Antimicrobial Drugs, Pathogenesis, and Infectious Disease: Study Guide

Controlling Microbial Growth in the Body: Antimicrobial Drugs

History and Types of Antimicrobial Agents

Antimicrobial drugs are essential tools in the treatment of infectious diseases. Their development and use have revolutionized medicine, but challenges such as resistance and side effects persist.

• Drugs: Chemicals that affect physiology in any manner.

• Chemotherapeutic agents: Drugs that act against diseases.

• Antimicrobial agents (antimicrobials): Drugs that treat infections.

• Sources: Many antibiotics are derived from Bacteria and Fungi.

• Semisynthetics: Chemically altered antibiotics that are more effective, longer lasting, or easier to administer than natural forms.

• Synthetics: Antimicrobials synthesized entirely in the laboratory.

Mechanisms of Action of Antimicrobial Drugs

Antimicrobial drugs target specific structures or functions in microbial cells, aiming for selective toxicity (harming microbes but not the host).

• Cell Wall Synthesis Inhibition: Prevents formation of peptidoglycan cross-links, leading to cell lysis. Beta-lactams (e.g., penicillins) are prominent, binding to enzymes that cross-link NAM subunits.

• Protein Synthesis Inhibition: Targets prokaryotic ribosomes (70S), which differ from eukaryotic ribosomes (80S). Some drugs may affect mitochondrial ribosomes (also 70S), causing side effects.

• Cytoplasmic Membrane Disruption: Drugs like polymyxin disrupt membrane integrity, especially in Gram-negative bacteria. Antifungals (e.g., nystatin, amphotericin B) target ergosterol in fungal membranes.

• Metabolic Pathway Inhibition: Sulfonamides and trimethoprim inhibit folic acid and nucleotide synthesis, blocking microbial metabolism.

• Nucleic Acid Synthesis Inhibition: Quinolones and fluoroquinolones inhibit DNA gyrase; nucleotide analogs interfere with DNA/RNA function, often used against viruses or cancer cells.

Clinical Considerations in Prescribing Antimicrobial Drugs

Choosing an antimicrobial involves evaluating its spectrum, effectiveness, administration route, and safety.

• Spectrum of Action: Narrow-spectrum drugs target few organisms; broad-spectrum drugs target many, but may disrupt normal microbiota and cause secondary infections.

• Effectiveness Testing: Includes diffusion susceptibility, minimum inhibitory concentration (MIC), and minimum bactericidal concentration (MBC) tests.

• Routes of Administration: Topical, oral, intramuscular, and intravenous routes are used depending on infection site and drug properties.

• Safety and Side Effects: Toxicity (to kidneys, liver, nerves), allergies (including anaphylactic shock), and disruption of normal microbiota are key concerns. The therapeutic index is the ratio of tolerated dose to effective dose: $\text{Therapeutic Index} = \frac{\text{Toxic Dose}}{\text{Effective Dose}}$

Antibiotic Resistance

Resistance to antimicrobial drugs is a growing threat, classified by the CDC into urgent, serious, and concerning levels.

• Mechanisms of Resistance:

  • Enzyme production that destroys or deactivates drugs

  • Prevention of drug entry

  • Alteration of drug targets

  • Altered metabolic pathways

  • Efflux pumps expelling drugs

  • Biofilm formation

  • Special proteins (e.g., MfpA in Mycobacterium tuberculosis)

• Multiple and Cross Resistance: Pathogens may resist multiple drugs, especially in hospital settings where R plasmids are exchanged.

• Slowing Resistance: Use drugs only when necessary, complete prescribed courses, use drug combinations (synergism), and develop new drugs.

CDC Resistance Threat Level	Description
Urgent	Potential for widespread, life-threatening disease
Serious	Alternatives available or incidence is low/declining
Concerning	Low threat or many treatment options

Infection, Infectious Diseases, and Epidemiology

Pathogens and Symbiosis

Pathogens are organisms capable of causing disease. Their interaction with hosts can take several forms, known as symbioses.

• Mutualism: Both species benefit (e.g., E. coli in the gut).

• Parasitism: One benefits, the other is harmed (e.g., Mycobacterium tuberculosis in humans).

• Commensalism: One benefits, the other is unaffected (e.g., skin bacteria).

• Antagonism: Both are harmed (e.g., Streptomyces vs. other soil bacteria).

Normal Microbiota

The human body hosts a vast number of microbes, most of which do not cause disease and are classified as normal microbiota.

• Resident microbiota: Always present on/in the body.

• Transient microbiota: Present temporarily; eliminated by competition, immune defenses, or environmental changes.

• Opportunistic pathogens: Normal microbiota that cause disease under certain conditions (e.g., immune suppression, unusual site introduction).

Reservoirs of Infection

Reservoirs are sources where pathogens persist and from which infection can spread.

• Animal reservoirs: Zoonoses are diseases naturally spread from animals to humans (e.g., via direct contact, consumption, or vectors).

• Human carriers: Asymptomatic individuals who can transmit pathogens.

• Nonliving reservoirs: Soil, water, and food contaminated with pathogens.

Pathogenicity and Virulence

Pathogenicity is the ability to cause disease; virulence is the degree of pathogenicity.

• Pathogenicity: Depends on invasion, multiplication, and evasion of host defenses.

• Virulence: Varies among species and strains; measured by severity of disease.

Transmission and Portals

Microbes enter and exit hosts through specific portals and are transmitted by various modes.

• Portals of entry/exit: Skin, mucous membranes, respiratory tract, gastrointestinal tract, urogenital tract.

• Modes of transmission:

  • Contact: Direct, indirect, or droplet

  • Vehicles: Air, water, food

  • Vectors: Arthropods (e.g., mosquitoes, ticks)

Virulence Factors and Disease Development

Virulence factors enable pathogens to infect and damage hosts.

• Adhesion: Attachment to host cells prevents removal by bodily fluids.

• Enzymes: Aid in invasion and evasion of host defenses.

• Toxins: Substances that damage host tissues.

• Direct action: Penetration, ingestion, inhalation, or sexual transmission.

Toxin Type	Source	Effect
Endotoxins	Part of Gram-negative cell wall (LPS)	Released on cell death/division; cause fever, shock
Exotoxins	Secreted by bacteria	Specific effects (e.g., neurotoxins, enterotoxins)

Hemolysins (Exotoxins)

• α-hemolysis: Partial hemoglobin breakdown (greenish ring)

• β-hemolysis: Complete hemoglobin breakdown (clear ring)

• γ-hemolysis: No hemolysis

Examples: Streptococcus pyogenes (β-hemolytic, causes strep throat, scarlet fever); Staphylococcus aureus (scalded skin syndrome, food poisoning)

Types and Stages of Infectious Disease

• Acute: Rapid onset, short duration

• Chronic: Slow development, long-lasting

• Latent: Periods of inactivity between episodes

Stage	Description
Incubation period	Time between infection and first symptoms
Prodromal phase	Mild, nonspecific symptoms
Invasive (Disease) phase	Severe symptoms; includes acme (peak)
Decline phase	Symptoms subside
Convalescence period	Recovery and return to health

Microbes in the Blood

• Septicemia: Pathogens present and multiplying in blood ("blood poisoning")

• Bacteremia: Bacteria in blood, not multiplying

• Viremia: Viruses in blood, not multiplying

Challenges in Eradicating Infectious Diseases

• Medical expertise may not be applied effectively

• Pathogens are highly adaptable

• Changes in human activity can make rare diseases significant

• Global travel introduces new strains

Nosocomial (Hospital-Acquired) Infections

• Definition: Infections acquired in healthcare settings.

• Precautions: Hand hygiene, sterilization, isolation procedures.

• Reduction: Infection control programs, surveillance, and antibiotic stewardship.

Summary Table: Major Modes of Transmission

Mode	Example	Notes
Direct contact	Touching, sexual contact	Person-to-person
Indirect contact	Fomites (doorknobs, utensils)	Via inanimate objects
Droplet	Coughing, sneezing	Short-range aerosols
Vehicle	Contaminated water, food	Common source outbreaks
Vector	Mosquitoes, ticks	Biological or mechanical transmission

Additional info:

• Antibiotic stewardship and infection control are critical in healthcare settings to prevent the spread of resistant organisms.

• Vaccination, sanitation, and public health measures have reduced mortality from many infectious diseases, but eradication remains difficult due to pathogen adaptability and global movement.