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The Effects of Antibiotics on Bacterial Growth

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 gram-negative.

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 surface.

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 diarrhea. 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 insects.

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 code. 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 other damages.
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 Luteus.

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 data chart.

The following is a table showing the different zones of inhibition of each antibiotic in the bacteria culture:

Tetracycline Chloramphenicol Kanamycin Neomycin Penicillin Streptomycin Erythromycin
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.

Bibliography
1) Encart Encyclopedia 1994, CD-ROM.
2) McGraw-Hill Encyclopedia of Science and Technology, 1992.
3) Physicians’ Desk Reference, 1996.

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