chapter 13 - control of microbial growth Flashcards
clean (term)
relative term, doesn’t necessarily mean clean of everything
- microbial load can be reduced by washing, vacuuming, and dishwashing but is this clean enough
whether it is clean enough or not depends on the intended use of an object or surface
why clean and not sterilize everything?
- sterilization often requires time
- is labor intensive
- may degrade the quality of the item being treated
- or even have toxic effects on users
biosafety levels ranked by the severity of disease and ease of transmission
(low) BSL-1→ BSL-2 → BSL-3 → BSL-4 (high)
low-risk microbes → high-risk microbes
BSL-1
biosafety level 1: low-risk microbes
- microbes are not known to cause disease in healthy hosts
- pose minimal risk to workers and the environment
e.g. nonpathogenic strains of E. coli
BSL-2
biosafety level 2
- microbes are typically indigenous
- are associated with diseases of varying severity
- pose moderate risk to workers and the environment
e.g. Staphylococcus aureus
BSL-3
biosafety level 3
- microbes are indigenous or exotic
- causes serious or potentially lethal diseases through respiratory transmission
e.g. Mycobacterium tuberculosis
BSL-4
biosafety level 4: high-risk microbes
- microbes are dangerous and exotic
- poses a high risk of aerosol-transmitted infections, which are frequently fatal without treatment or vaccines
- few labs are at this level
e.g. Ebola and Marburg viruses
terms used to describe antimicrobial measures
- sterilization
- disinfection
- antisepsis
- sanitation
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
- usually involves: cleaning and disinfecting an object
antimicrobials fall into 2 broad classes…
- cidal agents
- static agents
cidal agents
major class of antimicrobes
- kills microbes
bactericidal, algicidal, fungicidal, virucidal
- depending on what type of microbe is killed
static agents
major class of antimicrobials
- inhibit or control growth
bacteriostatic, algistatic, fungistatic, virustatic,
- depending on what type of microbe is inhibited
6 criteria for selection of disinfectant
1. must be fast-acting in the presence of organic materials
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
there is no perfect disinfectant that meets all criteria
microbial death curve
a logarithmic function which demonstrates the rate of microbial death
- e.g. can evaluate the degree of microbial control and to describe the progress and effectiveness of a particular protocol
several factors influence the speed at which lethal damage accumulates
1. the initial population size
- the larger the population, the longer it takes to decrease it to a specific number
2. population composition
- i.e. are spores involved?
3. concentration of agent or dose for radiation
4. duration of exposure
5. presence of organic material (e.g. blood, feces)
- ones that can inhibit disinfectant action
- organic load
can microbes be controlled without chemicals
yes, physical agents are often used to kill microbes
- or control their growth
commonly used physical control measures include…
- temperature extremes
- pressure (usually combined with temperature)
- filtration
- irradiation
high temperature and pressure ways of killing microbes
- moist heat
- dry heat
killing microbes: moist heat
uses water, high temperature and pressure to kill microbes
- very effective at killing microbes
steam autoclave is a type of moist heat method
killing microbes: dry heat
uses only high temperature and pressure to kill microbes
- requires higher temperatures than moist heat
- less effective than moist heat but is sometimes required
steam autoclave
form of moist heat to kill microbes
- to kill spores and thermophiles, a combination of heat and pressure is usually required
standard autoclave conditions
- 121 degrees C
- 15 psi (pounds per square inch)
- 20 minutes
heat killing
preferred sterilizing agent
- as long as it doesn’t damage the materials
associated terms:
- thermal death point (TDP)
- thermal death time (TDT)
- decimal reduction time (D value)
thermal death point (TDP)
temperature that kills all the bacteria in a 24 hour old culture in 10 min of exposure
- in reference to heat killing
thermal death time (TDT)
time required to kill all the bacteria in a culture at a specified temperature
- in reference to heat killing
decimal reduction time (D value)
length of time required to kill 90% of the organisms in a population at a specific temperature
- in reference to heat killing
moist heat diagram
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pasteurization
goal is 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:
1. low temperature, long time (LTLT)
2. high temperature, short time (HTST)
3. ultra high temperature (UHT)
cold methods to control microbial growth
- refrigeration, freezing
- freeze-drying
refrigeration, freezing
excellent to preserve food and other materials
- retards microbial growth
- does not effectively kill microorganisms
- microorganisms can be stored in this way
freeze-drying
freeze drying is used for storing many microorganisms
- many microorganisms are sensitive to drying
- but many are not
physical control measures: 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 0.2μm can remove microbial cells, but not viruses from solutions
- to remove viruses, pore sizes of 20nm are necessary (1nm = 0.001μm)
filtration diagram
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physical control measures: sonication
high frequency ultrasound waves to disrupt cell structures
- the disruption is achieved due to the rapid changes in pressure within the intracellular liquid
physical control measures: irradiation
method in which objects are bombarded with high-energy electromagnetic radiation
- e.g. ultraviolet (UV) light
- e.g. gamma rays, electron beams, X-rays
foods do not become radioactive when irradiated
- any reactive molecules produced when high-energy particles are absorbed by food dissipate almost immediately
examples of foods & purpose approved for irradiation in the US
1963: wheat flower
- control mold
1964: white potatoes
- inhibit sprouting
1986: pork
- kill trichina (Trichinella) parasites
1986: fruit and vegetables
- control insects; increase shelf life
1986: herbs and spices
- sterilize
1992: poultry
- reduce numbers of bacterial pathogens
1997: meat
- reduce numbers of bacterial pathogens
2006: shellfish
- reduce numbers of Vibrio species and other pathogens
2010: lettuce and spinach
- reduce numbers of bacterial pathogens
efficacy of a given chemical agent depends on…
- the presence of organic matter
- the kinds of organisms present
- corrosiveness
- stability, odor, and surface tension
efficacy of a given chemical agent: the presence of organic matter
chemicals will bind to inert organic material
- lowering its effectiveness against microbes
efficacy of a given chemical agent: the kinds of organisms present
ideally should be effective against a broad range of pathogens
efficacy of a given chemical agent: corrosiveness
should not corrode the surface
- nonliving or living
efficacy of a given chemical agent: stability, odor, and surface tension
stability
- should be stable upon storage
odor
- neutral or pleasant odor
surface tension
- low surface tension
commercial disinfectants + groups
used to reduce or eliminate microbial content from commercial products
six different groups:
1. ethanol, iodine, chlorine
2. surfactants (such as detergents)
3. aldehydes
4. phenolics
5. heavy metals
6. peroxygens
commercial disinfectant: ethanol, iodine, chlorine
highly reactive compounds
- damages proteins, lipids, and DNA
commercial disinfectant: surfactants (such as detergents)
help in the mechanical removal of microbes from surfaces
- meaning you need to put in effort to mechanically disinfect
commercial disinfectant: aldehydes
combines with proteins and nucleic acids
- inactivates them
commercial disinfectant: phenolics
denature proteins & disrupt membranes
Joseph Lister:
- carbolic acid (i.e. phenol) used for treatment of surgical wounds
commercial disinfectant: heavy metals
binds to proteins
- inhibit enzymatic activity
e.g. copper, silver
commercial disinfectant: peroxygens
strong oxidizers
- produce free radicals that damage cellular macromolecules
cellular macromolecules:
- nucleic acids, lipids, proteins, carbohydrates
what type of sterilization should be used for disposable plasticware?
gas sterilization (antimicrobial gases) OR gamma irradiation
- disposable plasticware such as petri dishes, syringes, sutures, and catheters are not amenable to heat sterilization or chemical disinfection
antimicrobial gases
used for gas sterilization (e.g. ethylene oxide [EtO])
- destroys proteins by alkylation
- microbicidal/sporicidal
- rapidly penetrates packing materials, including plastic wraps
- highly explosive
bacterial resistance to disinfectants
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
- e.g. iodine
disinfectants that have multiple targets at high concentrations may only have a single target at low concentrations
- a situation that can foster resistance
- e.g. triclosan
triclosan in hand soap
triclosan can find its way into wastewater and sewage systems from the handwashing process
- this can then make its way into the bodies of amphibians and fish
