Chapter 13

1. Introduction and Chapter Objectives

  • Main Goals of Microbial Control and Antimicrobial Therapy:

    • Compare disinfectants (used on non-living objects) and therapeutic drugs (used inside the body) based on how they work, which microbes they kill, and how they are used clinically.

    • Explain how healthcare providers decide if an antibiotic is clinically effective.

    • Understand clinical treatment strategies:

    • Empirical Therapy: Starting a broad treatment immediately based on clinical experience before the exact microbe is identified in the lab.

    • Targeted Drug Therapy: Switching to a specific, narrow-spectrum drug once the exact microbe and its sensitivities are known.

    • Synergism: Using two drugs together so that their combined effect is greater than the sum of their individual effects.

    • Understand how selective toxicity and bacterial resistance mechanisms drive the discovery and design of new antimicrobial drugs.

2. Core Concepts of Sterilization and Disinfection

  • Key Definitions:

    • Sterilization: The complete destruction or removal of all forms of microbial life. This includes living vegetative cells, highly resistant bacterial endospores, and viruses.

    • Disinfection: The killing, removal, or inhibition of disease-causing microbes on non-living (inanimate) surfaces and objects. It does not necessarily kill bacterial endospores.

    • Antisepsis: The removal or destruction of pathogens from the surface of living tissues (such as skin or gums) without damaging host tissues.

    • Sanitization: Lowering microbial populations to safe levels according to public health standards, usually by thorough cleaning and mild chemical treatment.

  • Microbial Control Classifications:

    • -cidal Agents: Kill microbes permanently and irreversibly.

    • Bactericidal: Kills bacteria.

    • Algicidal: Kills algae.

    • Fungicidal: Kills fungi and fungal spores.

    • Virucidal: Inactivates or destroys viruses.

    • Germicidal: A general term for any physical or chemical agent that destroys disease-causing germs (bacteria, fungi, and other pathogens).

    • -static Agents: Stop or slow down microbial growth without killing cells directly. If the agent is removed, the microbes can start growing again.

    • Bacteriostatic: Stops bacterial growth and replication.

    • Algistatic: Inhibits algae.

    • Fungistatic: Inhibits fungi.

    • Virustatic: Prevents viruses from replicating or entering host cells.

  • Factors Governing Microbial Death:

    • Microbial death is not instant. When exposed to a lethal agent, a population of microbes dies off at a steady, exponential rate over time.

    • Key factors that determine how fast microbes die and how long sterilization takes:

    • Initial Population Size: A larger starting number of microbes takes longer to sterilize.

    • Population Composition: Different microbes have different levels of resistance. Bacterial endospores, heat-loving thermophiles, and mycobacteria (with waxy cell walls) are much harder to kill than standard vegetative bacteria.

    • Agent Concentration or Dose: Higher concentrations of chemicals or higher doses of physical agents kill faster.

    • Duration of Exposure: The agent must stay in contact with the microbes long enough to produce lethal damage.

    • Presence of Organic Material: Blood, pus, saliva, feces, and biofilm slime can coat microbes, bind to disinfectants, and neutralize their killing ability. Surfaces must be cleaned mechanically before disinfection.

3. Physical Methods of Microbial Control

  • High Temperature and Pressure:

    • Moist Heat vs. Dry Heat: Moist heat kills microbes much faster and penetrates better than dry heat because water molecules help quickly denature and coagulate vital cellular proteins.

    • Boiling Water: Boiling at 100 ∘C100\,^\circ\text{C} kills most vegetative bacteria, fungi, and viruses rapidly, but it cannot kill bacterial endospores or thermophilic organisms.

    • Steam Autoclaving:

    • To kill endospores, steam must be trapped under pressure to reach temperatures above boiling (100 ∘C100\,^\circ\text{C}).

    • Standard Autoclave Parameters: Saturated steam at 121 ∘C121\,^\circ\text{C} (250 ∘F250\,^\circ\text{F}) under a pressure of 15 psi15\,\text{psi} (about 103.4 kPa103.4\,\text{kPa} above atmospheric pressure) for a minimum of 20 min20\,\text{min}.

    • Autoclave Functioning: Dense cold air is pushed downward and out through an automatic exhaust valve. When pure steam reaches the valve, the heat causes it to shut automatically. A steam jacket preheats the chamber walls to prevent water condensation from collecting inside.

  • Pasteurization:

    • Developed by Louis Pasteur to prevent French wine from spoiling.

    • Involves heating liquids to a moderately high temperature for a short time to kill foodborne pathogens (like Coxiella burnetii) without ruining the taste or texture of the food.

    • Pasteurization does not sterilize: Standard pasteurized milk can still contain about 20,00020{,}000 non-pathogenic bacteria per 1 mL1\,\text{mL}.

    • Ultra-High-Temperature (UHT) Treatment: Uses very high heat for a few seconds to achieve true sterility, allowing milk to stay fresh unrefrigerated for up to 6 months6\,\text{months}.

  • Cold Temperatures:

    • Cold is used to slow down microbial growth (temper growth) and preserve lab strains.

    • Most human pathogens are mesophiles (growing best at moderate temperatures). Their growth slows significantly or stops completely inside standard refrigerators (4−8 ∘C4-8\,^\circ\text{C} or 39−43 ∘F39-43\,^\circ\text{F}).

    • Important Exception: Listeria monocytogenes can continue to grow at low refrigeration temperatures, making it a serious food-safety hazard in deli meats and dairy.

  • Filtration:

    • Used for heat-sensitive liquids like liquid pharmaceutical medications, antibiotic solutions, and vitamin mixes.

    • Solutions pass through fine membrane filters with pore sizes small enough to physically trap microbes while letting liquid through:

    • Bacterial Filters: Pore size of 0.2 μm0.2\,\mu\text{m} stops bacteria.

    • Viral Filters: Pore size of 20 nm20\,\text{nm} stops viruses (1 nm=0.001 μm1\,\text{nm} = 0.001\,\mu\text{m}).

  • Irradiation:

    • Uses electromagnetic radiation to sterilize foods and single-use supplies.

    • Types of energy used: Ultraviolet (UV) light, gamma rays, electron beams, and X-rays.

    • Water molecules in the cells absorb the energy, causing free-radical production and severe DNA damage.

    • Highly effective against bacteria and parasites.

    • Limitation: Does not work against viruses or prions.

4. Chemical Disinfection and Antisepsis

  • Factors Affecting Chemical Disinfectant Potency:

    • Organic Matter: Dirt, blood, and fluids bind chemicals and use them up. Surfaces must be physically cleaned before applying a chemical disinfectant.

    • Types of Organisms: Spores and mycobacteria require strong sterilants, while enveloped viruses and Gram-positive bacteria die easily from lower-level germicides.

    • Corrosiveness: Chemicals should not corrode metals, melt plastics, or damage living skin.

    • Stability, Odor, and Surface Tension: Disinfectants must remain stable in storage, have low toxicity/odor, and have low surface tension so they can seep into cracks and spread evenly.

  • Specialized Chemical Methods:

    • Gas Sterilization (Ethylene Oxide - EtO):

    • Used for delicate, heat- and moisture-sensitive plastic medical supplies (such as plastic Petri dishes, surgical sutures, IV catheters, and syringes).

    • Acts as a strong alkylating agent that chemically cross-links proteins and DNA.

    • Microbicidal and sporicidal; penetrates through wrapped plastic and paper packaging.

    • Antimicrobial Touch Surfaces:

    • Incorporating metallic copper (Cu\text{Cu}) or copper alloys into high-touch surfaces in hospitals (like bed rails, door handles, and IV poles).

    • Continuously releases toxic copper ions (Cu+\text{Cu}^+) that damage bacterial membranes, create reactive oxygen species, and kill microbes.

  • Bacterial Resistance to Chemical Disinfectants:

    • Bacteria rarely develop broad resistance to chemical disinfectants because disinfectants attack many parts of the cell at once.

    • Low or sub-lethal concentrations can trigger resistance through:

    • Target Site Mutations: Low doses of triclosan can cause mutations in the bacterial fatty acid synthesis enzyme (fabI), leading to high resistance.

    • Multidrug Efflux Pumps: Bacteria can turn on active export pumps (such as the acrAB-tolC system) that pump out disinfectants. This can also cause cross-resistance to medical antibiotics.

    • Biofilms: Microbes encased in slimy extracellular polymers create a physical shield that keeps disinfectants from reaching dormant cells inside.

5. Concepts of Antibiotic Therapy

  • Selective Toxicity and Drug Classes:

    • Disinfectants are too toxic for internal use. Antibiotics, however, can be taken internally.

    • Antibiotics: Natural chemical substances made by certain microbes that kill or inhibit other competing microbes.

    • Chemotherapeutic Agents: Antimicrobials that are either synthesized entirely in a chemical lab or semi-synthetically modified from natural antibiotics to improve efficacy.

    • Selective Toxicity: The concept of a "magic bullet" (introduced by Paul Ehrlich). An ideal drug harms the infectious microbe without harming the patient by targeting structures or processes unique to bacteria (such as peptidoglycan walls and bacterial folic acid synthesis).

    • Spectrum of Activity:

    • Narrow-Spectrum: Targets a small, specific group of microbes (e.g., only Gram-positive bacteria).

    • Broad-Spectrum: Targets a wide variety of both Gram-positive and Gram-negative organisms.

  • Pharmacokinetics, Tissue Half-Life, and MIC:

    • Minimum Inhibitory Concentration (MIC): The lowest drug concentration needed to prevent visible bacterial growth.

    • Clinical Success: The concentration of an active antibiotic in tissue must remain higher than the MIC at all times during the dosing schedule.

    • Half-Life Factors:

    • Kidney Clearance: How fast the drug is filtered and excreted through the urine.

    • Liver Clearance: How fast liver enzymes break down and metabolize the drug.

  • Drug Safety and Interactions:

    • Therapeutic Dose: The minimum drug amount per kilogram of body weight needed to stop the pathogen from growing.

    • Toxic Dose: The maximum drug amount the patient can tolerate before serious side effects occur.

    • Chemotherapeutic Index: The ratio of toxic dose to therapeutic dose:
      Chemotherapeutic Index=Toxic DoseTherapeutic Dose\text{Chemotherapeutic Index} = \frac{\text{Toxic Dose}}{\text{Therapeutic Dose}}

    • A higher number means a safer drug.

    • Example A: Toxic dose = 11, Therapeutic dose = 0.10.1. Ratio is 1010 (safe/good).

    • Example B: Toxic dose = 0.50.5, Therapeutic dose = 0.50.5. Ratio is 11 (narrow safety margin/bad).

    • Drug Combinations:

    • Synergistic: Drugs work together to produce a stronger effect (e.g., aminoglycosides combined with vancomycin).

    • Antagonistic: Drugs interfere with each other, reducing overall effectiveness (e.g., combining bacteriostatic macrolides with bactericidal penicillins).

6. Survey of Antibacterial Drug Classes and Mechanisms

  • Cell Wall Inhibitors (Peptidoglycan Synthesis):

    • Beta-Lactam Antibiotics: Include penicillins and cephalosporins (available across five generations).

    • Contain a core beta-lactam ring that binds to transpeptidase enzymes (penicillin-binding proteins) responsible for linking cell wall peptides.

    • Without strong cross-links, bacterial cell walls burst under osmotic pressure (bactericidal).

    • Resistance: Many bacteria produce beta-lactamase enzymes that break open the beta-lactam ring and destroy the drug.

    • Other Cell Wall Inhibitors: Vancomycin, bacitracin, and monobactams.

  • Cell Membrane Disrupters:

    • Gramicidin: Composed of alternating D- and L-amino acids that insert into membranes to form open cation channels, destroying cell membrane polarity.

    • Polymyxins: Act like detergents that break apart phospholipid interactions. Highly toxic to human kidney and nerve cells, so they are primarily used in topical ointments.

  • DNA Replication and Integrity Inhibitors:

    • Metronidazole: A prodrug that is harmless until reduced by specific metabolic cofactors found only in anaerobic microbes. Once activated, it cuts DNA randomly, killing the cell. Aerobes and human cells lack the cofactors to activate it.

    • Quinolones: Block bacterial DNA gyrase and topoisomerases, stopping the progression of DNA replication forks.

  • RNA Polymerase Inhibitors:

    • Rifampin and Myxopyronins: Bind to bacterial RNA polymerase to block messenger RNA (mRNA) transcription.

  • Protein Synthesis Inhibitors (Targeting 70S Ribosomes):

    • Bacterial 70S ribosomes consist of 30S and 50S subunits, while human ribosomes are 80S. This allows for selective toxicity. Most protein synthesis inhibitors are bacteriostatic.

    • 30S Subunit Inhibitors:

    • Aminoglycosides (e.g., gentamicin): Broad-spectrum and bactericidal; cause errors in mRNA reading and block tRNA translocation.

    • Tetracyclines (e.g., doxycycline): Broad-spectrum and bacteriostatic; bind the 30S subunit to physically block charged transfer RNAs (tRNAs) from entering the A site.

    • 50S Subunit Inhibitors:

    • Chloramphenicol: Broad-spectrum and bacteriostatic; blocks peptidyltransferase, preventing new peptide bonds from forming.

    • Macrolides (e.g., erythromycin, clindamycin): Bacteriostatic; mainly target Gram-positive bacteria by blocking tRNA movement from the A site to the P site.

    • Oxazolidinones (e.g., linezolid): Bacteriostatic; reserved as a last resort against resistant Gram-positive pathogens; prevents the 30S and 50S subunits from joining into a functional 70S complex.

    • Streptogramins (e.g., quinupristin-dalfopristin): Inhibit protein assembly.

  • Metabolic Inhibitors (Antimetabolites):

    • Sulfonamides and Trimethoprim: Structurally mimic para-aminobenzoic acid (PABA) to competitively inhibit bacterial folic acid synthesis.

    • Because bacteria must synthesize folic acid internally while humans absorb it from food, these drugs are selectively toxic.

7. Challenges of Antibiotic Resistance

  • Mechanisms of Antibiotic Resistance:

    • Preventing Entry: Destroying the antibiotic before it enters (e.g., beta-lactamase), pumping it out immediately, or reducing outer membrane permeability.

    • Preventing Target Binding: Mutating the bacterial target structure or chemically adding groups to the drug to prevent binding.

    • Dislodging the Antibiotic: Using protective proteins to free the antibiotic from the ribosome.

    • Preventing Accumulation: Expressing multidrug resistance (MDR) efflux pumps that actively push drugs out of the cytoplasm.

  • How Resistance Develops and Spreads:

    • Spontaneous Genetic Mutations: Random mutations that occur during replication can give a survival advantage in the presence of an antibiotic.

    • Horizontal Gene Transfer: Plasmids carrying resistance genes spread quickly between different bacteria through conjugation, transformation, or transduction.

    • Overuse and Misuse: Over-prescribing antibiotics when not medically necessary kills off sensitive strains and leaves resistant mutants to grow.

    • Agricultural Use: Adding subtherapeutic levels of antibiotics to animal feed turns farm animals into incubators for antibiotic-resistant bacteria.

  • Strategies to Overcome Resistance:

    • Dummy Decoy Compounds: Co-administering beta-lactamase inhibitors (such as clavulanic acid) alongside beta-lactam drugs. The decoy binds the bacterial beta-lactamase, leaving the antibiotic free to destroy the bacterial cell wall.

    • Chemical Modification: Altering existing antibiotic structures in the laboratory to prevent bacterial enzymes from recognizing or destroying them.

8. Antiviral and Antifungal Chemotherapy

  • Antiviral Drugs:

    • Viruses are obligate intracellular parasites that use host enzymes, ribosomes, and pathways to replicate. Finding targets that kill the virus without harming human host cells is extremely difficult.

    • Major antiviral targets:

    • Viral Uncoating and Entry: Blocking cell receptors or endosomal low-pH uncoating (e.g., amantadine, rimantadine).

    • Viral DNA/RNA Synthesis: Nucleoside analogues (e.g., acyclovir) that selectively block viral polymerases and terminate nucleic acid chains.

    • Reverse Transcriptase Inhibitors: Nucleoside and non-nucleoside reverse transcriptase inhibitors (NRTIs and NNRTIs) that stop retroviral DNA synthesis in HIV.

    • Viral Release and Maturation: Inhibiting viral proteases or surface neuraminidase enzymes (e.g., oseltamivir) to trap new virions at the host cell surface.

  • Antifungal Drugs:

    • Because fungi are eukaryotic cells, their protein synthesis, DNA replication, and cellular components are very similar to human cells.

    • The primary target for antifungal drugs is ergosterol, a sterol found exclusively in fungal cell membranes (similar to cholesterol in human membranes).

    • Major drug classes targeting ergosterol:

    • Azoles (e.g., fluconazole, itraconazole, ketoconazole): Inhibit lanosterol 14-alpha-demethylase, blocking ergosterol synthesis and causing toxic intermediate sterols to accumulate.

    • Allylamines (e.g., terbinafine): Inhibit squalene epoxidase, an early step in ergosterol synthesis, causing squalene buildup and membrane failure.

    • Polyenes (e.g., amphotericin B, nystatin): Bind directly to ergosterol in fungal membranes to form pore channels that leak potassium (K+\text{K}^+) ions and essential cell contents, causing the cell to lyse.