An Overview on Antibiotics


Antibiotics can prevent and treat certain infections. It is important that they are only given when necessary, to avoid breeding antibiotic resistance.

There are several types of antibiotics that exist. Read this article to learn more.

Antibiotics are a group of medicines given to manage infections caused by bacteria and some parasites. They are ineffective against other microorganisms such as viruses and fungi. They typically work by either killing the bacteria (bactericidal) or stopping their growth and replication to allow the body to get rid of them (bacteriostatic)

There are various classes of antibiotics, which are prescribed based on the kind of infection and bacteria. They may be administered orally, topically, or parenterally depending on their characteristics and indications for use. 

Misuse and overuse of antibiotics can give rise to antibiotic-resistant bacteria which make infections more difficult, and occasionally, impossible to treat.

The Different Types of Antibiotics

Antibiotics can be classified according to their chemical structures. The main groups of antibiotics include: 

1. Beta-lactams:

– Bactericidal (4 Types)

  • Penicillins: Have five sub-categories with varying properties and bacterial coverage.

Generally well tolerated, with little toxicity.


  • Amoxicillin
  • Ampicillin
  • Cloxacillin
  • Flucloxacillin
  • Amoxicillin/clavulanate
  • Ampicillin/sulbactam
  • Piperacillin/tazobactam
  • Penicillin G
  • and Penicillin V.


  • Cephalosporins: Five generations, with the newer ones targeting a broader range of bacteria. 

Relatively safe, but on rare occasions may trigger seizures and affect blood clotting.


  • 1st generation: Cefazolin, Cephalexin
  • 2nd generation: Cefuroxime, Cefoxitine
  • 3rd generation: Ceftriaxone, Ceftaizidime
  • 4th generation: Cefepime
  • 5th generation: Ceftaroline


  • Carbapenems: Possess the widest spectrum of activity and greatest potency of the beta-lactams. 

To prevent the development of antimicrobial resistance, it is often reserved as the “last line” for more severe infections.

Nephrotoxicity (toxicity to the kidneys), neurotoxicity (toxicity to the nervous system), and immune-related side effects have been reported.  


  • Meropenem
  • Doripenem
  • Ertapenem
  • and Imipenem (usually given with Cilastin which prevents its breakdown)


  • Monobactam: Aztreonam is the only antibiotic in this class. 

It is an alternative option for patients allergic to other beta-lactams. Also given in combination with other antibiotics to target more resistant bacteria. 

Fairly safe with little toxicity.

  • Beta-lactams have differing antibacterial activity spectrums against a wide range of infections.
  • There is potential cross-reactivity amongst most of the beta-lactams. Aztreonam is the only one that may be used in penicillin-allergic patients, but not in those with Ceftazidime allergy.

2. Macrolides

  • Primarily bacteriostatic, but bactericidal in high concentrations and depending on the strain of bacteria.
  • Possible substitute to manage certain infections in penicillin-allergic individuals.
  • Commonly used in the treatment of community-acquired pneumonia, pertussis, and uncomplicated skin infections. 
  • Risk for adverse cardiac-related side effects e.g. QT prolongation (abnormal heart rhythm)
  • Associated with many drug-drug interactions.
  • Examples: Erythromycin, Clarithromycin, Azithromycin


3. Fluoroquinolones (or Quinolones):

  • Bactericidal (concentration-dependent)
  • Broad-spectrum antibiotics are used to treat various infections. 
  • Potential toxicities like abnormal heart rhythm, tendon damage, central nervous system effects (e.g. seizures, hallucinations, anxiety), low blood glucose levels, and photosensitivity (heightened skin sensitivity to ultraviolet radiation which may cause exaggerated sunburn). If severe, patients should stop and avoid all quinolones in the future. 
  • Examples: Ciprofloxacin, Levofloxacin, Moxifloxacin, Norfloxacin, Ofloxacin.


4. Sulfonamides:

  • Bacteriostatic, but exhibit bactericidal activity when combined with the compound trimethoprim. 
  • Frequently utilized for the management of urinary tract infections, pneumocystis pneumonia, and ear infections
  • Rare possible haematological side effects (blood disorders) and renal (kidney) injury.
  • Associated with a greater incidence of allergic reactions because of its sulfonamide structure i.e. “Sulfa allergy”.
  • Example: Sulfamethoxazole/trimethoprim  


5. Glycopeptides:

  • Bactericidal or bacteriostatic depending on the bacterial strain
  • Mainly used against more complicated infections and as a substitute in cases of beta-lactam hypersensitivity.
  • May warrant therapeutic drug monitoring during therapy due to potential nephrotoxicity and ototoxicity (damage to the inner ear causing a hearing and balance impairment).
  • Also associated with a rare infusion reaction, “Red Man Syndrome”, when administered intravenously (via a vein).
  • Example: Vancomycin, Teicoplanin


6. Aminoglycosides:

  • Bactericidal, potency is greater at higher concentrations 
  • Typically reserved for when other antibiotic therapies fail, and can also be administered with other antibiotics to expand coverage
  • Narrow therapeutic index i.e. small differences in blood levels can lead to adverse reactions; requires precise dosing and monitoring of antibiotic concentrations during treatment 
  • Reports of nephrotoxicity and ototoxicity with use. 
  • Examples: Amikacin, Gentamicin, Kanamycin, Neomycin, Streptomycin, Tobramycin


7. Tetracyclines:

  • Bacteriostatic
  • Used for a variety of infections such as acne, urinary/intestinal tract infections, eye infections, sexually transmitted diseases, and gum disease.
  • Can also be used for parasitic infections (e.g., malaria, lice). 
  • Associated with drug-food interactions: Avoid taking products containing aluminium, calcium, magnesium, or iron (e.g., in antacids, and certain supplements) which may impair absorption and efficacy.
  • Potential to cause photosensitivity, tooth discolouration and bony growth retardation, increased brain pressure, teratogenicity (fetal developmental abnormalities), and DRESS (Drug Rash with Eosinophilia and Systemic Symptoms)
  • Examples: Chlortetracycline, Doxycycline, Minocycline, Tetracycline, Tigecycline (tetracycline analogue).


8. Lincosamides:

  • Usually bacteriostatic, but can become bactericidal against highly susceptible organisms and at high lincosamide concentrations
  • Narrow activity spectrum
  • Used to treat more severe infections like pelvic inflammatory disease, intra-abdominal infections, lower respiratory tract infections, bone and joint infections, and acne. 
  • More frequently linked to gastrointestinal effects like diarrhoea and colitis due to Clostridium difficile infection. 
  • Examples: Clindamycin, Lincomycin


9. Nitromidazole:

  • Bactericidal
  • Can be used to manage both parasitic and bacterial infections, particularly those in body areas such as the gums, pelvic cavity, stomach, and intestine. 
  • Cautionary use in patients with the central nervous system or blood disorders. 
  • Examples: Metronidazole, Tinidazole

The above list is not exhaustive and is just some of the more commonly prescribed antibiotics, their classes and characteristics. 

To sum up, there are many different classes of antibiotics available to manage infections. Their use is governed by their properties and spectrum of activity to most effectively treat susceptible infections while minimising harm from side effects. More potent antibiotics as well as the use of various combinations are reserved for more serious infections or against resistant organisms where the benefits outweigh the risks of more serious side effects and toxicity. 

Tailored use of antibiotics for the adequate duration is also important in preventing the development of microbial resistance to ensure these antibiotics remain effective treatments in the future.

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