What organisms can be killed by antibiotics?

Antibiotics are drugs used to treat infections caused by bacteria in humans and animals. They work by killing bacteria or preventing them from reproducing and spreading. Antibiotics have been instrumental in treating many deadly bacterial infections and saving countless lives since their discovery in the early 20th century. However, not all antibiotics work against all types of bacteria, and some organisms have developed resistance to certain antibiotics over time.


Antibiotics are primarily designed to kill bacteria or inhibit their growth. Many different types of bacteria can be susceptible to antibiotics, including both Gram-positive and Gram-negative bacteria. Some examples of bacterial infections that can be treated with antibiotics include:

  • Streptococcus – causes strep throat, pneumonia
  • Staphylococcus aureus – causes skin infections, food poisoning, pneumonia
  • Escherichia coli – causes urinary tract infections, diarrhea
  • Klebsiella – causes pneumonia, urinary tract infections, wound infections
  • Pseudomonas aeruginosa – causes wound, blood, lung, and urinary tract infections
  • Salmonella – causes food poisoning, typhoid fever
  • Clostridium difficile – causes diarrhea and colitis
  • Mycobacterium tuberculosis – causes tuberculosis
  • Neisseria gonorrhoeae – causes gonorrhea

Different classes of antibiotics work against bacteria in various ways:

  • Penicillins (amoxicillin, ampicillin) – inhibit cell wall synthesis
  • Cephalosporins – inhibit cell wall synthesis
  • Tetracyclines (doxycycline, minocycline) – inhibits protein synthesis
  • Fluoroquinolones (ciprofloxacin, levofloxacin) – inhibit DNA synthesis
  • Macrolides (azithromycin, erythromycin) – inhibit protein synthesis
  • Clindamycin – inhibits protein synthesis
  • Trimethoprim-sulfamethoxazole – inhibits folate synthesis
  • Metronidazole – disrupts DNA

Determining which antibiotic is most appropriate depends on the type of bacteria causing the infection, the site of infection, and antibiotic resistance patterns. Bacteria can either be naturally resistant to certain antibiotics or can acquire resistance through genetic mutations over time.


Antibiotics have no effect on viruses. This is because viruses differ from bacteria in many ways:

  • Viruses are much smaller and simpler than bacteria
  • Viruses lack cell walls, cell membranes, or most of the cellular machinery that antibiotics target
  • Viruses replicate through host cells, while bacteria can replicate on their own

Some examples of viral infections that are unaffected by antibiotics include:

  • Common cold
  • Influenza
  • Hepatitis
  • Herpes
  • Measles
  • Mumps
  • Rubella
  • Viral meningitis
  • Viral pneumonia
  • Viral gastroenteritis
  • Chickenpox and shingles

Antibiotics are ineffective against these illnesses. Using antibiotics for viral infections is also counterproductive because it contributes to antibiotic resistance in bacteria while providing no benefit to the patient.


Similar to viruses, antibiotics also cannot kill fungal infections. Fungi differs significantly from bacteria:

  • Fungi have cell walls composed of chitin rather than peptidoglycan
  • Fungi do not have cytoplasmic membranes
  • Fungal cells are eukaryotic rather than prokaryotic

Some examples of fungal infections that do not respond to antibiotics include:

  • Athlete’s foot (tinea pedis)
  • Ringworm (tinea corporis)
  • Candidiasis (thrush)
  • Nail infections
  • Systemic fungal infections like histoplasmosis or coccidioidomycosis

Instead, antifungal medications like fluconazole, nystatin, or amphotericin B are needed to treat fungal infections. Using antibiotics for fungal infections can also contribute to antibiotic resistance.


Parasites are organisms that live on or inside a host organism and derive nutrients from the host. Some examples of parasites include:

  • Protozoa like Giardia, Cryptosporidium, Entamoeba histolytica
  • Helminths like tapeworms, roundworms, hookworms
  • Arthropods like lice, scabies mites

Like fungi and viruses, parasites have very different biology compared to bacteria. As a result, antibiotics have no effect on parasitic infections. Some examples of parasitic diseases that do not respond to antibiotics include:

  • Malaria
  • Amebiasis
  • Giardiasis
  • Ascariasis
  • Hookworm infection
  • Tapeworm infection
  • Pinworm infection
  • Scabies
  • Lice

Instead, anti-parasitic drugs like metronidazole, tinidazole, ivermectin, albendazole or praziquantel are needed to treat parasites. Using antibiotics for parasites can also contribute to antibiotic resistance.


Prions are misfolded proteins that can cause deadly and incurable brain diseases such as Creutzfeldt-Jakob disease in humans, scrapie in sheep, and bovine spongiform encephalopathy in cows.

Unlike bacterial or viral infections, there are currently no treatments that can cure, stop progression, or provide prolonged remission of prion diseases. Antibiotics have no effect on prions because:

  • Prions are simply proteins, lacking any cellular structure for antibiotics to disrupt
  • Prions replicate by inducing normal proteins to misfold, rather than through any metabolic or replicative process antibiotics could disrupt
  • The mechanisms by which prions cause brain damage are not fully understood

While antibiotic resistance is not an issue with prion diseases, they remain untreatable and fatal. Ongoing research seeks to uncover potential treatments for these diseases in the future.

Other Considerations

While antibiotics cannot directly kill viruses, fungi, parasites, or prions, they are sometimes used in certain situations involving these pathogens:

  • Secondary bacterial infections – Viruses like influenza can damage respiratory tissues and allow secondary bacterial pneumonia infections, which antibiotics could treat
  • Immune deficiency – Immunocompromised patients are at greater risk for opportunistic fungal and parasitic infections that may warrant antibiotic prophylaxis
  • Cutaneous anthrax – Antibiotics can treat cutaneous anthrax infections from Bacillus anthracis bacteria that produce anthrax toxin
  • Surgery/invasive procedures – Antibiotics may be given prophylactically before procedures to prevent bacterial infections of surgical wounds

However, antibiotics should only be used for these situations if secondary bacterial infections are likely. Their ability to treat only bacterial infections should always be considered.


Antibiotics are effective for treating a wide range of bacterial infections, but they cannot kill viral, fungal, parasitic infections or prions. This is because these other pathogens differ greatly from bacteria in their biology and in how they cause disease.

Using antibiotics when they are not indicated can contribute to antibiotic resistance without benefiting the patient. However, antibiotics still serve a vital role in treating secondary bacterial infections that may arise from viral illness or invasive medical procedures. Understanding the appropriate uses and limitations of antibiotics helps preserve their effectiveness for the bacterial infections they treat best.

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