A Guide To Common Antibiotics

  • Harry White Master of Science - MS, Biology/Biological Sciences, General, University of Bristol, UK
  • Zayan Siddiqui BSc in Chemistry with Biomedicine, KCL, MSc in Drug Discovery and Pharma Management, UCL

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Introduction to antibiotics

Antibiotics are powerful medications that treat and prevent bacterial infections. Antibiotics were first discovered in 1928 by Sir Alexander Fleming, a Scottish bacteriologist when he first made a groundbreaking discovery as he observed the antibacterial properties of the mould Penicillium notatum. He noticed a mould contaminating one of his Petri dishes, inhibiting Staphylococcus bacteria's growth. This observation led to the identification of penicillin, the first widely used antibiotic.

Understanding antibiotics is crucial for a few reasons:

  • Patients and individuals can make informed decisions and actively participate and collaborate with healthcare providers to ensure the best possible treatment outcome.
  • Knowledge regarding antibiotics is crucial in avoiding unnecessary antibiotic use, as not all illnesses are caused by bacteria. Knowing when antibiotics are and can appropriately be used minimises the risk of side effects, antibiotic resistance, and drug-to-drug interactions.
  • Knowledge and awareness regarding antibiotic use, such as administering the right antibiotics with suitable doses to target specific illnesses and completing the entire course of antibiotics, also help to effectively treat infections and prevent antibiotic resistance.

What everyday readers need to know regarding antibiotics

  • Antibiotics are prescribed medications: Antibiotics should only be taken under the supervision of qualified healthcare professionals. It is crucial to consult a medical professional first who can determine the appropriate medication, dosage, and duration of treatment based on the specific bacterial infection and individual health factors to avoid ineffective treatment and antibiotic resistance. Hence, it is essential to not self-prescribe antibiotics or buy antibiotics as over-the-counter medications.
  • To complete the entire course of antibiotics: It is crucial to finish the prescribed course, even if symptoms improve before completion. Incomplete courses may not fully eradicate the bacteria, potentially leading to a recurrence of the infection.
  • Side effects and allergies: Antibiotics can have side effects like any medication. Common side effects include nausea, diarrhoea, and allergic reactions. It's essential to be aware of potential side effects and report any unusual reactions to a healthcare provider.1 

How do antibiotics work to combat infections?

Antibiotics work using specific mechanisms to combat bacterial infections. It is mainly dependent on the type of antibiotics and the target bacteria. Below are common mechanisms by which antibiotics fight bacterial infections:2

Inhibition of cell wall synthesis 

  • Antibiotics can be effective by inhibiting the synthesis or production of bacteria cell walls. Bacteria cell walls act as structural support and protection. Inhibiting bacterial cell wall synthesis can weaken the bacteria itself, leading to bacterial cell lysis.
  • Examples of this type of antibiotic are penicillin and cephalosporins

Disruption of protein synthesis

  • Bacteria depend on protein synthesis for their vital functions. Hence, disruptions of protein synthesis can disrupt bacterial growth and reproduction.
  • Examples of antibiotics that disrupt protein synthesis are tetracycline and aminoglycosides.

Inhibition of nucleic acid synthesis

  • Bacteria can replicate and spread through DNA or RNA replication. Inhibiting nucleic acid synthesis interferes with DNA and RNA replication and prevents bacterial growth.
  • Examples of such antibiotics are fluoroquinolones

Disruption of cell membrane function

  • Antibiotics can disrupt the integrity of cell membrane function, causing leakage of cellular contents, leading to bacterial death.
  • Antibiotics that can target bacterial cell membranes are polymyxins.

Metabolic pathway interference

  • Once taken, this antibiotic can mimic the essential components for bacterial metabolism, inhibiting crucial enzymes and interfering with bacterial replication and growth.
  • Antibiotics like sulfonamides and trimethoprim can disrupt metabolic pathways.

Common types of antibiotics

Penicillin group

The first antibiotics to be discovered were from the mould genus Penicillium, which mostly works by inhibiting bacterial cell wall synthesis.

Common side effects are nausea, diarrhoea, and allergic reactions that range from mild rashes to life-threatening anaphylaxis.

Common penicillin antibiotics include the following:

Penicillin G (benzylpenicillin): This is used for bacteria from the group of Streptococcus and Staphylococcus species, which are mostly found in infections of the skin, throat, oral cavity, respiratory tract, groin, and axillary region. It is administered through injections.3

  • Penicillin V: This is mainly used for mild to moderate infections such as streptococcal throat infections and it can be administered orally to be used outside of hospital settings.3
  • Amoxicillin: This is a penicillin group with a broader coverage; hence, it is effective for a wider range of bacterial infections, such as ear infections, respiratory infections, and urinary infections.4
  • Ampicillin: This is similar to Amoxicillin but with extended coverage. Hence it could be used to treat various infections, such as bacterial meningitis
  • Penicillinase-resistant penicillins / beta-lactamase-resistant penicillins: Mainly used in cases of penicillinase-resistant bacteria, such as Staphylococcus aureus, that produce beta-lactamase enzymes which inactivate the effectiveness of traditional penicillins.5

Cephalosporin group

Class of broad-spectrum antibiotics that belong to the beta-lactam group of antibiotics (similar to penicillins) that act upon the synthesis of bacterial cell walls.6

Although cephalosporin is generally well tolerated,  common side effects include gastrointestinal symptoms and allergic reactions (individuals allergic to penicillin should also be cautious about cephalosporin antibiotics use), and could worsen kidney conditions in individuals with low and affected kidney functions.7 Cephalosporin is widely used to treat various bacterial infections and is effective against gram-positive and gram-negative bacteria. It is effective for infections of the respiratory tract, urinary tract, skin and soft tissue, and more.

There are five generations of the cephalosporins group:

  • First-generation cephalosporin: Antibiotics such as cefazolin and cephalexin (the most common) are used to treat infections from gram-positive bacteria
  • Second-generation cephalosporin: Antibiotics such as cefuroxime (often used in skin and soft tissue infections) and cefaclor are a broader spectrum of cephalosporin covering gram-positive and some gram-negative bacterial infections
  • Third-generation cephalosporin: Antibiotics such as ceftriaxone, cefotaxime, and ceftazidime have greater activity against gram-negative bacteria. They are effective against a broader range of bacteria, including some respiratory tract infections.
  • Fourth-generation cephalosporin: These antibiotics are an extended spectrum of cephalosporin antibiotics, such as cefepime, used to treat infections from gram-positive and gram-negative bacteria and some severe infections, including meningitis and certain respiratory and urinary infections

Macrolides group

Macrolides are antibiotics effective against a broad spectrum of bacteria, covering gram-positive (e.g. Staphylococcus pneumoniae and Staphylococcus aureus) and some gram-negative bacteria (e.g. Haemophilus influenzae and Moraxella catarrhalis) by inhibiting protein synthesis within the bacteria.8 Hence, macrolides are often used for respiratory tract infections, including community-acquired pneumonia, bronchitis, and sinusitis; soft tissue and skin infections, including cellulitis; and sexually transmitted infections, such as chlamydia.

Common side effects of macrolides antibiotics are nausea, diarrhoea, and allergic reactions as well as interaction with other medications.

Common macrolide antibiotics include:

  • Erythromycin: These are effective against various bacterial infections, including respiratory infections.
  • Azithromycin: These are usually used in respiratory, skin, and sexually transmitted infections.
  • Clarithromycin: These are used chiefly for Helicobacter pylori gastrointestinal infections.9

Tetracycline group

This is a class of broad-spectrum antibiotics with a wide coverage of various bacterial infections, covering both gram-positive and gram-negative bacteria. It works through the inhibition of protein synthesis, which consequently inhibits bacterial growth. Tetracycline can be used for respiratory, urinary tract, skin, and sexually transmitted infections.

While taking tetracycline, one should not take it with certain foods such as dairy products as it can interfere with the absorption of the antibiotics.10 It also needs to be avoided in pregnant women and children less than 8 years old. Side effects of tetracycline include interactions with other medications, photosensitivity (especially with Doxycycline), tooth discolouration, and enamel hypoplasia.11

Common tetracycline antibiotics include:

  • Tetracycline: Commonly used in respiratory infections such as bronchitis and pneumonia.
  • Doxycycline:  A tetracycline derivative with better bioavailability (the rate at which the active drug is absorbed and is available at the site of action). It is used mainly for respiratory tract infections, skin infections such as acne, and some sexually transmitted infections such as chlamydia and syphilis. Doxycycline is also commonly used to treat Lyme disease and as prophylaxis for malarial infections.
  • Minocycline: Often used for respiratory, acne, and skin infections.

Fluoroquinolones group

Fluoroquinolones are synthetic antibiotics that target bacterial DNA replication, inhibiting bacterial growth. Fluoroquinolones are effective on gram-positive and gram-negative bacteria and are commonly used for respiratory, urinary, skin and soft tissue infections, and certain gastrointestinal infections.

Side effects of fluoroquinolones include gastrointestinal disturbances, photosensitivity, central nervous system effects, tendonitis, and tendon rupture.12 It should also be used cautiously in pregnant women and children due to its potential adverse effects on developing cartilage. It should also not be taken unnecessarily to prevent antibiotic resistance.

Common fluoroquinolone antibiotics include:


Antibiotics are helpful and powerful medications used to treat bacterial infections. Knowledge regarding antibiotics is crucial for informed decisions, avoiding microbial resistance, and ensuring effective treatment. Antibiotics are not to be taken lightly; they have to be prescribed by a healthcare practitioner before being administered. Once taken, its course needs to be completed fully.

Each group of antibiotics combat bacterial infections in specific ways. For example, penicillins (e.g. amoxicillin) and cephalosporins (e.g. ceftriaxone) act on the synthesis of bacterial cell walls; macrolides (e.g. azithromycin) and tetracyclines (e.g. doxycycline) inhibit protein synthesis, while fluoroquinolones (e.g. ciprofloxacin) hinder DNA replication.

Understanding antibiotics empowers individuals to collaborate with healthcare providers, promoting effective treatment and reducing the risk of antibiotic resistance.


  1. Mohsen S, Dickinson JA, Somayaji R. Update on the adverse effects of antimicrobial therapies in community practice. Can Fam Physician [Internet]. 2020 Sep [cited 2023 Nov 19];66(9):651–9. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7491661/
  2. O’Rourke A, Beyhan S, Choi Y, Morales P, Chan AP, Espinoza JL, et al. Mechanism-of-action classification of antibiotics by global transcriptome profiling. Antimicrob Agents Chemother [Internet]. 2020 Feb 21 [cited 2023 Nov 19];64(3):e01207-19. Available from: https://journals.asm.org/doi/10.1128/AAC.01207-19
  3. Yip DW, Gerriets V. Penicillin. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 [cited 2023 Nov 19]. Available from: http://www.ncbi.nlm.nih.gov/books/NBK554560/
  4. Akhavan BJ, Khanna NR, Vijhani P. Amoxicillin. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 [cited 2023 Nov 19]. Available from: http://www.ncbi.nlm.nih.gov/books/NBK482250/
  5. Pandey N, Cascella M. Beta-lactam antibiotics. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 [cited 2023 Nov 19]. Available from: http://www.ncbi.nlm.nih.gov/books/NBK545311/
  6. Bui T, Preuss CV. Cephalosporins. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 [cited 2023 Nov 19]. Available from: http://www.ncbi.nlm.nih.gov/books/NBK551517/
  7. Barza M. The nephrotoxicity of cephalosporins: an overview. Journal of Infectious Diseases [Internet]. 1978 May 1 [cited 2023 Nov 19];137(Supplement):S60–73. Available from: https://academic.oup.com/jid/article-lookup/doi/10.1093/infdis/137.Supplement.S60
  8. Patel PH, Hashmi MF. Macrolides. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 [cited 2023 Nov 19]. Available from: http://www.ncbi.nlm.nih.gov/books/NBK551495/
  9. Leung WK, Graham DY. Clarithromycin for Helicobacter pylori infection. Expert Opinion on Pharmacotherapy [Internet]. 2000 Mar [cited 2023 Nov 19];1(3):507–14. Available from: http://www.tandfonline.com/doi/full/10.1517/14656566.1.3.507
  10. Neuvonen PJ. Interactions with the absorption of tetracyclines: Drugs [Internet]. 1976 [cited 2023 Nov 19];11(1):45–54. Available from: http://link.springer.com/10.2165/00003495-197611010-00004
  11. Matis BA, Wang Y, Eckert GJ, Cochran MA, Jiang T. Extended bleaching of tetracycline-stained teeth: a 5-year study. Operative Dentistry [Internet]. 2006 Nov 1 [cited 2023 Nov 19];31(6):643–51. Available from: https://meridian.allenpress.com/operative-dentistry/article/31/6/643/106943/Extended-Bleaching-of-TetracyclineStained-Teeth-A
  12. Van Der Linden, Van De Lei, Nab, Knol, Stricker. Achilles tendinitis associated with fluoroquinolones. Brit J Clinical Pharma [Internet]. 1999 Sep [cited 2023 Nov 19];48(3):433–7. Available from: https://bpspubs.onlinelibrary.wiley.com/doi/10.1046/j.1365-2125.1999.00016.x

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Dr. Izyani Hashim is a Malaysian medical doctor with several years of experience and passion for palliative and primary care and health communications. Having completed her medical degree in Russia and served as a medical doctor in Malaysia, rotating into different fields of medicine, she has gained invaluable experience in clinical care and patient interactions, enabling her to bridge the gap between complex medical information and everyday readers.

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