Antibiotics
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Antibiotics, also known as antibacterials, are medications designed to combat bacterial infections by either killing bacteria (bactericidal) or inhibiting their growth (bacteriostatic). Their application is a vital component of modern medicine, particularly in dentistry, where they are frequently used to prevent and treat oral infections.
BACTERIOSTATIC VS BACTERICIDAL
Bacteriostatic antibiotics act by inhibiting bacterial growth and reproduction, allowing the host’s immune system to eliminate the pathogen. They are often used in patients with competent immune systems. Examples include:
- Tetracyclines
- Macrolides
- Sulfonamides.
Bactericidal Antibiotics act by directly killing bacteria by targeting essential bacterial structures or processes. They are ideal for patients with weakened immune systems or severe infections. Examples include
- Penicillins
- Cephalosporins
- Fluoroquinolones.
Bacteriostatic and bactericidal drugs should not be used together, as they can antagonize each other’s effects. For example, a bacteriostatic drug slows bacterial growth, which reduces the efficacy of a bactericidal drug that targets actively dividing bacteria. Antibiotics are grouped into classes based on their mechanisms of action and molecular targets within bacterial cells.
Sulfonamides (Folate Synthesis Inhibitors) act by inhibiting the bacterial enzyme dihydropteroate synthase, which is necessary for the synthesis of folate (a vital nutrient for DNA synthesis). These drugs mimic para-aminobenzoic acid (PABA), a natural substrate for the enzyme, thereby competitively inhibiting folate production effective against urinary tract infections and some respiratory infections. They are often combined with trimethoprim (e.g., in the drug Bactrim) for synergistic effects and are Examples include:
- Sulfamethoxazole
- Sulfadiazine
Penicillins and Cephalosporins (Cell Wall Synthesis Inhibitors) inhibit transpeptidase, an enzyme required for bacterial cell wall synthesis. This weakens the bacterial cell wall, causing it to rupture and leading to bacterial death. Penicillins contain a beta-lactam ring, which is crucial for their activity.
Examples include:
- Penicillin G and Penicillin V: Narrow-spectrum, effective against gram-positive bacteria.
- Amoxicillin: Broad-spectrum, commonly used in dentistry.
- Augmentin: A combination of amoxicillin and clavulanic acid (a beta-lactamase inhibitor).
Cephalosporins are structurally similar to penicillins, with a beta-lactam ring, and are divided into generations (1st to 5th), with increasing activity against gram-negative bacteria. Example include Cephalexin (1st generation). Penicillins and cephalosporins are cross-allergenic, meaning patients allergic to penicillin may also react to cephalosporins.
Tetracyclines (Protein Synthesis Inhibitors) bind to the 30S ribosomal subunit in bacteria, preventing the attachment of transfer RNA (tRNA). This halts bacterial protein synthesis and inhibits growth. They are frequently used in the treatment of periodontal infections due to their ability to concentrate in the gingival crevicular fluid, and are effective against actinomycetes and spirochetes in periodontal disease. Tetracyclines bind to calcium, leading to tooth discoloration and enamel hypoplasia. They should not be used in children under 8 years old or pregnant patients. Examples include:
- Tetracycline
- Doxycycline
- Minocycline
Macrolides bind to the 50S ribosomal subunit, inhibiting bacterial protein elongation. They are a common alternative for patients allergic to penicillin and are effective against gram-positive bacteria and some gram-negative species. Macrolides can inhibit cytochrome P450 enzymes, leading to drug interactions (e.g., with warfarin). Examples include:
- Erythromycin
- Clarithromycin
- Azithromycin
DRUG INTERACTIONS AND ADVERSE EVENTS
Antibiotics are powerful tools for treating bacterial infections, but their use is not without risks. Many antibiotics can interact with other medications, leading to altered drug efficacy or increased toxicity. Additionally, antibiotics can cause a range of adverse effects, from mild gastrointestinal upset to severe allergic reactions.
One of the most well-known drug interactions involves antibiotics and oral contraceptives. Broad-spectrum antibiotics, such as ampicillin and tetracyclines, can disrupt the intestinal flora responsible for enterohepatic recirculation of active estrogen metabolites. This interference can reduce the effectiveness of oral contraceptives, potentially leading to unintended pregnancies. While the risk is not significant with all antibiotics, it is a key consideration in patient counseling. Women using oral contraceptives should be advised to use additional contraceptive methods, such as condoms, while taking antibiotics that could interfere with their efficacy.
The simultaneous use of bacteriostatic and bactericidal antibiotics should generally be avoided because their mechanisms of action can antagonize each other. Bacteriostatic drugs, such as tetracyclines and macrolides, inhibit bacterial growth by halting protein synthesis. Bactericidal drugs, such as penicillins and cephalosporins, kill bacteria by targeting actively dividing cells, typically through disruption of cell wall synthesis. When these two types of antibiotics are used together, the bacteriostatic drug slows bacterial growth, reducing the effectiveness of the bactericidal drug, which relies on actively dividing bacteria to exert its effects. For example, using tetracycline alongside penicillin can compromise the efficacy of both drugs, particularly in severe infections where rapid bacterial eradication is critical.
Many antibiotics, including amoxicillin and macrolides, can cause gastrointestinal (GI) symptoms such as nausea, vomiting, diarrhea, and abdominal pain. These effects are often mild and self-limiting but can be bothersome for patients. Patients can be advised to take antibiotics with food (if appropriate) or use probiotics to minimize GI upset.
Broad-spectrum antibiotics, such as tetracyclines and fluoroquinolones, can lead to superinfections by disrupting the body’s normal microbial flora. This disruption creates an environment where opportunistic pathogens, such as Candida albicans or Clostridioides difficile (C. diff), can overgrow. Candida albicans overgrowth can result in oral or vaginal candidiasis (thrush). Clostridioides difficile: Broad-spectrum antibiotics can suppress normal gut flora, allowing C. diff to proliferate, leading to pseudomembranous colitis, a severe and potentially life-threatening condition.
Allergic reactions to antibiotics are common and can range from mild rashes to severe anaphylaxis.
Beta-lactams (Penicillins and Cephalosporins): These are the most common culprits for allergic reactions. Cross-reactivity between penicillins and cephalosporins occurs in approximately 10% of allergic patients.
Patients with a history of severe allergic reactions to penicillins should avoid cephalosporins and be prescribed alternatives, such as clindamycin or macrolides.
Tetracyclines can cause tooth discoloration and enamel hypoplasia in children under 8 years old or in fetuses when taken during pregnancy. Fluoroquinolones can cause tendonitis and increase the risk of tendon rupture, particularly in older adults. Sulfonamides are associated with Stevens-Johnson Syndrome (SJS), a rare but serious hypersensitivity reaction involving severe skin blistering and mucosal involvement.
