chapter 13

how can microbial load be reduced?

washing, vacuuming and
dishwashing

Why clean and not sterilize everything?

Sterilization often requires time, is labor intensive and, may
degrade the quality of the item being treated, or even have
toxic effects on users

biosafety levels ranked by?

severity of disease and ease of transmisison

BSL-4

dangerous and exotic posing high risk of aerosol transmitted infections, which are frequently fatal without treatments or vaccines

Ebola and Marbug virus

BSL3

microbes are indigenous or exotic and cause serious or potentially lethal diseases through respiratoru transmission

ex. Mycobacterium tuberculosis

BSL-2

typically indigenous and are associated with diseases of varying severity

pose moderate risk to workers and environment

ex. staphylococcus aureus

BSL-1

not known to cause disease in healthy host and pose minimal risk to workers and environment

ex. nonpathogenic strains of e coli

sterilization

Process by which ALL living cells, spores, and viruses are
destroyed on an object.

disinfection

The killing or removal of DISEASE-PRODUCING organisms
from inanimate surfaces; it does not necessarily result in
sterilization.

antisepsis

Similar to disinfection but applies to removing pathogens from the
surface of LIVING tissues, such as skin

sanitation

Consists of reducing the microbial population to safe levels
and usually involves cleaning an object as well as disinfection.

antimicrobials fall into 2 broad classes:

cidal agents

static agents

cidal agents

kill microbes
Bactericidal, algicidal, fungicidal, virucidal,
depending on what type of microbe is killed.

static agents

inhibit or control growth
Bacteriostatic, algistatic, fungistatic, virustatic,
depending on what type of microbe is inhibited.

selection of disinfectant

. 1. Must be fast-acting in the presence of organic materialsaterials
2. Must be effective against all microorganisms without
destroying tissue or acting as a toxin if ingested.
3. Easily penetrate the material to be disinfected without
discoloration or damage.
4. Easy to prepare and stable in the environment where it
is to be used.
5. Inexpensive and easy to use.
6. Not have an unpleasant odor.

Several factors influence the speed at which lethal
damage accumulates

• the initial population size (The larger the population, the longer it
takes to decrease it to a specific number.)
• population composition (i.e., are spores involved?)
• agent concentration or dose for radiation
• duration of exposure
• presence of organic material (blood, feces) that can inhibit
disinfectant action; organic loads

physical agent control measures

temperature extremes
pressure (usually combined with temperature)
filtration
irradiation

what type of heat is very effective at killing microbes?

moist heat

dry heat is less effective but sometimes required

To kill spores and thermophiles, a combination of… is usually required

heat and pressure

Standard autoclave conditions are…

121°C at 15 psi
(pounds per square inch) for 20 minutes.

Heat killing

Preferred sterilizing agent as long as it doesn’t damage the
materials

Thermal death point (TDP)

temperature that kills all the bacteria
in a 24 hour old culture/ 10 min

Thermal death time (TDT)

Time required to kill all the bacteria in
a culture at a specified temperature.

Decimal reduction time (D value)

length of time required to kill
90% of the organisms in a population at a specific temperature

goal of pasteurization

not to sterilize, but to kill
pathogens without affecting the texture, color, or taste of
the product.

Different Time and Temperature combinations can be
used with pasteurization:

long temperature, long time (LTLT)

▪ High temperature, short time (HTST)
▪ Ultra high temperature (UHT)

Refrigeration, freezing

Cold
Refrigeration, freezing
•excellent to preserve food and other materials - retards
microbial growth
•but does not effectively kill microorganisms.
•Microorganisms can be stored in this way.

Freeze-drying

•many microorganisms are sensitive to drying - but, many are
not.
•Freeze drying is used for storing many microorganisms

Filtration

Filtration
• Many drugs/chemicals are sensitive to heat or other
chemical sterilization methods.
• These solutions can be sterilized by passing them
through sterile filters with tiny pore sizes that
effectively “sift” the microbes out of the fluid.

filtration through micropore filters of ____ can remove microbial cells, but not viruses, from solutions

0.2 microliters

to remove viruses, pore sizes of ____ are necessary

20 nm (1 nm = 0.001 microliters)

Sonication

High frequency ultrasound waves to disrupt cell
structures
• The disruption is achieved due to the rapid changes in
pressure within the intracellular liquid

irradiation

Method in which objects are bombarded with high-
energy electromagnetic radiation.
▪ Foods do not become radioactive when irradiated,
and any reactive molecules produced when high-
energy particles are absorbed by food dissipate
almost immediately.
▪ Ultraviolet (UV) light
▪ Gamma rays, electron beams, X-rays

Efficacy of a given chemical agent depends upon

1. presence of organic matter

2. kinds of organisms present

3. corrosiveness

4. stability, odor, and surface tension

the presence of organic matter

Chemicals will bind to inert organic material, lowering its
effectiveness against microbes

the kinds of organisms present

Ideally should be effective against a broad range of
pathogens.

Corrosiveness

Should not corrode the surface (nonliving or living)

stability, odor, and surface tension

Should be stable upon storage, neutral or pleasant odor, low
surface tension

ethanol, iodine, chlorine

highly reactive compounds that damage proteins, lipids,
and DNA

surfactants (such as detergents)

help in the mechanical removal of microbes from surfaces

aldehydes

combine with and inactivate proteins and nucleic acids

Phenolics

denature proteins and disrupt membranes

Heavy Metals

bind to proteins and inhibit enzymatic activity

Peroxygens

strong oxidizers, produce free radicals that damage cellular
macromolecules

Gas Sterilization

Disposable plasticware
These types of products are best sterilized by gamma
irradiation or antimicrobial gases, such as ethylene
oxide (EtO).
• destroys proteins by alkylation
microbicidal/sporicidal
rapidly penetrates packing materials, including plastic
wraps
highly explosiv

Bacterial resistance to disinfectant

Bacteria can develop resistance to chemical disinfectants
used to prevent infections.
• It is difficult for bacteria to develop resistance to
chemical agents that have multiple targets and can
easily diffuse into a cell (example: iodine).
• Disinfectants that have multiple targets at high
concentrations may only have a single target at low
concentrations—a situation that can foster resistance
(example: triclosan).