Controlling Microbial Growth in the Body: Antimicrobial Drugs

Controlling Microbial Growth in the Body: Antimicrobial Drugs

The History of Antimicrobial Agents

The development of antimicrobial drugs revolutionized the treatment of infectious diseases. Key historical figures contributed to the discovery and advancement of these agents.

• Paul Ehrlich: Proposed the concept of "magic bullets"—chemicals that selectively target pathogens. Discovered arsenic compounds effective against microbes.

• Alexander Fleming: Discovered penicillin, the first true antibiotic, produced by the mold Penicillium chrysogenum.

• Gerhard Domagk: Discovered sulfanilamide, the first widely used synthetic antimicrobial.

• Selman Waksman: Coined the term "antibiotics" for antimicrobial agents produced naturally by organisms.

Types of Antimicrobial Agents

• Antibiotics: Naturally produced by microorganisms (e.g., penicillin from fungi).

• Semisynthetics: Chemically modified antibiotics to improve efficacy, stability, or spectrum.

• Synthetics: Entirely synthesized in the laboratory, not derived from natural sources.

Microorganism	Antimicrobial
Penicillium chrysogenum	Penicillin G
Penicillium griseofulvum	Griseofulvin
Acremonium spp.	Cephalosporin
Streptomyces spp.	Streptomycin, Tetracycline, Chloramphenicol, etc.
Bacillus polymyxa	Polymyxin

Mechanisms of Antimicrobial Action

Principle of Selective Toxicity

Selective toxicity refers to the ability of an antimicrobial drug to harm the pathogen without damaging the host. This principle is fundamental to effective chemotherapy.

Major Mechanisms of Action

• Inhibition of cell wall synthesis

• Inhibition of protein synthesis

• Disruption of cytoplasmic membrane

• Inhibition of metabolic pathways

• Inhibition of nucleic acid synthesis

• Prevention of pathogen attachment or entry into host cell

Inhibition of Cell Wall Synthesis

Many antibacterial drugs target the synthesis of peptidoglycan, a key component of bacterial cell walls. Beta-lactam antibiotics (e.g., penicillins, cephalosporins) prevent cross-linking of NAM subunits, weakening the cell wall and causing cell lysis.

• Beta-lactams: Bind to enzymes that cross-link NAM subunits.

• Vancomycin and cycloserine: Interfere with bridges between NAM subunits in Gram-positive bacteria.

• Bacitracin: Blocks transport of NAG and NAM from cytoplasm.

• Isoniazid and ethambutol: Disrupt mycolic acid formation in mycobacteria.

Inhibition of Fungal Cell Wall Synthesis

• Echinocandins: Inhibit synthesis of glucan, a polysaccharide in fungal cell walls.

Inhibition of Protein Synthesis

Antimicrobials can selectively target prokaryotic ribosomes (70S) without affecting eukaryotic ribosomes (80S), though mitochondrial ribosomes may be affected.

• Aminoglycosides: Cause misreading of mRNA.

• Tetracyclines: Block docking site of tRNA.

• Chloramphenicol: Blocks peptide bond formation.

• Macrolides and lincosamides: Block ribosomal movement.

• Oxazolidinones: Block initiation of translation.

• Mupirocin: Inhibits isoleucyl-tRNA synthetase in Gram-positive bacteria.

Disruption of Cytoplasmic Membranes

Some drugs compromise membrane integrity, leading to cell death.

• Polymyxins: Disrupt membranes of Gram-negative bacteria (toxic to kidneys).

• Nystatin and amphotericin B: Bind to ergosterol in fungal membranes, forming pores.

• Azoles and allylamines: Inhibit ergosterol synthesis in fungi.

Inhibition of Metabolic Pathways

Antimetabolic agents target pathways unique to pathogens.

• Sulfonamides: Inhibit folic acid synthesis by acting as structural analogs of PABA.

• Trimethoprim: Inhibits a later step in folic acid synthesis.

• Atovaquone: Interferes with electron transport in protozoa and fungi.

• Antiviral agents: Amantadine and rimantadine prevent viral uncoating; protease inhibitors block HIV replication.

Inhibition of Nucleic Acid Synthesis

Some drugs block DNA replication or RNA transcription, often affecting both prokaryotic and eukaryotic cells.

• Quinolones and fluoroquinolones: Inhibit DNA gyrase in bacteria.

• Nucleotide/nucleoside analogs: Distort nucleic acid structure, preventing replication and transcription (used against viruses and cancer cells).

• Reverse transcriptase inhibitors: Block HIV replication; do not affect human cells.

Prevention of Virus Attachment, Entry, or Uncoating

• Attachment antagonists: Block viral attachment or receptor proteins (e.g., pleconaril).

• Uncoating inhibitors: Prevent viral genome release (e.g., arildone).

Clinical Considerations in Prescribing Antimicrobial Drugs

Spectrum of Action

Antimicrobial drugs vary in the range of pathogens they affect.

• Narrow-spectrum: Effective against a limited group of organisms.

• Broad-spectrum: Effective against a wide variety of organisms, but may disrupt normal microbiota and lead to superinfections.

Effectiveness

Several laboratory tests assess the efficacy of antimicrobial agents:

• Diffusion susceptibility (Kirby-Bauer) test: Measures zones of inhibition around antibiotic disks.

• Minimum inhibitory concentration (MIC) test: Determines the lowest concentration that inhibits visible growth.

• Minimum bactericidal concentration (MBC) test: Identifies the lowest concentration that kills the organism.

• Etest: Combines aspects of Kirby-Bauer and MIC tests using a gradient strip.

Routes of Administration

• Topical: For external infections.

• Oral: Convenient, but variable absorption.

• Intramuscular (IM): Delivers drug via injection into muscle.

• Intravenous (IV): Directly into bloodstream for rapid, high levels.

Safety and Side Effects

• Toxicity: Some drugs are toxic to kidneys, liver, or nerves; therapeutic index (TI) is the ratio of tolerated dose to effective dose.

• Allergies: Rare but potentially life-threatening (e.g., anaphylactic shock).

• Disruption of normal microbiota: May lead to secondary infections or superinfections, especially in hospitalized patients.

Resistance to Antimicrobial Drugs

The Development of Resistance in Populations

Microbial resistance can arise through new mutations or acquisition of resistance (R) plasmids via transformation, transduction, or conjugation.

Mechanisms of Resistance

• Enzymatic destruction or deactivation of the drug (e.g., beta-lactamase).

• Prevention of drug entry into the cell.

• Alteration of drug target site.

• Alteration of metabolic pathways.

• Efflux pumps expel the drug from the cell.

• Biofilm formation increases resistance.

• Production of proteins that protect target enzymes (e.g., MfpA in Mycobacterium tuberculosis).

Multiple Resistance and Cross Resistance

• Multiple resistance: Pathogens resistant to three or more antimicrobial agents, often due to R plasmid exchange.

• Cross resistance: Resistance to drugs with similar structures or mechanisms.

Retarding Resistance

• Maintain high drug concentrations in the patient for sufficient time.

• Use antimicrobial combinations (synergism enhances effect; antagonism reduces efficacy).

• Limit antimicrobial use to necessary cases.

• Develop new drugs and modify existing ones to overcome resistance.

Additional info: Understanding the mechanisms of action and resistance is crucial for effective clinical use of antimicrobials and for combating the rise of drug-resistant pathogens.