13.2 Physical Means of Control

What do the suffixes -cides and -static indicate?

-cides kill microbes, while -static stop their growth.

What is the difference between bacteriocidal and bacteriostatic?

Bacteriocidal kills bacteria, while bacteriostatic stops their growth.

What is the difference between viricidal and viristatic?

Viricidal kills viruses, while viristatic stops their growth.

What is the difference between fungicidal and fungistatic?

Fungicidal kills fungi, while fungistatic stops their growth.

What can be observed to measure the degree of microbial control?

The microbial death curve.

What factors affect the success of microbial control?

Length of exposure, concentration of agent, and population level.

Percentage of kill in microbial death curve

Measure of percentage of kill

Decimal reduction time (DRT)

How much time it takes to kill 90% (1 log reduction) of population

1 x 10^6 -> 1 x 10^5 CFU/mL

represents a 1-log reduction, which is equivalent to a 90% decrease in the viable microbial population

Physical Means of Control

Most are applied to non-living (i.e. fomites)

Types of Physical Means of Control

1. Temperature 2. Radiation 3. Filtration 4. Desiccation 5. Pressure

Heat Sterilization

Oldest and most common method of sterilization.

Thermal Death Point

Lowest temp that will kill in 10 min.

Thermal Death Time

Length of time to kill at a certain temperature.

Dry Heat

Incineration; direct application of high heat (>250C) ex: bunsen burner

Moist Heat

Application of high temperature liquid/vapor; beneficial because it penetrates cells better than dry heat ex: autoclave

Autoclave

Raises temperature of water above boiling point (~121C) by raising pressure to 15 psi for ~20 minutes; kills viruses & endospores

Pasteurization

Semi-sterilizes food but does not ruin food quality; many methods rely on 'flash' heating foods to kill most microbes

pressure sterilization Pascalization

High pressure used in food industry to kill microbes

pressure sterilization Hyperbaric chambers

Can be used to treat infections by inducing high levels of oxygen to saturate infection site

Autoclaves with pressure is ____

Used in combination with temperature for sterilization

Desiccation

AKA dehydration; creates difference in osmotic pressure through salt or lyophilization to remove water

High energy radiation

Used to kill or inhibit microbes

Ionizing radiation

Enters into cells and disrupts molecular structures such as DNA

X-rays & gamma rays

Can be used to sterilize non-autoclavable items and can be an alternative to pasteurization in canned foods

Non-ionizing radiation

Does not penetrate glass, plastics, etc. but can damage cells with direct exposure.

UV irradiation

Forms thymine dimers in DNA causing lethal mutations.

Sonication

High frequency sound waves to disrupt cell structure; causes bubbles to form inside cells and induce lysis.

Filtration

Use of barrier to physically separate microbes; useful when media cannot be autoclaved (e.g., Urea broth).

HEPA filters

Capable of filtering to virus level.

Filtration pore size

Filters usually have pore size of 0.2 µm (or smaller for viruses).