Chapter 11 - Physical and Chemical Agents for Microbial Control

Early protection attempts laid the foundation for microbial control methods today:

• Burning wood releases formaldehyde

• Herbs, perfume, and vinegar contain mild antimicrobial substances

Decontamination

Physical, chemical, and mechanical methods to destroy or reduce undesirable microbes in a given area

Primary targets of decontamination are microorganisms capable of causing infection or spoilage:

• Vegetative bacterial cells (metabolically active) and endospores

• Fungal hyphae and spores, yeast

• Protozoan trophozoites and cysts

• Worms

• Viruses

• Prions (misfolded or infectious protein)

Antisepsis

Application of chemical agents (antiseptics) directly to exposed body surfaces, wounds, and surgical incisions to destroy or inhibit vegetative pathogens

Disinfection

Use of a physical process or a chemical agent (disinfectant) to destruction or removal of vegetative pathogens but not bacterial endospores

• Usually used only on inanimate objects

Sterilization

The complete removal or destruction of all viable microorganisms

• Used on inanimate objects

Highest resistance of microbes

• Prions

• Bacterial endospores (Bacillus, Clostridium)

Moderate resistance of microbes

• Protozoan cysts; naked viruses

• Bacteria with no endospores but resistant walls:

  • Mycobacterium, Pseudomonas, Acinetobacter, other gram (-)

  • Staphylococcus (most heat- and chemical-resistant bacteria)

Least resistance or microbes

• Most bacterial vegetative cells

• Fungal spores and hyphae

• Yeasts

• Enveloped viruses

• Protozoan trophozoites

Microbicidal agents (germicides)

Antimicrobial agent aimed at destroying a certain group of microorganisms (bactericide, fungicide, virucide, sporicidal)

Agents that cause microbistasis

Antimicrobial agent aimed at temporarily prevent microbes from multiplying

Sanitation

Any cleansing technique that removes microorganisms from inanimate surfaces to reduce the potential for infection and spoilage

Degermation

Reduction of microbial load from living tissue by mechanical means

Microbial death

Permanent loss of reproductive capability, even under optimum growth conditions

Hard to detect, microbes often reveal…

no conspicuous vital signs to begin with

Factors that affect microbial death:

• Number of microbes

• Nature of microbes in the population

• Temperature and pH of environment

• Concentration or dosage of agent

• Mode of action of the agent

• Presence of solvents, organic matter, or inhibitors

Selection of method of control depends on circumstances:

• Does the application require sterilization?

• Is the item to be reused?

• Can the item withstand heat, pressure, radiation, or chemicals?

• Is the method suitable?

• Will the agent penetrate to the necessary extent?

• Is the method cost- and labor-efficient and is it safe?

Cellular targets of physical and chemical agents:

• Cell wall

• Cell membrane

• Protein and nucleic acid synthesis

• Protein function

Cell wall and physical/ chemical agents:

becomes fragile and cell lyses 

(some antimicrobial drugs, detergents, and alcohol)

Cell membrane and physical/ chemical agents:

loses integrity

(surfactants)

Protein and nucleic acid synthesis and physical/ chemical agents:

prevention of replication, transcription, translation, peptide bond formation, protein synthesis

(chloramphenicol, ultraviolet radiation, formaldehyde)

Protein function and physical/ chemical agents:

disrupt or denature proteins

(alcohols, phenols, acids, heat)

The vast majority of microbes are readily controlled by abrupt changes in their environment.

Some physical methods are:

• Heat (moist and dry)

• Cold temperatures

• Desiccation

• Radiation

• Filtration