Antimicrobial Drugs and Resistance: Study Notes (Chapter 15)

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Penicillin was the first true antibiotic discovered and marked a turning point in medical microbiology.

Alexander Fleming discovered penicillin in 1928 when he observed that the mold Penicillium notatum inhibited the growth of Staphylococcus bacteria on a petri dish.
This observation led to the isolation of penicillin, which was later developed into a widely used antibiotic.
Example: Penicillin is effective against many Gram-positive bacteria.

Antimicrobial drugs are classified based on the range of microorganisms they target.

Broad-spectrum drugs act against a wide variety of microorganisms, including both Gram-positive and Gram-negative bacteria.
Narrow-spectrum drugs target specific types or groups of bacteria.
Reason for use: Broad-spectrum drugs are used when the causative agent is unknown, but they can disrupt normal microbiota and increase risk of secondary infections.
Example: Tetracycline (broad-spectrum), Penicillin G (narrow-spectrum).

Empirical therapy involves starting treatment before the causative organism is identified, while susceptibility testing guides drug selection.

Empirical therapy is based on clinical experience and knowledge of likely pathogens.
Susceptibility testing (e.g., disk diffusion, MIC tests) determines which antibiotics are effective against a specific pathogen.
Example: Empirical use of vancomycin for suspected MRSA infection, followed by susceptibility testing to confirm.

Selective toxicity is the ability of a drug to target microbial cells without harming host cells.

Definition: The drug affects structures or processes unique to microbes (e.g., cell wall synthesis, ribosomes).
Example: Penicillins inhibit peptidoglycan synthesis in bacterial cell walls, which is absent in human cells.

Antimicrobial drugs act on specific targets within microbial cells.

Cell wall synthesis inhibitors: e.g., Penicillins, cephalosporins.
Protein synthesis inhibitors: e.g., Tetracyclines, aminoglycosides.
Nucleic acid synthesis inhibitors: e.g., Quinolones, rifampin.
Metabolic pathway inhibitors: e.g., Sulfonamides.
Membrane function disruptors: e.g., Polymyxins.
Example: Tetracycline inhibits the 30S ribosomal subunit, blocking protein synthesis.

Drugs targeting viruses and fungi must exploit differences between these pathogens and host cells.

Antiviral drugs: Target viral enzymes (e.g., reverse transcriptase, protease) or block viral entry/exit.
Antifungal drugs: Target ergosterol in fungal cell membranes (e.g., azoles, polyenes).
Example: Acyclovir inhibits herpesvirus DNA polymerase; fluconazole inhibits ergosterol synthesis.

The Kirby-Bauer test is a standardized method to assess microbial susceptibility to antibiotics.

Procedure: Disks impregnated with antibiotics are placed on an agar plate inoculated with the test organism.
Interpretation: Zones of inhibition around disks indicate susceptibility; size is compared to standard charts.
Example: Large zone of inhibition for ampicillin indicates sensitivity; small or no zone indicates resistance.

Microorganisms may be naturally resistant (intrinsic) or develop resistance through genetic changes (acquired).

Intrinsic resistance: Natural, due to inherent structural or functional characteristics (e.g., Gram-negative bacteria resistant to vancomycin).
Acquired resistance: Results from mutation or horizontal gene transfer (e.g., plasmids carrying resistance genes).
Example: MRSA (Methicillin-resistant Staphylococcus aureus) acquired resistance via mecA gene.

Antimicrobial resistance arises and spreads due to selective pressure from drug use.

Selection pressure: Antibiotic use kills susceptible bacteria, allowing resistant ones to survive and multiply.
Example: Overuse of antibiotics in hospitals leads to increased prevalence of resistant strains.

Human activities in medicine and agriculture accelerate the development of antimicrobial resistance.

Clinical practices: Overprescribing antibiotics, incomplete courses, lack of infection control.
Agricultural practices: Use of antibiotics in livestock for growth promotion and disease prevention.
Example: Resistant Salmonella strains emerging from antibiotic use in poultry farms.

Drug Class	Target	Example
Beta-lactams	Cell wall synthesis	Penicillin
Aminoglycosides	Protein synthesis (30S ribosome)	Streptomycin
Quinolones	DNA synthesis	Ciprofloxacin
Azoles	Fungal cell membrane (ergosterol)	Fluconazole
Antivirals	Viral enzymes	Acyclovir

Minimum Inhibitory Concentration (MIC): The lowest concentration of an antimicrobial that inhibits visible growth of a microorganism.
Zone of Inhibition (Kirby-Bauer): Diameter measured in millimeters; compared to standard values for interpretation.

*Additional info: Expanded explanations and examples provided for each question topic to ensure completeness and academic context.*