Antibiotic Therapy – Effects, Uses and Resistance

What Is Antibiotic Therapy?

Antibiotic therapy refers to the use of antibiotics to treat infectious diseases caused by bacteria. Antibiotics are substances that either inhibit the growth of bacteria (bacteriostatic) or kill them directly (bactericidal). They represent one of the most significant advances in modern medicine and have fundamentally transformed the treatment of once life-threatening infections.

It is important to understand that antibiotics are effective only against bacterial pathogens. They have no effect on viral infections such as the common cold, influenza, or COVID-19 and should not be used in these cases.

When Is Antibiotic Therapy Used?

Antibiotic therapy is prescribed when a bacterial infection is confirmed or reasonably suspected. Common indications include:

• Respiratory tract infections: e.g., bacterial pneumonia, strep throat (scarlet fever), bacterial bronchitis
• Urinary tract infections: e.g., cystitis (bladder infection), pyelonephritis (kidney infection)
• Skin infections: e.g., wound infections, erysipelas, deep tissue infections (cellulitis)
• Sexually transmitted infections: e.g., chlamydia, gonorrhea, syphilis
• Gastrointestinal infections: e.g., certain bacterial gastroenteritis, Helicobacter pylori infection
• Perioperative prophylaxis: prevention of infections before and after surgical procedures

Mechanisms of Action

Different classes of antibiotics target specific structures or processes within bacterial cells:

• Inhibition of cell wall synthesis: Penicillins, cephalosporins, carbapenems, and glycopeptides (e.g., vancomycin) disrupt the construction of the bacterial cell wall, causing the bacteria to lyse and die.
• Inhibition of protein synthesis: Macrolides, tetracyclines, aminoglycosides, and lincosamides prevent the production of essential proteins within the bacterial cell.
• Inhibition of DNA synthesis: Fluoroquinolones (e.g., ciprofloxacin) block enzymes required for the replication of bacterial DNA.
• Inhibition of folate metabolism: Sulfonamides and trimethoprim interfere with the folate metabolism that bacteria need to grow.
• Disruption of the cell membrane: Polymyxins destabilize the outer membrane of gram-negative bacteria.

Major Antibiotic Classes at a Glance

Beta-Lactam Antibiotics

This group includes penicillins, cephalosporins, carbapenems, and monobactams. They have a broad spectrum of activity and are generally well tolerated, but can cause hypersensitivity reactions in patients with known penicillin allergy.

Macrolides

Drugs such as azithromycin and clarithromycin are frequently used for respiratory tract infections and as an alternative in patients with penicillin intolerance.

Fluoroquinolones

This class (e.g., ciprofloxacin, levofloxacin) is used primarily for urinary tract infections, respiratory infections, and gram-negative bacteria. Due to side effects affecting tendons and the nervous system, their use is increasingly restricted.

Tetracyclines

Doxycycline and other tetracyclines have a broad spectrum of activity and are used for conditions such as chlamydia infection, Lyme disease (caused by Borrelia), or acne vulgaris.

Dosage and Duration of Treatment

The dosage and duration of antibiotic therapy depend on the pathogen, the site of infection, the severity of the illness, and the general health of the patient. Key principles include:

• Antibiotics should always be taken for the full recommended course, even if symptoms improve early.
• Stopping treatment prematurely may allow surviving bacteria to develop resistance.
• The prescribed daily dose and dosing interval must be followed strictly to maintain effective drug levels in the blood.

Side Effects

Like all medications, antibiotics can cause side effects. Common adverse effects include:

• Gastrointestinal complaints: Nausea, diarrhea, abdominal pain, or vomiting are particularly common.
• Allergic reactions: These range from skin rashes to severe anaphylactic reactions, especially with penicillins.
• Disruption of gut flora: Antibiotics do not only destroy pathogens but also beneficial gut bacteria, which can lead to fungal infections (e.g., Candida) or infection with Clostridioides difficile.
• Photosensitivity: Increased sensitivity of the skin to sunlight can occur, particularly with tetracyclines and fluoroquinolones.
• Kidney and liver toxicity: With certain antibiotics (e.g., aminoglycosides), kidney function must be monitored closely.

Antibiotic Resistance

Antibiotic resistance is one of the greatest challenges facing modern medicine. It develops when bacteria evolve mechanisms that make them insensitive to one or more antibiotics. Key drivers of resistance include:

• Overuse and misuse of antibiotics in human and veterinary medicine
• Incomplete antibiotic courses
• Self-medication without a prescription
• Inadequate hygiene measures in healthcare settings

Resistant pathogens such as MRSA (methicillin-resistant Staphylococcus aureus) and multidrug-resistant gram-negative bacteria pose a serious threat to public health. The World Health Organization (WHO) classifies antibiotic resistance as one of the most urgent global health threats.

Rational Antibiotic Use and Antibiotic Stewardship

To preserve the long-term effectiveness of antibiotics, health authorities worldwide promote the concept of antibiotic stewardship. This refers to coordinated programs and strategies that ensure antibiotics are only used when truly necessary, and that the most appropriate antibiotic is prescribed at the correct dose and for the appropriate duration.

Advice for Patients

• Take antibiotics only when prescribed by a doctor.
• Do not stop the course of treatment early, even if you feel better.
• Do not share leftover antibiotics with others.
• Inform your doctor about known allergies or other medications you are taking.
• If you experience severe side effects, seek medical attention promptly.

References

1. World Health Organization (WHO): Antimicrobial Resistance. Global Action Plan on Antimicrobial Resistance. Geneva: WHO, 2015. Available at: https://www.who.int/publications/i/item/9789241509763
2. European Centre for Disease Prevention and Control (ECDC): Antimicrobial resistance in the EU/EEA (EARS-Net) - Annual Epidemiological Report 2023. Stockholm: ECDC, 2023.
3. Mandell GL, Bennett JE, Dolin R (eds.): Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. 9th edition. Philadelphia: Elsevier, 2020.