Control of microorganisms
Many bacteria cause disease
and food spoilage
Need exists to kill or inhibit
the growth of these bacteria
Frequently used terms
Sterilization - removal or destruction of all living cells, viable
spores, viruses and viriods
Disinfection - removal or destruction of pathogens (spores and
some other microorganisms remain)
Sanitization - reduction of microbial population to safe levels
Antisepsis - prevention of infection (accomplished by
antiseptics)
Frequently used terms
Bactericide - substance that kills bacteria
Bacteriostatic - substance that prevents growth of bacteria
Pattern of microbial death
Microorganisms usually die logarithmically (i.e. the population
will be reduced by the same fraction at regular intervals)
Pattern of microbial death
Microorganisms usually die logarithmically (i.e. the population
will be reduced by the same fraction at regular intervals)
Conditions influencing effectiveness of antimicrobial agents
Population size
Population composition
Concentration or intensity of agent
Duration of exposure to agent
Temperature
Local environment (e.g. pH, presence of organic material)
Physical methods of control
Heat
Low temperature
Filtration
Radiation
Measuring heat-killing efficiency
Thermal death point (TDP) - lowest temperature at which all
microorganisms in a suspension are killed in 10 minutes
Decimal reduction time (D value) - time it takes to kill 90% of
the cells or spores in a sample at a specific temperature
Decimal reduction time (D value)
Measuring heat-killing efficiency
Z value - the increase in
temperature required to
reduce D to 1/10 its value
F value - time in minutes at
a specific temperature
required to kill a population
of spores or cells
Measuring heat-killing efficiency
Moist heat
Effective against all types of microorganisms
Degrades nucleic acids, denatures proteins, and disrupts
membranes
Moist heat
Autoclaves are used to kill endospores
Uses steam under pressure to achieve temperatures above boiling
Pasteurization
Controlled heating at temperatures below boiling
Does not sterilize
Kills pathogens and reduces levels of spoilage microorganisms
First used to preserve wine, now used for milk, beer, juice and
other beverages
Pasteurization
Traditional method: 63 ºC for 30 minutes
Flash pasteurization: 72 ºC for 15 seconds
Ultrahigh temperature (UHT) sterilization
Sometimes used by the milk industry
Milk heated at 140 to 150 ºC for 1 to 3 seconds
Products can be stored at room temperature for 1 to 2 months
Dry heat sterilization
Less effective, requiring higher temperatures and longer exposure
times
Does not corrode metal and glass instruments
Oxidizes cell constituents and denatures proteins
Low temperature
Refrigeration- storage at 4 ºC slows microbial growth (only used
for short-term storage)
Freezing- storage at - 20 ºC stops microbial growth (does not kill
microorganisms)
Freezing at -30 to -70 ºC used to preserve microbial samples
Filtration
Can be used to sterilized or reduce the microbial population of
heat-sensitive liquids
Removes microorganisms rather than destroying them
Solutions often forced through filters by pressure or a vacuum
Depth filters
Thick layers of fibrous or granular material with small diameter
channels
Made of various materials (e.g. diatomaceous earth or asbestos)
Membrane filters
Porous membrane about 0.1 mm thick
Entrap microbes on their surface
Pore size of 0.2 um diameter removes most cells but not viruses
Air filtration
Surgical masks
Cotton plugs on culture vessels
Air filtration
Laminar flow biological safety cabinets
Employ high efficiency particulate air (HEPA) filters
Remove 99.97 % of particles larger than 0.3 um
Air forced through filter forms a curtain of sterile air
across the opening of the cabinet
Laminar flow biological safety cabinets
Radiation
Ultraviolet (UV) radiation
Ionizing radiation
Ultraviolet (UV) radiation
Wavelength of about 260 nM
Lethal but does not penetrate
glass
Used to sterilize air or
exposed surfaces
Can sterilize water in a thin
layer
Burns the skin
Ionizing radiation
Penetrates deep into objects
Not always effective against
viruses
Gamma radiation from
Cobalt 60 often used
Used to treat meat, fruits,
vegetables and spices
Chemical agents
Are most commonly used agents for disinfection and antisepsis
Phenolics
Phenol first used by Lister
Phenol and derivatives used
as disinfectants in hospitals
and labs
Effective in the presence of
organic material
Can cause skin irritation
Phenolics
Act by denaturing proteins
and disrupting cell
membranes
Alcohols
Not effective against spores
or lipid-containing viruses
Ethanol and isopropanol
most commonly used (at 7080 %)
Act by denaturing proteins
and possibly dissolving
membrane lipids
Halogens
Include fluorine, chlorine,
bromine, iodine and astatine
Iodine used as a skin
disinfectant
Chlorine used to disinfect
water
Both act by oxidizing cell
material and iodinating or
chlorinating molecules
Aldehydes
Formaldehyde and
gutaraldehyde are the most
commonly used
Are highly reactive
molecules
Inactivate proteins and DNA
by cross-linking alkylating
molecules
Quaternary ammonium compounds (QUATS)
Are a class of detergents
Consist of a positively
charged quaternary nitrogen
with a long hydrophobic
chain
Act by disrupting
membranes and denaturing
proteins
Sterilizing gases
Ethylene oxide (EtO) is used
to sterilize heat-sensitive
material
Kills by combining with
proteins
Rapidly penetrates packing
material
Sterilizing gases
Betapropiolactone (BPL) is
occasionally used
Less difficult to eliminate
and kills more quickly
Does not penetrate well and
may be carcinogenic
Heavy metals
Ions of mercury, silver, arsenic, zinc and copper were used for
many years as germicides (less toxic and more effective
compounds have generally replaced them)
Silver nitrate can be used in the eyes of newborns (antibiotics
more common now)
Silver sulfadiazine used on burns
Copper sulfate used as an algicide
Heavy metals
Act by combining with proteins and inactivating or precipitating
them
Evaluation of antimicrobial agent effectiveness
The Environmental protection agency (EPA) regulates
disinfectants
The Food and Drug Administration (FDA) regulates agents used
on humans and animals
Phenol coefficient
Potency of disinfectant
compared to phenol
Coefficient greater than 1
indicates agent is more
potent than phenol
Not always indicative of
potency during normal use
Other evaluation methods
Use dilution test
Measures rate at which selected bacteria are destroyed by
various chemical agents
In use test
Testing done using conditions that approximate normal
use of disinfectant