Unit 4: Control of microorganisms

Microbial control means reducing or eliminating microbial populations to prevent infection, spoilage or contamination. Practices range from the physical (Pasteur's heat treatment of wine, 1864) to the chemical (Lister's carbolic-acid antisepsis, 1867) and the pharmacological (Fleming's penicillin, 1928). All later sections rest on a shared vocabulary and on the fact that no single method is universally effective; the choice depends on the target organism, the surface, and the tolerable damage.

• Sterilization: complete destruction or removal of all viable microbes including endospores, e.g. autoclaving at 121 °C.
• Disinfection: killing of vegetative pathogens on inanimate objects; spores may survive (phenol on floors).
• Antisepsis: disinfection applied to living tissue (iodine on skin).
• Sanitization: lowering microbial counts to safe public-health levels (hot-water washing of cutlery).
• -cidal vs -static: a bactericidal agent kills; a bacteriostatic agent only halts growth, so removal allows regrowth.
• Death is logarithmic: a fixed fraction of the population dies per unit time, giving the decimal reduction time (D-value) — the minutes needed to kill 90% of cells at a set temperature.

II. Conditions Influencing Effectiveness

Factors that decide whether an agent works

Whether an antimicrobial succeeds depends less on the agent alone than on the conditions under which it acts.

• Population size: larger populations take longer to kill because a constant fraction dies per interval — 10⁶ cells need more D-value cycles than 10³.
• Population composition: endospores (Bacillus, Clostridium), mycobacteria with waxy mycolic acids, and Pseudomonas are far more resistant than ordinary vegetative cells.
• Concentration/intensity of agent: usually higher is more effective, but not linearly — 70% ethanol kills better than 95% because water is needed to denature protein.
• Contact time: longer exposure kills more; sterilization protocols specify both dose and time (e.g. 121 °C for 15 min).
• Temperature: raising temperature speeds chemical killing; many disinfectants act faster when warm.
• Local environment:
  • Organic matter: blood, faeces and biofilm shield microbes and neutralize agents like chlorine.
  • pH: affects ionization; some agents penetrate cells only in a particular charge state.
  • Biofilm: embedded cells resist penetration and tolerate far higher doses.

III. Characteristics of Ideal Antimicrobial Agents

The benchmark against which real agents fall short

An ideal chemical control agent meets a set of properties no real product fully satisfies; the list is a selection guide.

• Broad activity at low concentration: effective against bacteria, fungi and viruses in small doses.
• Soluble and stable: dissolves in water or lipid, retains potency in storage.
• Non-toxic to hosts: harmless to human tissue and animals — the property antiseptics most often lack.
• Penetrating: reaches microbes through organic films and surface irregularities.
• Non-corrosive and non-staining: does not damage metals, fabrics or surfaces.
• Compatible and residual: not inactivated by soaps or organic matter, and leaves lasting protection.
• Deodorizing and cheap: controls odour and is economical for large-scale use.

IV. Physical and Chemical Agents for Controlling Microorganisms

Two families of control: what they are and how they act

Physical agents alter the microbe's environment (heat, radiation, filtration); chemical agents are reactive molecules applied as disinfectants or antiseptics.

1. Physical agents

• Moist heat: denatures proteins and melts membranes.
  • Autoclave: saturated steam at 121 °C, 15 psi, 15 min — sterilizes, kills spores.
  • Pasteurization: 63 °C/30 min or 72 °C/15 s (HTST) — reduces pathogens without sterilizing.
  • Boiling: 100 °C/10 min kills vegetative cells, not all spores.
• Dry heat: oxidation; hot-air oven at 160–170 °C for 2 h; incineration for loops and carcasses.
• Radiation:
  • Ionizing (gamma, X-ray): breaks DNA, sterilizes plastics and food.
  • Non-ionizing (UV, 260 nm): forms thymine dimers in DNA; surface use only, poor penetration.
• Filtration: physically removes microbes; HEPA filters (0.3 µm) clean air; 0.22 µm membrane filters sterilize heat-labile fluids like sera.

2. Chemical agents

• Phenolics: denature protein and disrupt membranes; act with organic matter (Lister's phenol; hexachlorophene).
• Alcohols: 70% ethanol/isopropanol denature protein and dissolve lipids; no sporicidal action.
• Halogens: chlorine (hypochlorite) oxidizes; iodine/iodophors (Betadine) as skin antiseptics.
• Heavy metals: silver nitrate, mercurials — oligodynamic action, inactivate enzyme thiol groups.
• Aldehydes: glutaraldehyde and formaldehyde cross-link proteins and nucleic acids; sterilizing.
• Gases: ethylene oxide alkylates proteins — sterilizes heat-sensitive plastics and instruments.
• Surfactants (quats): quaternary ammonium compounds disrupt membranes; effective on Gram-positives.

V. Antibiotics and Therapeutic Agents: Historical Highlights

The origins of chemotherapy

Chemotherapy is the systemic use of drugs against pathogens inside the host; its history moves from synthetic dyes to natural microbial products.

• Paul Ehrlich (1910): coined the "magic bullet" concept and introduced Salvarsan (arsphenamine), the first synthetic drug against syphilis.
• Gerhard Domagk (1935): discovered Prontosil, a sulfonamide dye active against streptococci — the first broadly useful synthetic antibacterial.
• Alexander Fleming (1928): observed Penicillium notatum inhibiting Staphylococcus on a contaminated plate — the discovery of penicillin.
• Florey and Chain (1940–41): purified penicillin and proved its clinical value, opening the "antibiotic era".
• Selman Waksman (1943): coined the term antibiotic and isolated streptomycin from Streptomyces griseus — the first drug effective against tuberculosis.

VI. Characteristics of Antimicrobial Drugs and Mechanism

What defines a good drug and how drugs act

An effective antimicrobial drug exploits differences between microbe and host, giving selective toxicity — harming the pathogen more than the patient.

A. Characteristics of antimicrobial drugs

• Selective toxicity: measured by the therapeutic index = toxic dose ÷ therapeutic dose; higher is safer.
• Spectrum: narrow-spectrum (isoniazid, TB only) vs broad-spectrum (tetracycline, many taxa).
• Cidal or static: bactericidal (penicillin) vs bacteriostatic (sulfonamides); measured by MIC (minimum inhibitory concentration) and MBC (minimum bactericidal concentration).

B. Mechanisms of action

• 1. Inhibition of cell-wall synthesis: β-lactams (penicillins, cephalosporins) block transpeptidase, preventing peptidoglycan cross-linking; vancomycin binds D-Ala-D-Ala. Selective because human cells lack walls.
• 2. Disruption of the cell membrane: polymyxins bind lipopolysaccharide and increase permeability, causing leakage.
• Inhibition of protein synthesis: exploits the 70S bacterial ribosome vs the host 80S.
  • Aminoglycosides (streptomycin) and tetracyclines act on the 30S subunit.
  • Chloramphenicol, macrolides (erythromycin) act on the 50S subunit.
• Inhibition of nucleic-acid synthesis: fluoroquinolones inhibit DNA gyrase; rifampin inhibits bacterial RNA polymerase.
• Antimetabolites: sulfonamides competitively block dihydropteroate synthase; trimethoprim blocks dihydrofolate reductase — together they arrest folic-acid synthesis, a pathway humans lack.

VII. Antibiotics and Therapeutic Agents: Development of Resistance

How microbes escape drugs and how resistance spreads

Resistance is the ability of a microbe to grow despite a drug once effective against it; it arises genetically and spreads through selection.

A. Origins of resistance

• 1. Chromosomal mutation: spontaneous change in a target gene, e.g. altered RNA polymerase conferring rifampin resistance; vertically inherited.
• 2. Acquired (horizontal) transfer: resistance genes carried on R-plasmids move between cells by conjugation, transformation or transduction, spreading rapidly across species.

B. Biochemical mechanisms

• Drug inactivation: enzymes destroy the drug — β-lactamase hydrolyses the penicillin β-lactam ring.
• Altered target: modified penicillin-binding proteins in MRSA; ribosomal methylation blocks macrolides.
• Reduced uptake / efflux pumps: porin loss or active pumping lowers intracellular drug, as in tetracycline resistance.
• Bypass pathway: an alternative enzyme replaces the inhibited step.

C. Drivers and consequences

• Selective pressure: overuse, incomplete courses and agricultural use kill susceptible cells, leaving resistant ones to multiply — natural selection in action.
• Multidrug resistance: R-plasmids can carry several genes, producing "superbugs" such as MRSA and MDR-TB.
• Countermeasures: combination therapy, β-lactamase inhibitors (clavulanic acid), narrow-spectrum choice, and completing full courses to prevent survivors.