Chapter 29 Bacterial Infection_4dcbc1d82f74174579e88ef4420bc73f

Introduction

Objectives:

• Understand terminology associated with treatments for infection.

• Differentiate between bacteriostatic and bactericidal.

• Describe the morphology of bacterial cells.

• Explain microbial resistance and factors leading to its development.

Key Terminology

• Antibiotic: A substance from microorganisms that kills or inhibits other microorganisms (e.g., penicillin).

• Antimicrobial: Destroys or inhibits growth of microorganisms.

• Bactericidal: Destroys bacteria.

• Bacteriostatic: Inhibits bacterial growth; relies on host defenses for bacteria elimination.

• Broad-spectrum antibiotic: Effective against a large variety of bacteria.

• Microbial resistance: Ability of bacteria to resist effects of anti-infectives; resistance genes can be shared among bacteria.

Overview of Infectious Diseases

Impact: Leading cause of morbidity and mortality worldwide.Risk Factors: Include poverty, malnutrition, lack of clean water, poor sanitation, and inadequate housing.Infant Mortality: Infectious diarrhea is a significant cause in developing countries.

Mechanisms of Antimicrobial Action

• Culture and Sensitivity Testing: Identifies effective antibiotics for specific bacterial infections.

• Targeting Bacterial Structures: Inhibit bacterial cell wall synthesis, vital for bacterial survival, without harming human cells. Examples include penicillins, cephalosporins.

Types of Antimicrobial Agents

Classes:

1. Aminoglycosides:

   • Used primarily against Gram-negative bacteria.

   • Examples include gentamicin, amikacin, and tobramycin.

   • Mechanism of action: Inhibit protein synthesis by binding to ribosomal RNA, causing misreading of mRNA.

2. Cephalosporins:

   • Divided into generations, each with varying spectrum of activity.

   • First Generation: Effective against Gram-positive bacteria (e.g., cephalexin, cefazolin).

   • Second Generation: Broader spectrum, including some Gram-negative coverage (e.g., cefuroxime, cefoxitin).

   • Third Generation: Enhanced activity against resistant Gram-negative bacteria (e.g., ceftriaxone, ceftazidime).

   • Fourth Generation: Broad-spectrum, including both Gram-positive and Gram-negative organisms (e.g., cefepime).

3. Fluoroquinolones:

   • Broad-spectrum antibiotics that affect DNA replication.

   • Examples include ciprofloxacin, levofloxacin, and moxifloxacin.

   • Mechanism of action: Inhibit DNA gyrase, preventing bacterial cell division.

4. Macrolides:

   • Effective against Gram-positive and some Gram-negative bacteria.

   • Examples include azithromycin, erythromycin, and clarithromycin.

   • Mechanism of action: Inhibit protein synthesis by binding to the 50S ribosomal subunit.

5. Oxazolidinones:

   • Used to treat infections caused by multidrug-resistant Gram-positive bacteria.

   • Linezolid is a common example.

   • Mechanism of action: Inhibit the initiation of protein synthesis.

6. Penicillins and Carbapenems:

   • Penicillins (e.g., amoxicillin, ampicillin, piperacillin) are effective primarily against Gram-positive bacteria.

   • Carbapenems (e.g., meropenem, ertapenem, imipenem) have a broad spectrum, including both Gram-positive and Gram-negative organisms.

   • Mechanism of action for both: Inhibit cell wall synthesis by preventing cross-linking of peptidoglycan layers.

7. Sulfonamides:

   • Inhibit the bacterial synthesis of folic acid, essential for nucleic acid synthesis.

   • Examples include sulfamethoxazole (often combined with trimethoprim in cotrimoxazole).

8. Tetracyclines:

   • Broad-spectrum antibiotics effective against a wide range of bacteria, often used for acne and respiratory infections.

   • Examples include tetracycline, doxycycline, and minocycline.

   • Mechanism of action: Inhibit protein synthesis by binding to the 30S ribosomal subunit.

Specific Antibiotic Mechanisms

• Inhibit Cell Wall Synthesis:

  • β-lactams: Penicillins and cephalosporins prevent cross-linking in cell walls.

  • Glycopeptides: Such as vancomycin, also target cell walls.

• Inhibit Protein Synthesis:

  • Aminoglycosides bind to ribosomes, causing translation errors.

• DNA/RNA Synthesis Inhibition:

  • Fluoroquinolones interfere with enzyme DNA gyrase; vital for cell replication.

  • Antifolates block folic acid synthesis, essential for bacterial growth.

Adverse Reactions and Precautions

• Common side effects include gastrointestinal disturbances, dizziness, and others specific to drug classes.

• Warnings: Complete therapy, avoid sunlight, monitor for severe reactions, and take as prescribed.

Conclusion

Understanding antimicrobials and their mechanisms helps inform effective treatment for bacterial infections, addressing both efficacy and resistance concerns.