Microbial Control Physical Means
Nestor T. Hilvano, M.D., M.P.H.
Images Copyright by Bauman, Robert. 2009. Microbiology, With
Diseases by Taxonomy, 3rd edition, Pearson Benjamin Cummings
Learning Objectives
You should be able to:
1. Contrast sterilization, disinfection, antisepsis, sanitization
, and pasteurization .
2. Compare the effects of "-static" versus "-cidal" control
agents on microbial growth.
3. Define microbial death rate, and describe its significance
in microbial control.
4. Describe how antimicrobial agents act against cell
boundaries, proteins, and nucleic acids.
5. Identify the three most-resistant groups of microbes, and
explain why they are resistant to many antimicrobial
agents.
6. Describe types of physical methods of microbial control
and discuss their advantages and disadvantages.
Microbial Control
• Antisepsis – reduction in number of microorganisms and
viruses on living tissue; ex: alcohol, iodine
• Disinfection – destruction of most microorganisms and
viruses on non-living tissue; ex: alcohol, phenol
• Pasteurization – use of heat to destroy pathogen; ex:
pasteurized milk and fruit juice
• Sanitation – removal of pathogens from objects to meet
public health standards; ex: washing table wares in
restaurants
• Sterilization – destruction of all microorganisms and
viruses in or on an object; by autoclave, incineration, or
ethylene oxide gas; ex: preparing culture media
Microbial Death Rates
• Microbial death –
permanent loss of
reproductive ability
• MDR is constant over
time for any microbes
under a particular set of
conditions
• Ex in graph: 90%/min.;
requires 9 mins. to
achieve sterilization
Physical Methods of Microbial Control
1.
2.
3.
4.
5.
6.
Heat – moist; dry
Low temperature – refrigeration; freezing
Filtration
Desiccation
Osmotic pressure
radiation
• Heat – oldest and
Heat
common means
• Thermal death point lowest temp. that kills all
cells in10 mins.
• Thermal death time –
minimum time required to kill
all organisms at given temp.
(ex: 22.5 mins)
• Decimal reduction time
(D) - time required to kill
90%of organisms; useful in
food processing to eliminate
endospores of C. botulinum
Moist Heat
Moist Heat – used to disinfect, sanitize, sterilize, &
pasteurize:
1.Boiling – vegetative bacteria & fungi, trophozoites
of protozoas,and most viruses in 10 mins.
2. Autoclave – uses steam heat under pressure
(121˚C, 15 psi at 15 mins.) to sterilize chemicals
and objects.
3.Pasteurization – heating food/beverages to kill
pathogens and control spoilage organisms.
Dry Heat
• Dry Heat – at higher temp. for longer
times:
1. Oven - 160˚C for 2 hrs., for powders and
oils
2. bunsen burner
3. complete incineration
Refrigeration and Freezing
• Refrigeration = between 0˚C and 7˚C, halts
growth of most pathogens
• Slow Freezing = below 0˚C , inhibits
microbial metabolism; but many vegetitive
bacterial cells, bacterial endospores, and
viruses can survive for years
Desiccation & Lyophilization
• Desiccation – drying to preserve foods;
inhibits growth because metabolism
requires liquid water
• Lyophilization – freeze-drying; preserve
microbes and other cells for many years;
freeze culture in liquid nitrogen or frozen
CO2, then remove water via a vacuum
Filtration & Osmotic Pressure
• Filtration – passage of air
or liquid through material
that traps and removes
microbes; 0.3 µm for O.R.;
0.22 µm for bacteria; 0.01
µm for viruses
• Osmotic pressure – use of
high concentration of
salt/sugar; drawing out of
water; ex. honey, jams,
salted fish, and pickles
Radiation
1. Ionizing radiation – wavelengths shorter than 1
nm; electron beams, gamma rays, X-rays;
effects leading to denaturation (destroy DNA);
medical/lab equipment sterilization
2. Nonionizing radiation – wavelengths longer
than 1 nm; UV light, visible light, infrared light,
radio waves; UV causes pyrimidine dimers
which kills cells; sterilization of surfaces and of
transparent fluids and gases
3. Microwaves – heat kills microbes; Trichinella
spiralis in pork resist heating
Homework
1.
2.
3.
4.
5.
Define terms: sterilization, disinfection, antisepsis,
sanitation, bacteriostatic, and bactericidal.
Compare moist heat and dry heat method of microbial
control and discuss advantages and disadvantages.
Compare uses/advantages of microbial control of
desiccation and lyophilization; and filtration and
osmotic pressure.
Compare ionizing and non-ionizing radiation.
What is microbial death rate? Decimal reduction time?