Discovery of Antibiotics
In 1929, Alexander Fleming discovered that the mold Penicillium notatum inhibited the growth of the bacterium Staphylococcus aureus. Further studies led to the discovery that the mold produced the drug penicillin, which was responsible for this inhibitory action.
Antibiotics may be classified as bactericidal and bacteriostatic. Streptomycin is an antibiotic produced by the soil microbe Streptomyces griseus. American biochemists, Albert Shcatz, Elizabeth Bugie and Selman Waksman isolated the microbe in 1943, and reported its antimicrobial activities in 1944. This was the first antimicrobial agent developed after penicillin.
Activity Spectrum
- This antibiotic is inhibitory for a gamut of microorganisms, both gram-positive and gram-negative.
- It is widely is used in the treatment of infective diseases, especially in infections by gram-negative bacteria, like septic sore throat.
- It is active against species that are resistant to other antibiotics, e.g. penicillin-resistant Staphylococci and bacteria of the genera Proteus.
- It is the treatment of choice in infections of the heart, i.e. bacterial endocarditis.
- It is used extensively as the primary medicine in the treatment of Tuberculosis, in combination with other drugs like isoniazid, ethambutol and aminosalicylic acid.
Streptomycin consists of three components, linked glycosidically:
a) N-methyl – L – glycosamine
b) Streptose
c) Streptydine
Mode of Action
Streptomycin is characterized chemically as an aminoglycosidic antibiotic. Other aminoglycosides include kanamycin, neomycin, tobramycin and amikain. All aminoglycoside exert their inhibitory action by blocking protein synthesis in bacteria.
Streptomycin is bactericidal in action. It inhibits protein synthesis by combining irreversibly with the 30S subunit of the70S ribosomes, found typically in prokaryotes. Specifically, it binds with the S12 protein involved in the initiation of protein synthesis.
Experiments have shown that streptomycin prevents the initiation of protein synthesis by blocking the binding of initiator N-formylmethionine tRNA to the ribosome.
Streptyomycin is also known to prevent the normal dissociation of 70S ribosomes into their 50S and 30S subunits. Thus formation of polysomes is inhibited.
The overall effect of streptomycin seems to involve distorting the ribosome so that transition from initiation complex (30S-mRNA-tRNA) to chain elongating ribosome is blocked.
Thus, the normal sequence of translation is disrupted, the bacteria is unable to synthesize proteins vital for its cell growth and thereby fails to survive. The drug also disrupts the cell membrane of susceptible bacteria.
Many bacterial species, e.g. Bacillus subtilis Strain SRB15T+ have undergone mutations in their genome that enable these to bypass the streptomycin-mediated inhibition of cell growth.
Possible Side Effects of Streptomycin
- headache
- nausea
- difficulty in hearing, ringing sensation in the ears
- loss of balance
- fatigue
- difficulty in passing urine
- itchy rashes on the skin
