Control of Microbial Growth Control of Microbial Growth Kathy

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					                    Control of Microbial Growth
                          Kathy Huschle
            Northland Community and Technical College

Microbial Growth
scientific control of microbial growth has only been around for about
100 years
mid-1800’s was the first time an association between
microorganisms and disease was made
first microbial practice for medicine was simple hand washing
microbial growth affects
–product quality

Antimicrobial Definitions
–removing or destroying all microorganisms and viruses
–microorganism cannot be revived
–free of all forms of microbial life
–control directed at destroying harmful microbes
–some viable microbes may persist
–kills bacteria
–prevents growth of bacteria
–disinfectants formulated for use on skin
–brief heat treatment to reduce the # of spoilage organisms
–item that has been treated to reduce # of disease-causing
–implies reduced microbial population
–delaying spoilage of foods

Method of Control
physical or chemical?
–physical control includes heat, irradiation, filtration and mechanical
–chemical control involves the use of antimicrobial chemicals
–depends on the situation
–degree of control required

Method of Control
daily life
–soap and water
–important to minimize nosocomial infection (hospital acquired) due
weakened condition of patients
breaching of intact skin
high concentration of pathogens from patients and workers
microbiology labs utilize
–sterile equipment
–aseptic technique
–diligent workers due to the nature of their work
foods/food production industry
–physical removal
–adding chemicals
toxicity may be a result
–clean surfaces/machinery

Selection of Control Method
antimicrobial procedure used for control of microbial growth is based
–type of microbe
–extent of contamination
–environmental conditions
–potential risk
type  of microorganism
–some organisms are more resistant and require stronger measures
for control
endospores require chemical treatment for 10 hours
Mycobacterium’s waxy cells are resistant to chemicals
extent of microbial population
–larger population take more time to destroy
–only a fraction of the microbial population is destroyed in a given
time interval
if 90% of population is destroyed in the first 3 minute
90% or remaining population is destroyed in the next 3 minutes
and so on
environmental conditions
–pH, temperature
– presence of
organics: blood
the potential risk of transmitting infectious agents
–critical items
–semicritical items
–non-critical items

criticalitems have
–direct contact with body tissues
needles, scalpels
semicritical items have
–contact with mucous membranes but do not penetrate
endoscopes, endotrachial tubes
non-critical items have
–contact with unbroken skin
countertops, stethoscopes

Physical Microbial Controls
Around    350 BC Aristotle suggested to Alexander the Great to boil
his armies’ drinking water. He did, and that is one of the reasons he
is Alexander the Great.
Heat as a microbial control
–is fast, reliable, inexpensive
–does not introduce potential toxic substances
types of heat control include
–moist heat
–pressurized steam
–dry heat
moist heat
causes irreversible coagulation of proteins found in microorganisms
–with 10 minutes of boiling
most microorganisms and viruses will be destroyed
–exception: endospores
 some can survive hours of boiling
–reduces the number of heat-sensitive organisms
–widely used in milk and juices
increases shelf life
does not alter quality
–original pasteurization protocol was
62 C for 30 minutes
–protocol used more commonly now
–72 C for 15 seconds
pressurized steam
–pressure cooker or autoclave
–higher air pressure increases the temperature at which steam forms
–15psi (pounds/square inch) at 121 C for 15 minutes
effective temperature to kill endospores
dry heat
–heating in the absence of moisture
–burns the cell constituents
oxidized to ash
irreversibly denatures proteins
–takes longer
       0                          0
200 C for 1.5 hours (dry) = 121 C for 15 minutes (moist)
–advantage over moist heat
can be used for powders
does not dull sharp instruments
does not corrode metals

Physical Microbial Control:
used for heat sensitive fluids
–depth filters
ability to retain microorganisms while allowing the suspended fluid
to pass through a complex, tortuous passage
–membrane filters
the use of graded pore size to retain microorganisms: 0.4-.02 um
 HEPA: high efficiency particulate air
–removes nearly all airborne microorganisms greater than 0.3 um

Physical Microbial Control:
 gamma irradiation
–causes biological damage to the microorganism
–used to sterilize heat sensitive materials
–will not alter the flavor of food
–is not radioactive
ultraviolet light
–damages the structure and function of nucleic acids
–penetrates poorly
–used to disinfect surfaces
–can cause damage to human cells
–does not affect microorganisms directly
–kills by the heat generated

Chemical Controls
germicidal chemicals
–are less reliable than heat
–can be used for disinfecting and sometimes sterilization
–chemical germicides
need to be used exactly as directed
materialbeing treated must be free of organic matter before the
chemical germicide is used
–organic matter negates the effectiveness of the germicide

Chemical Controls
–grouped according to potency
destroy microorganisms, endospores and viruses
used for critical equipment: scalpels
destroy viruses and vegetative microorganisms (no endospores)
–used for semicritical equipment: endoscopes
 destroy vegetative microorganisms, some viruses
used for non critical equipment: stethoscopes
destroy fungi, vegetative microorganisms
used for general purpose disinfectants

Selecting Germicidal Chemical
toxicity to humans or environment?
–weigh the benefits vs. the risks
presence of organic material
–hypochlorite (household bleach) is inactivated by the presence of
organic matter
–electrical equipment with a liquid???
–some have to be rinsed with sterile water
cost and availability
storage and stability
–may come in a concentrated form for ease in storing
–those have to be mixed
environmental risk
–is neutralization necessary before disposal?
Classes of Germicidal Chemicals
–coagulate enzymes and proteins
–damage lipid membranes
–no residue
–inactivate proteins and nucleic acids
–toxic to humans
–extensive antiseptic use
–adheres and persists on skin, mucous membranes
–low toxicity
ethylene oxide
–reacts with proteins
–gas: penetrable
halogens: oxidize proteins
irritating to skin
organic compounds consume free chlorine
–metal compounds
interfere with protein function
–oxidizing agents
–hydrogen peroxide
–leaves no residue
phenolic compound
–historically important
–irritant, unpleasant odor
–destroy cytoplasmic membrane
–denatures proteins
Preservation of Perishables
extends shelf life
–slows or halts to growth of microbes to delay spoilage
chemical preservatives
–some germicidal preservatives are used in non-food items
–food preservatives must be non-toxic to humans
benzoic, sorbic, propionic acids are commonly used
–inhibits germination of C. botulinum endospores
low temperature storage
–temperature dependent
most microorganisms do not reproduce in ordinary refrigerator
temps (0 – 70C)
low temperature storage
ice crystals can cause irreversible damage to many microorganisms
–killing up to 50%
freezing stops all growth, but may start up again once food is
reducing water
draw water out of cell
less available for microorganism
reducing water
–removing the water: powdered milk
–freeze drying
freeze food first
then dry in a vacuum

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