Antimicrobial Drugs: Principles, Mechanisms, and Resistance

Antimicrobial Drugs

Definition and Historical Impact

Antimicrobial drugs are therapeutic compounds designed to kill or inhibit the growth of microbes, thereby preventing or treating infectious diseases. Their development revolutionized modern medicine, drastically reducing mortality from infections that were once often fatal.

• Antimicrobial drug: Any substance that kills or inhibits the growth of microorganisms, including bacteria, viruses, fungi, and parasites.

• Historically, minor infections could be deadly; the introduction of purified antimicrobials in the 20th century changed this.

Discovery of Penicillin

Alexander Fleming discovered penicillin in 1928 when he observed that a mold (genus Penicillium) inhibited the growth of Staphylococcus aureus on a culture plate. This led to the development and mass production of the first clinically useful antibiotic in the 1940s.- PENICILLIN

• Zone of inhibition: Area around the mold where bacterial growth is prevented, indicating antimicrobial activity.

Spectrum of Activity and Empiric Therapy

1. Basic Criteria for drugs:

• Effective against microbes

• sufficiently non-toxic to the host

• able to be purified in high amounts

2. Spectrum of Antimicrobial Activity

The spectrum of activity refers to the range of microbes an antimicrobial agent can affect.

• Broad-spectrum: Effective against a wide variety of microbes; useful when the pathogen is unknown but can disrupt normal microbiota.

• Narrow-spectrum: Targets a limited group of microbes; preferred when the pathogen is identified to minimize collateral damage. It can take days.

• Limited-spectrum: Effective against a single organism or disease.

  Empiric Therapy

  1. Empiric therapy = of medical treatment with a broad-spectrum drug initiation while waiting for culture/ID results

     –”Educated guess” based on clinical experience and observation in the absence of complete information

  2. NO initial empiric choice is perfect

     –Send for culture! Identify microbe! Ensure follow-up!

     –Switch to a narrow-spectrum drug specific to the pathogen

Classification and Modification of Antimicrobial Drugs

Sources of Antimicrobials

• Naturally occurring: Produced by microorganisms (e.g., soil bacteria, fungi).

• Synthetic: Fully manufactured by chemical processes.

• Semisynthetic: Chemically modified natural antibiotics to improve efficacy, spectrum, or pharmacokinetics.

Drug Modification and Generations

1. Antimicrobial compounds can be modified by chemical means (e.g., adding R groups

   – Next-generation drugs result from each successive round of chemical modification (1st Gen, 2nd Gen, etc)

2. Drugs in later generations have expanded capabilities over their predecessors:

   – Altered spectrum of activity

   – Better penetration of different cell wall types

   – Increased stability (longer half-life)

   – Higher potency

   – Circumvent resistance mechanisms

   – Easier synthesis/purification/administration

   – Lower toxicity

• Example: Ampicillin and amoxicillin are semisynthetic derivatives of penicillin G, with broader activity due to added chemical groups.

Principles of Antimicrobial Drug Safety

Selective Toxicity and Therapeutic Index

Selective toxicity is achieved by exploiting vulnerable targets for the drug in the microbe that do not exist in the patient.

Selective toxicity is the ability of a drug to target microbial processes without harming the host. The therapeutic index (TI) quantifies drug safety:

• Therapeutic Index (TI): $TI = \frac{\text{Toxic Dose}}{\text{Therapeutic Dose}}$

  • TI= ratio of the toxic dose (to the patient) to the theraputic dose (to eliminate the infection).

• High TI = safer drug; low TI = higher risk of toxicity.

• Drugs with a wide/high TI have a large margin of safety and less danger of producing toxic effects vs drugs with a narrow/low TI.

Toxicity Considerations

Antimicrobials can cause toxicity, especially to the liver (hepatotoxicity), kidneys (nephrotoxicity), and gut microbiome. They metabolize and eliminate drugs.

• Hepatotoxicity: Drug-induced liver injury; a common reason for discontinuing drugs.

• Nephrotoxicity: Kidney damage; antimicrobials are a leading cause.

• Gut microbiome toxicity: Disruption of normal intestinal flora.

Routes of Administration

• Oral: Preferred for convenience and cost;

  • must be stable in stomach acid and absorbed in the intestines, but not irritating to GI mucosa.

• Parenteral: Non-oral routes (not involving the intestines)

  • (e.g., intravenous, intramuscular, subcutaneous, intradermal); used for rapid or targeted delivery.

  • Downside: needles are used, and needle phobia; injection discomfort; infection risk

Drug Half-Life and Interactions

• Half-life: Time required for half the drug to be eliminated from the body; determines dosing frequency; time to reach steady state; duration of drug activity.

  • Short half-life = frequent administration

• Drug interactions: Some antimicrobials interact with other drugs, foods, or supplements, affecting efficacy or safety.

Survey of Antimicrobial Drugs

Antibiotics vs. Antimicrobials

• Antibiotics: Subset of antimicrobials that specifically target bacteria.

• Antimicrobials: Broader term including agents against bacteria, viruses, fungi, and parasites.

Bacteriostatic vs. Bactericidal Drugs

• Bacteriostatic: Inhibit bacterial growth; rely on the immune system to clear infection.

• Bactericidal: Kill bacteria directly; useful in immunocompromised patients or severe infections.

CATEGORY	BACTERICIDAL	BACTERIOSTATIC
Definition	Kills bacteria	Inhibits bacterial growth
Minimum Concentration	MBC (minimum bactericidal concentration): lowest conc. needed to kill bacteria.	MIC (minimum inhibitory concentration): lowest conc. to inhibit growth
# of bacterial cells	number decreases	# remains the same
Viability	Bacteria die	Bacteria remain viable
Activation of Immune System	No effect	Helps immune system deal with infection
High dose effect	kills all bacterial cells	may act as a bactericidal at high dose
Low dose effect	acts as bacteriostatic	not effective
Reversibility	Irreversible	Reversible

Mechanisms of Action

Antibacterial drugs exploit differences between prokaryotic and eukaryotic cells. Major targets include:

Bacteriostatic: inhibition of...

• Metabolic pathways- folic acid synthesis (e.g., sulfa drugs, trimethoprim)

• Protein synthesis (e.g., tetracyclines, macrolides)

Bactericidal: inhibition of...

• Nucleic acid synthesis (e.g., quinolones, rifamycins)

• Cell membrane integrity (e.g., polymyxins)

• Cell wall synthesis (e.g., penicillins, cephalosporins)

Antiviral, Antifungal, and Antiparasitic Drugs

• Antivirals: Target steps in viral replication or stimulate host immune responses; difficult to design due to reliance on host cell machinery.

  • Do not destroy their target pathogen

  • Goal – inhibit viral development to treat active infection

    • Antiviral drugs available today designed against HIV, herpes viruses, hepatitis viruses (B,C), influenza viruses, SARS-CoV2

  • Difficult to design safe and effective antiviral drugs

    • Viruses use host cells to replicate

    • Challenging to ID drug targets specific to the virus that won’t also harm the host organism's cells

  • Aim – target and disable essential virus-specific proteins shared among multiple viral strains and unlike host-cell components

  • Antiviral drugs are mainly effective against actively replicating viruses

    • Latent viruses are difficult to treat

  Antiviral Drug Targets

  1. 1st category – target viral replication cycle: 1. Attachment 2. Penetration 3. Uncoating 4. Viral replication 5. Viral assembly 6. Viral release

  2. 2nd category – stimulate the host’s anti-viral immune responses (e.g., interferons)

  3. Vaccination remains the primary and most effective way to reduce severe viral infection and disease (prevent vs treat)

• Antifungals: Target fungal-specific structures (e.g., ergosterol in membranes, cell wall synthesis); toxicity is a concern due to eukaryotic similarities.

• Antiparasitics: Target unique features of protozoa or helminths; often limited by toxicity and complex life cycles.

  Antifungal Drug Targets

  1. Mycoses – range from common benign infections like 'jock itch’ and ’athlete’s foot’ to serious, life-threatening infections such as cryptococcal meningitis

  2. Most antifungal drugs target fungal-specific compounds not in host cells

     A. Interfere with nucleic acid synthesis ▪ Leads to failure of fungal cell replication

     B. Interfere with cell wall synthesis resulting in lysis ▪ Targets synthesis of essential fungal CW components (ex. glucans, chitin)

     C. Interfere with cell membrane stability and structure causing death ▪ Targets essential CM sterol ergosterol (not cholesterol)

  Antiparasitic Drug Targets

  1. intracellular components– Function to ‘stun’ (paralyze, detach) or ‘kill’

Assessing Sensitivity to Antimicrobial Drugs

Antibiotic Susceptibility Testing (AST)

1. Patient specimens are sent for culture and A ntibiotic S T esting (AST) usceptibility

   – Samples assessed include blood, urine, cerebrospinal fluid, sputum, wound exudates, stool, and other body fluids/discharge

2. AST is performed by clinical microbiology laboratories

   – Determine organism causing the specific infection and which antimicrobials will inhibit the growth of the infecting microbe

   – Identify and report drug resistance in pathogens

3. AST methods include drug diffusion and minimum inhibitory concentration (MIC) assays

   – Large healthcare facilities also utilize commercial molecular systems to characterize bacterial resistance

Agar Diffusion Tests: Kirby Bauer vs Etest Agar Assays

Kirby- Bauer (Disk diffusion)	E-test (epsilometer test)
Drug-infused disks are placed on a freshly inoculated agar surface	Strips infused with a drug gradient are placed on a freshly inoculated agar surface
Plates are incubated to allow for bacterial growth and drug diffusion from the disc	Plates are incubated to allow for bacterial growth and drug diffusion from the strip
Zone of inhibition appears if the bacteria are prevented from growing – Diameter of zone is measured and interpreted using a table of standards – Abt sensitivity is determined to be susceptible, resistant, or intermediate	An ellipse-shaped inhibition zone intersects the test strip at the minimal inhibitory concentration (MIC) of the antibiotic – A quantitative method to determine MIC

Broth Dilution Tests

Differentiate between static (inhibit) vs cidal (kill) drug concentrations

1. Minimum Inhibit ory Concentration (MIC) assays

• Determine the lowest concentration of an antimicrobial agent that prevents visible growth

1. Antibiotic is serially diluted in growth media and a standardized amount of bacterial culture is added to each tube

2. Following incubation, MIC is determined

   –Level of growth is measured by assessing turbidity or colorimetric indicators

   –MIC = Lowest concentration of drug at which growth is completely inhibited (absence of visible turbidity/color)

2. Minimum Bactericidal Concentration (MBC) assays

• Determine the lowest concentration of an antimicrobial agent required to kill the organism .

• Defined as a ≥99.9% reduction of the initial inoculum within a fixed period of time (usually 24 hrs)

1. Plate the dilution where the MIC was observed along with at least 2 adjacent samples onto agar media that does not contain antibiotics

2. Only live cells will be able to grow

3. MBC = minimum concentration of drug that kills at least 99.9% of bacteria present in the assay

Antimicrobial Drug Resistance and Stewardship

Types o1f Resistance

• Intrinsic resistance: Natural, due to inherent structural or functional traits (e.g., lack of target, impermeable cell wall).

  • Natural resistance to antimicrobial drugs based on inherent microbial structure/function

  • Makes certain pathogens/infections harder to treat/eliminate

• Acquired resistance: Due to genetic mutations or horizontal gene transfer (conjugation, transformation, transduction).

  • 3 main types of acquired resistance to evade antimicrobial drugs:

    • (1) Alter drug’s target

    • (2) Inactivate drug

    • (3) Reduce drug concentrations inside the cell

      • A. Block drug entry = decrease cell permeability

      • B. Pump the drug out = efflux pumps

Superbugs Cause Superinfections

• Superbugs: Microbes resistant to multiple antimicrobials that are commonly used

• Superinfection: result when superbugs emerge from the initial infection resistant to the treatment and become the primary cause of a second infection

  • Long-term use of broad-spectrum antimicrobials sets the stage for superinfections

Factors Accelerating Resistance

• Noncompliance with dosing

• Misuse in healthcare and agriculture

• Unregulated use in developing countries

Antimicrobial Stewardship

Stewardship programs promote appropriate use of antimicrobials to improve outcomes, reduce resistance, and limit the spread of multidrug-resistant organisms. Key strategies include:

• Using narrow-spectrum drugs when possible

• Educating healthcare workers and patients

• Following proper dosing and infection control practices

Summary Table: Key Concepts

Concept	Explanation
Antimicrobial drug	Compound that kills or inhibits microbes
Spectrum of activity	Range of microbes affected (broad, narrow, limited)
Therapeutic index	Ratio of toxic to therapeutic dose; higher is safer
Bacteriostatic	Inhibits growth
Bactericidal	Kills bacteria
Intrinsic resistance	Natural, structural resistance
Acquired resistance	Genetic mutation or gene acquisition
Superinfection	Secondary infection by resistant microbes
Stewardship	Appropriate use to limit resistance