposed to every one of
the antibiotics listed above. The bacteria used in the experiment were:
Bacillus Cerus, Escerichia Coli, Seratia Marscens, and Micrococcus
After a 24 hour incubation period, the results were measured. In order
to determine which antibiotic had the most effect their zones of
inhibition were recorded. The zone of inhibition refers to the distance
from the disc to the outermost section around the disc where no
bacterial growth was present. The results can be seen on the graph and
The following is a table showing the different zones of inhibition of
each antibiotic in the bacteria culture:
Tetracycline Chloramphenicol Kanamycin Neomycin Penicillin Streptomycin
B. Cerus 5.5 9 5 6.6 1 7 13
E. Coli 7 4.2 5.5 4.5 no effect 4.6 no effect
S. Marscens no effect no effect 4.5 4 no effect 3 no effect
M. Luteus 23 22 10 11 23.5 11.5 19
After analysis of the data obtained it is obvious that each antibiotic
had a distinct effect on the growth of the different bacteria. The
results of this experiment are very important, since they teach of how
each bacteria reacts to different antibiotics. This is very valuable
because it is the information which assists physicians in prescribing
certain medications to cure diseases caused by bacteria.
1) Encart Encyclopedia 1994, CD-ROM.
2) McGraw-Hill Encyclopedia of Science and Technology, 1992.
3) Physicians’ Desk Reference, 1996.
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