Chapter 9 - Physical & Chemical Control of Microbes

Controlling microorganisms - outcomes

• The methods of microbial control (used outside of the host body) are designed to result in 4 possible outcomes:

  • 1. Sterilization

  • 2. Disinfection

  • 3. Antisepsis (also called degermation)

  • 4. Decontamination (also called sanitization)

1. Sterilization

• A process that destroys or removes all viable microorganisms, including bacterial endospores & all viruses

  • Doesn’t include removal of prions

• Only result that’s 100% removal

2. Disinfection

A process that destroys vegetative pathogens (anything other than spores or cysts) but not bacterial endospores

3. Antisepsis/degermation

A process that reduces the number of microbes on the human skin (so goal is not to destroy all of them)

4. Decontamination/sanitization

• A process that mechanically removed microorganisms as well as other debris to reduce contamination to safe levels

• Different than antisepsis b/c it mechanically removes (outside human body)

• Considered clean, but not sterile

Relative resistance of different microbial types to microbial control agents

• Prions = most resistant (made up of just proteins, not considered microbes)

• Microorganisms:

  • 1. Bacterial endospores

  • 2. Mycobacterium (due to their acid-fast cell walls)

  • 3. Protozoa (cysts & trophozoites)

  • 4. Gram-negative bacteria

  • 5. Fungi & fungal spores

  • 6. Non-enveloped viruses (naked; only capsid layer)

  • 7. Gram-positive bacteria

  • 8. Enveloped viruses (alcohol easily damages envelope)

Sterilization & disinfection agent used in the processes…

• Bactericide

• Fungicide

• Virucide

• Sporicide

• Germicide/microbicide

“cide” = kill

Bacteristatic vs. Bactericidal effect

• A cidal agent does not necessarily result in sterilization → it just needs to have the intent to kill

  • Often applies high temps

• A static agent means to stop the growth of microbes without killing them

  • Often applies low temps

Devices for microbial control

1. Critical medical devices

2. Semicritical devices

3. Noncritical devices

1. Critical medical devices

• Expected to come into contact w/ sterile tissues

• Must be sterilized before use

2. Semicritical devices

• Come into control w/ mucosal membranes

• Must receive high-level disinfection, preferably sterilized

3. Noncritical devices

• Do not touch the patient or only touch intact skin

• Require only low-level disinfection, unless they become contaminated w/ blood or body fluids

Factors that influence action of antimicrobial agents

• 1. Number of microbes

  • Higher load of contaminants take longer to destroy

  • Microbial death continues in logarithmic manner as time or concentration is increased → active cells tend to die more quickly than less metabolically active cells

• 2. Nature of microorganisms in the population

  • Target population is usually a mixture of bacteria, fungi, spores, & viruses w/ broad spectrum of resistance

• 3. Temp & pH of environment

• 4. Concentration (dose & intensity) of agent

  • UV radiation is most effective at 260 nm

  • Most disinfectants are more active at higher concentrations

  • **Alcohol is the only one that still works well at lower concentration → also can’t be 100% concentration**

• 5. Mode of action of agent

  • How does it kill or inhibit microorganism (cidal vs. static)

• 6. Presence of solvents, interfering organic matter, & inhibitors

  • Saliva, blood, & feces can inhibit actions of disinfectants & even heat

  • So it makes them harder to kill

Modes of action of antimicrobial agents

• Antimicrobials have a range of cellular targets

• Cellular targets of physical & chemical agents:

  • 1. Cell wall

  • 2. Cytoplasmic membrane

  • 3. Cellular synthetic processes (DNA, RNA)

  • 4. Proteins

  • (Same chemical can have multiple effects → hit different targets)

Mode of action of Surfactants on the Cytoplasmic Membrane

• Alcohol-based hand sanitizers are a mainstay of disinfection of surfaces & antisepsis of hands during the Covid-19 pandemic

  • SARS-CoV-2 is an enveloped virus & its envelope is susceptible to both detergents & alcohol

Microbial control methods - summary chart

Methods of physical control - heat

• High temps are microbicidal, lower temps are microbistatic

• 2 types of heat:

  • 1. Moist heat

  • 2. Dry heat

Moist heat

• Penetrates microbial cell better → lower temps & shorter exposure times to achieve the same effectiveness as dry heat

• Microbicidal effect is the coagulation & denaturation of proteins

• Ex. hot water, boiling water, or steam b/w 60°C-135°C (applying pressure allows it to go above boiling, aka >100°C)

Dry heat

• Dehydrates the cell, removing water necessary for metabolic reactions

• Denatures proteins

• At very high temps, oxidizes cells, burning them to ashes (w/ incinerator)

• Ex. hot air or an open flame, which ranges from 160°C-several thousand °C

Comparison of time & temps to achieve sterilization w/ moist & dry heat