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Antimicrobial Agents in the Treatment of Infectious Disease
(page 4)

(This chapter has 6 pages)

© Kenneth Todar, PhD

Protein synthesis inhibitors

Many therapeutically useful antibiotics owe their action to inhibition of some step in the complex process of protein synthesis. Their attack is always at one of the events occurring on the ribosome and never at the stage of amino acid activation or attachment to a particular tRNA. Most have an affinity or specificity for 70S (as opposed to 80S) ribosomes, and they achieve their selective toxicity in this manner. The most important antibiotics with this mode of action are the tetracyclines, chloramphenicol, the macrolides (e.g. erythromycin) and the aminoglycosides (e.g. streptomycin).

The aminoglycosides are products of Streptomyces species and are represented by streptomycin, kanamycin, tobramycin and gentamicin. These antibiotics exert their activity by binding to bacterial ribosomes and preventing the initiation of protein synthesis.

Streptomycin binds to 30S subunit of the bacterial ribosome, specifically to the S12 protein which is involved in the initiation of protein synthesis. Experimentally, streptomycin has been shown to prevent the initiation of protein synthesis by blocking the binding of initiator N-formylmethionine tRNA to the ribosome. It also prevents the normal dissociation of ribosomes into their subunits, leaving them mainly in their 70S form and preventing the formation of polysomes. The overall effect of streptomycin seems to be one of distorting the ribosome so that it no longer can carry out its normal functions. This evidently accounts for its antibacterial activity but does not explain its bactericidal effects, which distinguishes streptomycin and other aminoglycosides from most other protein synthesis inhibitors.

Streptomycin is the first aminoglycoside antibiotic to be discovered, and was the first antibiotic to be used in treatment of tuberculosis. It was discovered in 1943, in the laboratory of Selman Waksman at Rutgers University. Waksman and his laboratory discovered several antibiotics, including actinomycin, streptomycin, and neomycin. Streptomycin is derived from the bacterium, Streptomyces griseus. Streptomycin stops bacterial growth by inhibiting protein synthesis. Specifically, it binds to the 16S rRNA of the bacterial ribosome, interfering with the binding of formyl-methionyl-tRNA to the 30S subunit. This prevents initiation of protein synthesis.

Kanamycin and tobramycin have been reported to bind to the ribosomal 30S subunit and to prevent it from joining to the 50S subunit during protein synthesis. They may have a bactericidal effect because this leads to cytoplasmic accumulation of dissociated 30S subunits, which is apparently lethal to the cells.

Aminoglycosides have been used against a wide variety of bacterial infections caused by Gram-positive and Gram-negative bacteria. Streptomycin has been used extensively as a primary drug in the treatment of tuberculosis. Gentamicin is active against many strains of Gram-positive and Gram-negative bacteria, including some strains of Pseudomonas aeruginosa. Kanamycin is active at low concentrations against many Gram-positive bacteria, including penicillin-resistant staphylococci. Gentamicin and Tobramycin are mainstays for treatment of Pseudomonas infections. An unfortunate side effect of aminoglycosides has tended to restrict their usage: prolonged use is known to impair kidney function and cause damage to the auditory nerves leading to deafness.

Gentamicin is an aminoglycoside antibiotic, used mostly to treat Gram-negative infections. However, it is not used for Neisseria gonorrhoeae, Neisseria meningitidis or Legionella pneumophila infections. It is synthesized by Micromonospora, a genus of Gram-positive bacteria widely distributed in water and soil. Like all aminoglycosides, when gentamicin is given orally, it is not systemically active because it is not absorbed to any appreciable extent from the small intestine. It is useful in treatment of infections caused by Pseudomonas aeruginosa.

The tetracyclines consist of eight related antibiotics which are all natural products of Streptomyces, although some can now be produced semisynthetically or synthetically. Tetracycline, chlortetracycline and doxycycline are the best known. The tetracyclines are broad-spectrum antibiotics with a wide range of activity against both Gram-positive and Gram-negative bacteria. Pseudomonas aeruginosa is less sensitive but is generally susceptible to tetracycline concentrations that are obtainable in the bladder. The tetracyclines act by blocking the binding of aminoacyl tRNA to the A site on the ribosome. Tetracyclines inhibit protein synthesis on isolated 70S or 80S (eucaryotic) ribosomes, and in both cases, their effect is on the small ribosomal subunit. However, most bacteria possess an active transport system for tetracycline that will allow intracellular accumulation of the antibiotic at concentrations 50 times as great as that in the medium. This greatly enhances its antibacterial effectiveness and accounts for its specificity of action, since an effective concentration cannot be accumulated in animal cells. Thus a blood level of tetracycline which is harmless to animal tissues can halt protein synthesis in invading bacteria.

The tetracyclines have a remarkably low toxicity and minimal side effects when taken by animals. The combination of their broad spectrum and low toxicity has led to their overuse and misuse by the medical community and the wide-spread development of resistance has reduced their effectiveness. Nonetheless, tetracyclines still have some important uses, such as the use of doxycycline in the treatment of Lyme disease.

Some newly discovered members of the tetracycline family (e.g. chelocardin) have been shown to act by inserting into the bacterial membrane, not by inhibiting protein synthesis.

The tetracycline core structure. The tetracyclines are a large family of antibiotics that were discovered as natural products of Streptomyces bacteria beginning in the late 1940s. Tetracycline sparked the development of many chemically altered antibiotics and in doing so has proved to be one of the most important discoveries made in the field of antibiotics. It is a classic "broad-spectrum antibiotic" used to treat infections caused by Gram-positive and Gram-negative bacteria and some protozoa.

Doxycycline is a semisynthetic tetracycline developed in the 1960s. It is frequently used to treat chronic prostatitis, sinusitis, syphilis, chlamydia, pelvic inflammatory disease, acne and rosacea. In addition, it is used in the treatment and prophylaxis of anthrax and in prophylaxis against malaria. It is also effective against Yersinia pestis (the infectious agent of bubonic plague) and is prescribed for the treatment of Lyme disease, ehrlichiosis and Rocky Mountain spotted fever. Because doxycycline is one of the few medications that is effective in treating Rocky Mountain spotted fever (with the next best alternative being chloramphenicol), it is indicated even for use in children for this illness.

Chloramphenicol is a protein synthesis inhibitor that has a broad spectrum of activity but it exerts a bacteriostatic effect. It is effective against intracellular parasites such as the rickettsiae. Unfortunately, aplastic anemia develops in a small proportion (1/50,000) of patients. Chloramphenicol was originally discovered and purified from the fermentation of a Streptomyces species, but currently it is produced entirely by chemical synthesis. Chloramphenicol inhibits the bacterial enzyme peptidyl transferase, thereby preventing the growth of the polypeptide chain during protein synthesis.

Chemical structure of chloramphenicol

Chloramphenicol is entirely selective for 70S ribosomes and does not affect 80S ribosomes. Its unfortunate toxicity towards the small proportion of patients who receive it is in no way related to its effect on bacterial protein synthesis. However, since mitochondria originated from procaryotic cells and have 70S ribosomes, they are subject to inhibition by some of the protein synthesis inhibitors including chloramphenicol. This likely explains the toxicity of chloramphenicol. The eucaryotic cells most likely to be inhibited by chloramphenicol are those undergoing rapid multiplication, thereby rapidly synthesizing mitochondria. Such cells include the blood forming cells of the bone marrow, the inhibition of which could present as aplastic anemia. Chloramphenicol was once a highly prescribed antibiotic and a number of deaths from anemia occurred before its use was curtailed. Now it is seldom used in human medicine except in life-threatening situations (e.g. typhoid fever).

The macrolide family of antibiotics is characterized by structures that contain large lactone rings linked through glycoside bonds with amino sugars. The most important members of the group are erythromycin and oleandomycin. Erythromycin is active against most Gram-positive bacteria, Neisseria, Legionella and Haemophilus, but not against the Enterobacteriaceae. Macrolides inhibit bacterial protein synthesis by binding to the 50S ribosomal subunit. Binding inhibits elongation of the protein by peptidyl transferase or prevents translocation of the ribosome or both. Macrolides are bacteriostatic for most bacteria but are cidal for a few Gram-positive bacteria.

Chemical structure of a macrolide antibiotic, erythromycin.

Azithromycin, shown above, is a subclass of macrolide antibiotics. Azithromycin is one of the world's best-selling antibiotics. It is s derived from erythromycin, but it differs chemically from erythromycin in that a methyl-substituted nitrogen atom is incorporated into the lactone ring, thus making the lactone ring 15-membered. Azithromycin is used to treat certain bacterial infections, most often bacteria causing middle ear infections, tonsillitis, throat infections, laryngitis, bronchitis, pneumonia and sinusitis. It is also effective against certain sexually transmitted diseases, such as non-gonococcal urethritis and cervicitis.

Lincomycin and clindamycin are a miscellaneous group of protein synthesis inhibitors with activity similar to the macrolides. Lincomycin has activity against Gram-positive bacteria and some Gram-negative bacteria (Neisseria, H. influenzae). Clindamycin is a derivative of lincomycin with the same range of antimicrobial activity, but it is considered more effective. It is frequently used as a penicillin substitute and is effective against Gram-negative anaerobes (e.g. Bacteroides).

Clindamycin is a lincosamide antibiotic. It is usually used to treat infections with anaerobic bacteria but can also be used to treat some protozoal diseases, such as malaria. It is a common topical treatment for acne, and can be useful against some methicillin-resistant Staphylococcus aureus (MRSA) infections. The most severe common adverse effect of clindamycin is Clostridium difficile-associated diarrhea (the most frequent cause of pseudomembranous colitis). Although this side-effect occurs with almost all antibiotics, including beta-lactam antibiotics, it is classically linked to clindamycin use.

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Kenneth Todar has taught microbiology to undergraduate students at The University of Texas, University of Alaska and University of Wisconsin since 1969.

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