Antimicrobial Chemotherapy

Introduction to Antimicrobials

Antimicrobials are substances used in therapy to treat infectious diseases due to their effectiveness and minimal toxicity. Their activity is defined by their ability to either kill microorganisms or inhibit their growth and multiplication. The origin of antimicrobials can be natural, chemically synthesized (chemotherapeutic), or semi-synthetic (modified natural molecules).

• Selective toxicity: Antimicrobials harm microorganisms but not human cells.

• Therapeutic targets: Fundamental structures of microorganisms are targeted to kill them.

• Drug concentration: Must be sufficient in tissues to inhibit or kill pathogens.

• Types: Antibiotic, Antibacterial, Antiviral, Antifungal, Antiparasitic.

Basic Characteristics of Antimicrobials!

Effective antimicrobials must be active against pathogens at low concentrations and have minimal toxicity in the host. The choice of antimicrobial depends on the relationship between the microorganism, the patient, and the drug.

• Virulence: Pathogen's ability to cause disease.

• Defenses: Patient's immune response.

• Sensitivity and resistance: Microorganism's susceptibility to the drug.

• Toxicity and pharmacokinetics: Drug's effect and distribution in the host.

Mechanisms of Action of Antimicrobials

Spectrum of Action and Antibacterial Effect

Antibiotics are classified by their spectrum of action and effect:

• Narrow spectrum: Active against specific species or genera.

• Broad spectrum: Active against a wide range of microorganisms.

• Bacteriostatic: Reversibly inhibit growth; patient defenses are essential.

• Bactericidal: Kill microorganisms; activity depends on concentration.

Classification by Mechanism of Action

Antibiotics act on bacteria depending on their target:

• Cell wall synthesis inhibitors: β-lactams (penicillins, cephalosporins, carbapenems, monobactam s), glycopeptides, fosfomycin.

• Protein synthesis inhibitors: Aminoglycosides, macrolides, streptogramins, lincosamides, oxazolidinones, tetracyclines, chloramphenicol.

• Membrane function inhibitors: Polymyxins, lipopeptides.

• Nucleic acid synthesis inhibitors: Quinolones, rifamycins, nitroimidazoles.

• Metabolic pathway inhibitors: Sulfonamides, trimethoprim.

Mode of Action of Key Antibiotic Classes

• β-lactams: Bind to PBPs (penicillin binding proteins), block cell wall synthesis, cause bacterial lysis. Bactericidal.

• Macrolides: Bind to 50S ribosomal subunit, inhibit protein translation by preventing translocation of aminoacyl tRNA.

• Aminoglycosides: Bind to 30S ribosomal proteins, block initiation of protein synthesis.

• Lincosamides: Bind to 50S ribosomal subunit, similar to macrolides.

• Tetracyclines: Bind to 30S ribosomal proteins, block entry of new aminoacyl-tRNA.

• Quinolones: Bind to DNA gyrase and topoisomerase IV, inhibit DNA replication. Broad spectrum.

• Rifamycins: Bind DNA-dependent RNA polymerase, block mRNA synthesis.

• Nitroimidazoles: Metabolized by anaerobes to toxic compounds that damage DNA.

• Sulfonamides: Compete with PABA, prevent folic acid synthesis.

• Trimethoprim: Inhibits dihydrofolate reductase.

Chemical Structure and Classification Table

Mechanisms of Bacterial Resistance to Antimicrobials

Overview of Antibiotic Resistance

Antibiotic resistance is the ability of bacteria to survive in the presence of antibiotics, rendering treatment ineffective. Resistance can be intrinsic (innate to all bacteria of a species) or acquired (due to mutations or acquisition of genetic material).

• Intrinsic resistance: Innate genetic traits (e.g., Gram-negative resistance to glycopeptides).

• Acquired resistance: Chromosome mutations or acquisition of plasmids, transposons, etc.

Mechanisms of Resistance

• Enzymatic inactivation: Antibiotic is hydrolyzed or chemically modified (e.g., β-lactamases, acetyl/adenylyl/phosphoryl transferases).

• Modification of target: Altered structure of target molecules (e.g., PBPs, ribosomal RNA, DNA gyrase).

• Active efflux pumps: Expel antibiotics from the cell, require energy, can affect multiple antibiotic classes.

• Reduced permeability: Changes in membrane porins, especially in Gram-negative bacteria, limit antibiotic entry.

• New metabolic pathways: Bypass the blocked pathway.

Resistance to β-lactams

• Modification of PBPs: Loss of affinity for β-lactams.

• β-lactamases: Enzymes that break the β-lactam ring, inactivating the antibiotic.

• Reduced permeability: Loss of porins.

• Efflux pumps: Expel β-lactams.

β-lactamase Inhibitors

• Clavulanic acid, sulbactam, tazobactam: Irreversibly bind β-lactamases, allowing β-lactam antibiotics to act.

• Examples: Amoxicillin + clavulanic acid, piperacillin/tazobactam.

Resistance to Macrolides and Quinolones

• Macrolides: Target modification (methylation of 23S rRNA, ribosomal protein mutations), efflux pumps, enzyme inactivation.

• Quinolones: Modification of DNA gyrase and topoisomerase IV.

Multiresistant Bacteria

Bacteria with multiple resistance mechanisms are resistant to almost all families of antibiotics, posing significant treatment challenges.

Antibiogram: Laboratory Evaluation of Antibiotic Sensitivity

Principles and Techniques

An antibiogram evaluates the in vitro response of microorganisms to antimicrobials, guiding appropriate antibiotic treatment. Main techniques:

• Diffusion technique: Disk-plate diffusion; qualitative method based on inhibition zone diameter.

• Dilution technique: Agar or broth microdilution; quantitative method based on minimum inhibitory concentration (MIC).

• Molecular biology: Detection of resistance genes.

E-test and MIC Determination

The E-test uses plastic strips with decreasing antibiotic concentrations to determine MIC. The intersection of the inhibition ellipse with the strip indicates the MIC value.

Dilution Techniques

MIC is determined by exposing bacteria to different antibiotic concentrations in liquid or solid media. The lowest concentration with no growth is the MIC.

Antibiotic Susceptibility Concepts

• Range: MICs found in the population.

• MIC50: Inhibits at least 50% of bacteria.

• MIC90: Inhibits at least 90% of bacteria.

Rational Use of Antibiotics

Principles and Toxicity

Antibiotic chemotherapy should act selectively on microorganisms without harming the host. Undesirable effects include toxicity, hypersensitivity, and disruption of normal microbiota.

• Toxicity: Effects on host biochemical systems.

• Hypersensitivity: Allergic reactions.

• Microbiota disruption: Killing normal flora.

Associations of Antibiotics

• Advantages: Increased action, extended spectrum, empirical treatment, prevention of resistance.

• Disadvantages: Increased toxicity, antagonism.

• Effects: Additive, synergistic, antagonistic.

Responsible Use in Dentistry

Antibiotics should be used only when necessary, with knowledge of common pathogens and their sensitivity. Correct dose, duration, and associations are essential. Most dental infections are polymicrobial, often involving anaerobes and gram-positive cocci.

• Empiric treatment: Based on likely pathogens and local sensitivity patterns.

• Directed treatment: Based on microbiological diagnosis.

• Prophylaxis: To prevent complications from oral microbiota.

Summary Table: Mechanisms of Bacterial Resistance

Key Equations

• Minimum Inhibitory Concentration (MIC):

• MIC50 and MIC90:

Exam Preparation Tips

• Understand the mechanisms of action and resistance for each antibiotic class.

• Be able to interpret antibiogram results and MIC values.

• Know the principles of rational antibiotic use and toxicity.

• Review the summary tables for quick comparison of antibiotic classes and resistance mechanisms.