Antibiotics - Microbiology

Antibiotics Like Penicillin, Streptomycin and Tetracycline

Antibiotics are the substances produced by microorganism which suppress the growth of microbes or kill other microorganism ,at very low concentration. This definition excludes

• other natural substances which also inhibit microbes but are produced by higher forms. Example: antibodies

• The chemicals which kill the microorganisms but are not obtained by living organisms. Example: dettol and phenyl

• chemicals produced by microbes which can kill the microbes but are needed in higher concentration. Example: lactic acid and ethanol

Antibiotics are obtained from various microorganisms like fungi ,bacteria and actinomycetes.

Some important antibiotics are explained below

Penicillin

This was the first antibiotic to be used clinically in 1941. It was originally obtained from a fungus Penicillium notatum but the present source is a high yielding nutrient of Penicillium chrysogenum. The penicillin nucleus consist of fused thiozoledone and beta lactam rings to which side chains are attached through an amide linkage.Four types of natural penicillium F,K,X and G were produced originally. Out of these penicillin G,having the benzyl side chain ,has a most desirable properties and is used clinically.

Mechanism of action

Penicillin interferes with the synthesis of bacterial cell walls during the synthesis transpeptidase and carboxypeptidase are inhibited. So that cross linking does not take place when the bacteria divides in presence of penicillin, the cell wall deficient (CWD) forms are produced. Because the interior of the bacterium is hyper osmotic. The cell wall swells and burst. This causes bacterial lysis. Rapid cell wall form occurs when the bacteria are actively multiply . So penicillin is more lethal in this phase. The peptidoglycon cell wall is unique to bacteria No such substance is synthesised in higher animals that is why penicillin is practically non toxic to men. Further in gram positive bacteria the cell wall is almost entirely made up of peptidoglycane. So the gram positive bacteria have a higher susceptibility to penicillin.

Uses of penicillin

Penicillin G is widely used for the cure of many infections caused by pathogens susceptible to penicillin. Some diseases where penicillin G is the drug of choice are as follows

• Streptococcal infections : infection caused by streptococcus like scarlet fever, rheumatic fever are cured by penicillin G given in ordinary doses.

• Pneumococcal infections : penicillin G is the drug of choice in Pneumonia meningitis etc

• Staphylococcal infection : In some of these infections, penicillin G is given but mostly these bacteria are penicillinase producing

• Syphilis : Penicillin G is used against the pathogen Treponema palladium .

• Tetanus : Tetanus and gas gangrene, in both cases, penicillin G is used to kill clostradium species.

• Trench mouth :due to fusobacterium and spirochetes ,is cured by penicillin G.

Streptomycin

It is an aminoglycoside antibiotic. These are the group of antibiotics having polybasic amino groups linked to amino sugars unlike penciline G, which was the chance of discovery, streptomycin discovery was the result of deliberate search for drugs effective against gram negative bacteria. It was the first member, discovered in 1944 by Waksman et al . It was an important discovery because it was effective against Mycobacterium tuberculosis. Streptomycin was obtained from Streptomyces griseus

Mechanism of action

Streptomycin is a bacteriacidel antibiotic i.e it kills prokaryotic cells. The mechanism of action is by inhibiting the protein synthesis, it combines with 30s subunit of ribosome and interferes with protein synthesis. This causes ultimate killing of the bacteria. Streptomycin is effective against most of the gram negative bacteria. It diffuses through the outer coat of gram negative bacteria.

Uses of streptomycin

The antimicrobial spectrum of streptomycin is relative narrow . It is active primarily against aerobic gram negative bacilli. It is used in the following infections:

• Tuberculosis : Streptomycin is effective against the tubercal bacilli but it has poor penetration into cells so it acts only on extracellular bacilli when given to a patient it penetrates tubercular cavities but does not cross to the cerebrospinal fluid.

• Bacterial endocarditis :in this infection streptomycin is given in conjunction with penicillin

• plague and tularemia, in both cases these streptomycin is given and shows rapid cure

• in some other infections example ordinary tract infection, septicemia etc . Streptomycin was used earlier but now the pathogen show resistant to this drugs.

Tetracycline

These are a class of antibiotics having a nucleus of four cyclic rings. All are obtained from soil actinomycetes . The first to be introduced was chlorotetracycline in 1948 . It was obtained from streptomyces. It is broad spectrum antibiotic. The second member to be introduced was Oxytetracycline and after that many other members were isolated.

Mechanism of action

Tetracycline inhibit protein synthesis . It binds to 30s ribosome, as a result the peptide chain fails to grow.

Uses

When originally introduced tetracycline inhibit practically all types of pathogenic microbes accept fungi and viruses. However indiscriminate use has narrowed the spectrum . the infections against which tetracycline are used are given below

• Tetracycline are given to the patients in cases where the nature and sensitivity of the infecting organism is not known and cannot be reasonably guess. They may also be used for initial treatment of mixed infection of wound infections etc

• Many pathogens have developed resistant to tetracycline but they are still the drug of choice in most of the diseases like gonorrhea and syphilis

• Tetracyclines are given in many rickettsial infection eg Rocky Mountain

• Tetracycline are given in chalamydal infection example trachoma and conjunctivitis.

• Tetracyclines are second choice drug to penicillin. In cases of tetanus, anthrax, actinomycosis.

• Some other infection in which the tetracycline may be used are urinary track infection ,amaebosis. In this case , tetracycline are given with other amoebiocides.

• Acne (pimples): In case of acne prolonged therapy with low doses may be used in severe cases.

Other important antibiotics

1. Bacitracin it is an antibiotic which is used to inhibit cell wall synthesis by a specific inhibiting action on pyrophosphitase. It also disorganise the cell membrane . It was first described in 1945, the sources Bacillus subtilis was found contaminating a wound in patient. Hence it acts mostly on gram positive bacteria.

2. Erythromycins it obtained from streptomyces species ,act against gram positive bacteria but also attack several species of gram negative bacteria. Their mode of action is to inhibit synthesis of protein by blocking the function of 50s ribosomal unit.

3. Chloramphenicol it is obtained from Streptomyces venezuelae . Now it is also obtained synthetically . It inhibits protein synthesis by combining with 50s ribosome. It is a broad spectrum antibiotic . It is also effective against chlamydae and richettisae in adults to gram positive and gram negative bacteria.

4. Grifamysin these are important antibiotics because these are effective against Mycobacterium tuberculosis as well as some gram negative bacteria obtained from Streptomyces mediteranei . Their mode of action is including inhibiting the function of bacterial RNA polymerase.

5. Polymyxins Polymixins A,B,C,D,E obtained from Bacillus polimyxa , they act by attacking cell wall and cell membrane, they affect mainly gram negative species (the cell wall of which are rich in lipids).

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