Q&A: Mechanism of Action of Tetracycline

Effective antimicrobials are required for preventive and curative measures, protecting patients from potentially fatal diseases, and ensuring that complex procedures can be provided at low risk of infection.

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This is a Question & Answer revision article designed for medical students and professionals preparing for the PLAB, MRCP or USMLE examinations. They are based on actual questions from these examinations. You may find the Antibiotic Resistance article more useful, or one of our many articles on Diseases & Conditions, Medical Syndromes, Health & Wellness or Home Remedies.

In this article:
Basic mechanisms of antibiotic action against bacterial cells
MCQ exam: clinical scenario
MCQ exam: answer
MCQ exam: explanation

Basic mechanisms of antibiotic action against bacterial cells

The struggle of mankind against infectious diseases is well known. The discovery of antibiotics led to optimism that infections can be controlled and prevented. However, infections are still the leading cause of death in the developing world. Irrespective of data on emergence of new infectious diseases, reemergence of diseases once controlled and more specifically the appearance of antimicrobial resistance, antibiotics continue to play a very important role in healthcare settings worldwide.

Antibiotics disrupt essential processes or structures in the bacterial cell. This either kills the bacterium or slows down bacterial growth. Depending on these effects an antibiotic is said to be bactericidal (kills the bacterium) or bacteriostatic (lows down bacterial growth). There are five basic mechanisms of antibiotic action against bacterial cells, including:

• Inhibition of Cell Wall Synthesis (most common mechanism)
• Inhibition of Protein Synthesis (Translation) (second largest class)
• Alteration of Cell Membranes
• Inhibition of Nucleic Acid Synthesis
• Antimetabolite Activity

A bactericidal antibiotic kills the bacteria while the bacteriostatic antibiotics stops bacterial growth but does not kill the cells. The human immune system is then needed to clear the infection.

MCQ exam: clinical scenario

In the mechanism of action of antibiotics, what is the specific mechanism of action of the Tetracyclines:

a) Inhibits protein synthesis by binding to a subunit of the bacterial ribosome
b) blocks cross-linking of the cell wall structure
c) inhibits enzymes needed in the biosynthesis of folic acid
d) inhibits bacterial DNA gyrase
e) inhibits DNA-dependent RNA polymerase

MCQ exam: answer

The correct answer is A.
Tetracyclines act by inhibiting protein synthesis.

MCQ exam: explanation

The tetracyclines inhibit protein synthesis by binding to the subunit of the bacterial ribosome (30S subunit).

The tetracyclines are broad-spectrum antibiotics whose value has decreased owing to increasing bacterial resistance. They remain, however, the treatment of choice for infections caused by chlamydia (trachoma, psittacosis, salpingitis, urethritis, and lymphogranuloma venereum), rickettsia (including Q-fever), brucella (doxycycline with either streptomycin or rifampicin), and the spirochaete, Borrelia burgdorferi (See Lyme disease). They are also used in respiratory and genital mycoplasma infections, in acne, in destructive (refractory) periodontal disease, in exacerbations of chronic bronchitis (because of their activity against Haemophilus influenzae), and for leptospirosis in penicillin hypersensitivity (as an alternative to erythromycin).

Tetracyclines have a role in the management of meticillin-resistant Staphylococcus aureus (MRSA) infection.

Reference(s)
1). British National Formulary (BNF): Treatment Summaries - Tetracyclines. Available online: https://bnf.nice.org.uk/treatment-summary/tetracyclines.html
2). Garima Kapoor, Saurabh Saigal, and Ashok Elongavan. Action and resistance mechanisms of antibiotics: A guide for clinicians. J Anaesthesiol Clin Pharmacol. 2017 Jul-Sep; 33(3): 300–305. doi: 10.4103/joacp.JOACP_349_15
3). University of Maryland: Basic Mechanisms of Antibiotic Action and Resistance. Available online: https://science.umd.edu/classroom/bsci424/Chemotherapy/AntibioticMechanisms.htm