Antibacterial Agents: Action and Resistance 📚

Source Information: This study material has been compiled from a lecture audio transcript, personal notes, and PDF/PowerPoint texts provided by Prof. Dr. Rıza Durmaz (dated March 4, 2026, and March 12, 2019).

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1. Introduction to Antimicrobial Agents 🌍

Antimicrobial agents are chemical substances designed to kill or inhibit the growth of microorganisms. This broad category encompasses various types, including:

• Antibacterial agents (antibiotics and other antibacterials)
• Antifungal agents
• Antiparasitic agents
• Antiviral agents

A key distinction within antibacterial agents is their mode of action:

• Bactericidal agents: Directly kill bacteria. ✅
• Bacteriostatic agents: Inhibit bacterial growth, allowing the host's immune system to clear the infection. ✅

Antibiotics are a specific type of antimicrobial agent. They are substances produced by microorganisms that inhibit the growth of other microorganisms (generally bacteria) at low concentrations.

💡 Important Note: All antimicrobial drugs are chemotherapeutic agents, but not all chemotherapeutic agents are antimicrobial agents. Chemotherapeutic agents are broadly defined as drugs used to treat diseases, including cancer.

2. Core Concepts in Antibacterial Therapy 📊

2.1. Resistance vs. Susceptibility

• Susceptible bacteria: Their growth is inhibited at achievable, non-toxic drug levels.
• Resistant strains: Their growth is not inhibited at achievable, non-toxic drug levels, meaning the antibiotic cannot be effectively used for treatment.

2.2. Quantifying Antibiotic Activity: MIC & MBC

To assess an antibiotic's effectiveness, two critical concentrations are measured:

• 📚 Minimum Inhibitory Concentration (MIC):

  • The lowest concentration of an antibiotic needed to inhibit the visible growth of an organism.
  • Example: If bacterial growth is observed at 2 µg/ml but not at 4 µg/ml, the MIC is 2 µg/ml. This is often determined using "MIC tubes" where different antibiotic concentrations are tested.

• 📚 Minimum Bactericidal Concentration (MBC):

  • The lowest concentration of an antibiotic that kills 99.9% of the bacterial population.
  • To determine MBC, sub-cultures are performed from tubes showing no visible growth (e.g., from the MIC test). If no growth is observed after sub-culturing from the 4 µg/ml tube, then the MBC is 4 µg/ml.

2.3. Antibiotic Spectrum of Activity

Antibiotics are categorized by the range of microorganisms they affect:

• Narrow-spectrum: Effective against a limited range of microbes.
• Broad-spectrum: Effective against a wider range of microbes. ⚠️ No single antibiotic is effective against all microbes.

2.4. Antibiotic Combinations

Sometimes, antibiotics are used in combination to:

1. Broaden the antibacterial spectrum: Useful for empirical therapy (treatment before specific pathogen identification) or polymicrobial infections.
2. Prevent emergence of resistant organisms: By targeting multiple pathways, it's harder for bacteria to develop resistance to all drugs simultaneously.
3. Achieve a synergistic killing effect:
   • Antibiotic Synergism: Combinations of two antibiotics show enhanced bactericidal activity compared to each drug alone.
   • Additive Effect: The combined potency is roughly equal to the sum of individual potencies.
   • Antibiotic Antagonism: The activity of one antibiotic interferes with the activity of the other, resulting in a less effective combination than the most active individual drug.

2.5. Selective Toxicity and Therapeutic Index 📈

• 📚 Selective Toxicity: A fundamental principle meaning an antibiotic must be highly effective against the microbe but have minimal or no toxicity to human cells. This is possible because bacterial cells have unique structures (like cell walls) or metabolic pathways not found in humans.
• 📚 Therapeutic Index (TI): The ratio of the toxic dose (to the patient) to the therapeutic dose (to eliminate the infection).
  • TI = Toxic Dose / Therapeutic Dose
  • A larger TI indicates a safer drug for human use. We must use antibiotics at concentrations within their therapeutic index.

2.6. Characteristics of a Clinically Useful Antibiotic 💡

An ideal antibiotic should possess as many of these characteristics as possible:

• Wide spectrum of activity.
• Nontoxic to the host and without undesirable side effects.
• Nonallergenic to the host.
• Should not eliminate the normal flora of the host.
• Able to reach the site of infection in the human body.
• Inexpensive and easy to produce.
• Chemically stable (long shelf-life).
• Microbial resistance should not easily develop against it.

3. Basic Mechanisms of Antibiotic Activity (Detailed Focus) 🔬

Antibiotics target essential bacterial processes, leading to their inhibition or death.

3.1. Inhibition of Cell Wall Synthesis 🧱

The bacterial cell wall, made of peptidoglycan, is crucial for structural integrity and is absent in human cells, making it an excellent target.

3.1.1. Beta-Lactam Antibiotics

• Types: Penicillins, Cephalosporins, Carbapenems, Monobactams, Cephamycins. All share a common beta-lactam ring structure.
• Mechanism of Action:
  1. The bacterial cell wall's backbone consists of N-acetylglucosamine (NAGA) and N-acetylmuramic acid (NAMA) chains.
  2. These chains are cross-linked by peptide bridges (e.g., pentaglycine bound) in the peptidoglycan layer.
  3. This cross-linking is catalyzed by specific enzymes called Penicillin-Binding Proteins (PBPs), which include DD-transpeptidases and DD-carboxypeptidases. PBPs are responsible for the final stages of cell wall assembly.
  4. Beta-lactam antibiotics bind to these PBPs, mimicking the natural substrate. This binding irreversibly inhibits the formation of cross-links between peptidoglycan layers.
  5. This leads to a weakened cell wall, osmotic lysis, and a bactericidal effect on growing bacteria.
• Generations of Cephalosporins:
  • First-generation (e.g., Cefazolin, Cephalexin): Narrow-spectrum, primarily active against Gram-positive organisms and some Enterobacteriaceae.
  • Second-generation (e.g., Cefoxitin, Cefaclor): Expanded-spectrum, more resistant to beta-lactamases of some Gram-negative organisms, with increased activity against Enterobacteriaceae.
  • Third-generation (e.g., Ceftriaxone, Cefotaxime, Ceftazidime): Broad-spectrum, highly active against Gram-negative organisms, but may still be ineffective against some multi-drug resistant Gram-negatives.
  • Fourth-generation (e.g., Cefepime): Extended-spectrum, enhanced ability to cross Gram-negative outer membranes, resistant to many Gram-negative beta-lactamases, active against P. aeruginosa.
  • Fifth-generation (e.g., Ceftolozane): Designed to kill highly drug-resistant Gram-negative bacteria, including P. aeruginosa.
• Carbapenems (e.g., Imipenem, Meropenem): Broadest spectrum of all beta-lactams due to easy penetration of bacterial cells and high resistance to beta-lactamases.
• Monobactams (e.g., Aztreonam): Primarily active against Gram-negative bacteria, highly resistant to Gram-negative beta-lactamases, but poor affinity for PBPs of Gram-positive organisms and strict anaerobes.
• Cephamycins (e.g., Cefoxitin): Closely related to cephalosporins, more stable to beta-lactamase hydrolysis, effective against anaerobic microbes and ESBL-producing organisms.
• Resistance to Beta-Lactam Antibiotics:
  1. Production of Beta-Lactamases: Enzymes that hydrolyze the beta-lactam ring, inactivating the antibiotic. These can be specific (penicillinase, cephalosporinase) or broad-spectrum (Extended-Spectrum Beta-Lactamases - ESBLs), which are often plasmid-encoded and transferable.
  2. Changes in Porin Proteins: Alterations in the size or charge of outer membrane channels (especially in Gram-negative bacteria like Pseudomonas species) can exclude beta-lactams from entering the cell.
  3. Modification of PBPs:
     • Acquisition of a new PBP (e.g., methicillin resistance in Staphylococcus aureus via mecA gene).
     • Modification of existing PBPs through recombination or point mutations (e.g., penicillin resistance in Streptococcus pneumoniae).
• Beta-Lactam/Beta-Lactamase Inhibitor Combinations: To overcome beta-lactamase resistance, inhibitors (e.g., clavulanic acid, sulbactam, tazobactam, avibactam) are combined with beta-lactams. These inhibitors bind to beta-lactamases, preventing them from inactivating the beta-lactam antibiotic, thereby restoring its activity.

3.1.2. Glycopeptides (e.g., Vancomycin)

• Mechanism of Action: Disrupts peptidoglycan synthesis in growing Gram-positive bacteria by binding to the D-alanine-D-alanine termini of the pentapeptide side chains. This binding prevents the formation of cross-linkages.
• Spectrum: Primarily used for Gram-positive bacteria, especially oxacillin-resistant staphylococci and other beta-lactam-resistant Gram-positives.
• Resistance to Vancomycin:
  1. Intrinsic Resistance (Gram-negative bacteria): Vancomycin's large molecule cannot pass through the outer membrane of Gram-negative bacteria.
  2. Acquired Resistance (e.g., Vancomycin-Resistant Enterococci - VRE): Bacteria modify the D-alanine-D-alanine terminus to D-alanine-D-lactate or D-alanine-D-serine, which significantly reduces vancomycin's binding affinity. This resistance can be plasmid-mediated and transferred, even to staphylococci.

3.1.3. Polypeptides (e.g., Bacitracin)

• Mechanism of Action: Inhibits cell wall synthesis by interfering with the movement of peptidoglycan precursors through the cytoplasmic membrane.
• Spectrum: Active against staphylococci (Gram-positive).
• Resistance: The antibiotic cannot penetrate into the bacterial cell.

3.1.4. Antibiotics for Mycobacterial Infections (e.g., Isoniazid, Ethionamide, Ethambutol, Cycloserine)

• Mechanism of Action: These antibiotics interfere with cell wall synthesis by blocking the synthesis of mycolic acid or arabinogalactan, which are unique components of the mycobacterial cell wall.
• Specificity: These are specific to mycobacteria and cannot be used to treat Gram-positive or Gram-negative bacterial infections because those bacteria lack mycolic acid and arabinogalactan.
• Resistance: Reduced drug uptake into the bacterial cell or alteration of the target sites.

3.2. Inhibition of Cell Membrane Function 🛡️

These agents disrupt the selective permeability of the bacterial cell membrane, leading to leakage and cell death.

3.2.1. Polymyxins (e.g., Colistin, Polymyxin B)

• Mechanism of Action: Cationic detergent-like effect. They bind to lipopolysaccharides (LPS) and phospholipids in the outer membrane of Gram-negative bacteria. This binding disrupts both the outer and inner membranes, increasing cell permeability and causing cell death.
• Spectrum: Bactericidal for many Gram-negative aerobic rods.

3.2.2. Daptomycin

• Mechanism of Action: Binds irreversibly to the cytoplasmic membrane, causing depolarization and disruption of ionic gradients, which leads to cell death.
• Spectrum: Potent activity against Gram-positive bacteria, including multidrug-resistant staphylococci, streptococci, and vancomycin-resistant enterococci.
• Resistance (Gram-negative bacteria): Gram-negative bacteria are resistant because the drug cannot penetrate their outer cell wall to reach the cytoplasmic membrane.

3.3. Inhibition of Protein Synthesis 🧬

These antibiotics target bacterial ribosomes, which differ structurally from eukaryotic ribosomes, providing selective toxicity.

3.3.1. Aminoglycosides (e.g., Kanamycin, Amikacin, Gentamicin, Tobramycin)

• Mechanism of Action: Bind irreversibly to both the 30S and 50S ribosomal subunits. This attachment has two main effects:
  1. Production of abnormal proteins: Due to misreading of messenger RNA (mRNA).
  2. Interruption of protein synthesis: By causing premature release of the ribosome from mRNA.
• Effect: Bactericidal.
• Spectrum: Used to treat infections caused by many Gram-negative rods and some Gram-positive bacteria.
• Resistance:
  1. Enzymatic modification of the antibiotic: The most common mechanism, involving phosphotransferases, adenyltransferases, and acetyltransferases.
  2. Mutations of the ribosomal binding site.
  3. Decreased uptake of the antibiotic: Due to smaller porin size.
  4. Increased expulsion of antibiotic from the cell.
  5. Intrinsic Resistance (Anaerobes): Penetration of aminoglycosides through the cytoplasmic membrane is an aerobic energy-dependent process. Anaerobes lack this process, making them resistant.
  6. Intrinsic Resistance (Streptococci and Enterococci): Aminoglycosides cannot penetrate their cell walls effectively.
     • 💡 Synergistic Effect: Treatment of streptococcal and enterococcal infections often requires co-administration of an aminoglycoside with a cell wall inhibitor (e.g., penicillin, ampicillin, vancomycin) to allow the aminoglycoside to reach its target.
• Selective Toxicity: Eukaryotic ribosomes are resistant to aminoglycosides, and the drugs are not actively transported into eukaryotic cells, explaining their ineffectiveness against intracellular bacteria like Rickettsia and Chlamydia.

3.3.2. Tetracyclines (e.g., Tetracycline, Doxycycline, Minocycline)

• Mechanism of Action: Inhibit protein synthesis by binding to the 30S ribosomal subunit, blocking the binding of aminoacyl-tRNA to the ribosome-mRNA complex.
• Effect: Reversible binding, bacteriostatic.
• Spectrum: Broad-spectrum, effective against Rickettsia, Chlamydia, and Mycoplasma.
• Resistance:
  1. Active efflux of the antibiotic out of the cell: The most common mechanism.
  2. Decreased penetration of the antibiotic into the bacterial cell.
  3. Alteration of the ribosomal binding site.
  4. Enzymatic modification of the antibiotic.
  5. Production of proteins similar to elongation factors.