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What antibiotic kills a bacterial infection?

Antibiotics are medications used to treat bacterial infections. They work by killing bacteria or preventing them from multiplying. The most effective antibiotic depends on the type of bacteria causing the infection. Doctors prescribe specific antibiotics based on the suspected bacteria, where the infection is located, and severity. Choosing the right antibiotic is critical to successfully treat the infection.

How do antibiotics work?

Antibiotics work in one of two ways:

  • Bactericidal antibiotics kill bacteria directly by interfering with bacterial cell wall production, protein production, or DNA replication. Penicillins, fluoroquinolones, and aminoglycosides are examples of bactericidal antibiotics.
  • Bacteriostatic antibiotics stop bacteria from multiplying by interfering with bacterial protein production. Tetracyclines and sulfa drugs are bacteriostatic antibiotics.

Both bactericidal and bacteriostatic antibiotics are effective at treating infections. Bactericidal antibiotics depend on the immune system to kill the bacteria after the antibiotic damages them. Bacteriostatic antibiotics allow the immune system to kill bacteria while their growth is suppressed.

What types of antibiotics treat common bacterial infections?

The most commonly prescribed antibiotics for common bacterial infections include:

  • Penicillins: Amoxicillin, ampicillin, penicillin. Used to treat strep throat, pneumonia, skin infections, ear infections, and urinary tract infections (UTIs).
  • Cephalosporins: Cefdinir, cefaclor, cefuroxime. Used for sinus infections, ear infections, strep throat, and skin infections.
  • Macrolides: Azithromycin, clarithromycin, erythromycin. Treat strep throat, pneumonia, sinus infections, ear infections, and bronchitis.
  • Tetracyclines: Doxycycline, minocycline, tetracycline. Used for Lyme disease, UTIs, acne, and pneumonia.
  • Fluoroquinolones: Ciprofloxacin, levofloxacin, moxifloxacin. Treat sinus infections, UTIs, skin infections, bronchitis, and pneumonia.
  • Sulfonamides: Bactrim, trimethoprim-sulfamethoxazole. Used for UTIs and sinus infections.

How are antibiotics matched to an infection?

Doctors select antibiotics based on:

  • Most likely causing bacteria
  • Location of infection
  • Severity of infection
  • Patient allergies

Identifying the bacteria causing the infection is key. Common bacteria behind infections include:

  • Streptococcus pneumoniae: Pneumonia, ear infections, sinus infections, meningitis
  • Staphylococcus aureus: Skin infections, pneumonia, bloodstream infections
  • Haemophilus influenzae: Pneumonia, ear infections, sinus infections, meningitis
  • Escherichia coli: UTIs, intra-abdominal infections, pneumonia
  • Klebsiella pneumoniae: Pneumonia, UTIs, wound infections, sepsis
  • Enterococcus faecalis: UTIs, wound infections, endocarditis, bloodstream infections

Doctors may take cultures or rapid tests to identify the bacteria. Otherwise, they choose an antibiotic that treats the most likely culprits.

Common infections and antibiotic choices

Infection Usual bacteria Antibiotic options
Skin infections Staph, Strep Cephalexin, dicloxacillin, clindamycin
UTIs E. coli, Klebsiella, Enterococcus Nitrofurantoin, ciprofloxacin, Bactrim
Strep throat Streptococcus pyogenes Penicillin, amoxicillin, cephalexin
Sinus infection Strep, Haemophilus, Moraxella Amoxicillin, doxycycline, cephalosporins

The infection location also guides treatment. Some antibiotics better penetrate certain tissues. For example, Bactrim treats UTIs well because it concentrates in the kidneys and bladder.

How long should antibiotics be taken?

It is critical to take antibiotics for the full prescribed course, even if feeling better. Stopping early can allow bacteria that have survived to multiply, causing recurrence or antibiotic resistance.

Typical antibiotic course lengths are:

  • Acute infections: 5-7 days
  • Abscessed tooth: 5-7 days
  • Pneumonia: 7-10 days
  • UTI: 3 days for cystitis, 7 days for pyelonephritis
  • Sinus infection: 5-7 days
  • Strep throat: 10 days

Taking antibiotics for shorter or longer than prescribed can be harmful. Always complete the full course as directed.

When are antibiotics ineffective?

While antibiotics are powerful medications, they do not treat every type of infection. Common instances where antibiotics are ineffective include:

  • Viral infections: Antibiotics cannot kill viruses that cause colds, flu, bronchitis, runny noses, or sore throats. However, antibiotics may be prescribed for complications of viral infections, like pneumonia.
  • Some fungal infections: Antibiotics do not treat ringworm, athlete’s foot, or yeast infections. Antifungal medications are required.
  • Parasitic infections: Diseases like malaria, amoebiasis, and giardiasis require anti-parasitic medications, not antibiotics.

Antibiotics also become ineffective if bacteria develop resistance. Overusing and misusing antibiotics contributes to antibiotic resistance.

What are antibiotic side effects?

While antibiotics can be lifesaving, they carry risks of side effects. Common side effects include:

  • Diarrhea or nausea/vomiting
  • Yeast infections
  • Rashes or hives
  • Dizziness
  • Photosensitivity
  • Liver damage
  • Kidney damage
  • Peripheral neuropathy
  • Myelosuppression

Certain populations have increased risk of side effects:

  • Elderly: Tendon damage, C. diff infection
  • Pregnant women: Birth defects, maternal hepatotoxicity
  • Children: Tooth discoloration, bone growth inhibition

Severe allergic reactions like anaphylaxis are possible but very rare. Mild side effects like diarrhea are common. Always weigh risks versus benefits before taking antibiotics.

Antibiotic risks

Antibiotic class Common side effects
Penicillins Diarrhea, rash, nausea, neurotoxicity (high doses)
Cephalosporins Rash, diarrhea, Clostridium difficile infection
Tetracyclines Photosensitivity, nausea, yeast infections
Sulfonamides Rash, nausea, kidney injury, hemolytic anemia
Fluoroquinolones Tendon rupture, CNS effects, photosensitivity, QT prolongation

When should antibiotics not be used?

Doctors should avoid prescribing antibiotics in situations where they offer little benefit, including:

  • Viral respiratory infections – colds, most sinus infections, bronchitis, flu
  • Non-bacterial diarrhea
  • Non-streptococcal sore throats
  • Asymptomatic bacteriuria
  • Colonizing organisms without active infection

Overusing antibiotics promotes antibiotic resistance without benefiting patients. However, for bacterial infections antibiotics significantly reduce symptom duration and complications.


Antibiotics are highly effective medications for treating bacterial infections. Choosing the optimal antibiotic depends on the suspected causative bacteria, location of infection, and patient factors. Completing the full antibiotic course is critical to prevent recurrence and resistance. While antibiotics carry risks, they greatly improve outcomes when used appropriately for bacterial diseases.