Control of Microbial Growth
Introduction
With the advent of the "Germ Theory" of Disease, the medical community gradually began to grow aware of the problem of nosocomial infections and that there was a need to practice "asepsis" in order to prevent contamination of wounds, dressings and surgical instruments. Ignatz Semmelweis (1816-1865) and Joseph Lister (1827-1912), are considered to be important pioneers for the promotion of "asepsis"; working in a germ free environment. Semmelweiss, an Hungarian obstetrician, recognized (and enforced) the need for hospital staff to frequently wash their hands and take other precautions to minimize transmitting infections to their patients. Semelweiss' interventions significantly reduced deaths due to "childbirth fever;" a serious streptococcal infection. Lister, a British physician, pioneered antiseptic surgery. He performed surgery while using a mist of phenolic compounds and endorsed the treatment of dressings with carbolic acid. Lister also promoted the heat sterilization of surgical instruments.
Today we take "aseptic" practices for granted, especially in a hospital environment. However, many thousands of people die each year in the U.S. and abroad of nosocomial (hospital-acquired) infections, many of which can be prevented!
We are also living in an excessively "germ phobic" culture. Many over-the-counter antimicrobial products are being marketed and sold but prolonged and frequent use of these products can be detrimental in several important ways:
Antibacterial skin lotions and "sanitizers" destroy beneficial normal skin microbes which help out-compete pathogens. Overuse of such agents can therefore create an opportunity for pathogens to cause infection.
Chemical pressure may "select" for the evolution of resistant microbes that are no longer susceptible to the activity of the antimicrobial agent.
Many of the "antimicrobial" products are more costly than equivalent brands.
"Antimicrobial" chemicals are being applied to commonly used products such as children's toys. These chemicals may be ingested by young children or cause sensitivity responses.
Although it is important and reasonable to expect a life free of infection and disease, the issues listed above stress that we should keep our expectations realistic. Microbes also inhabit and create important ecosystem: if we upset the balance of microbial ecology we can pay the price!
Terminology related to control of microorganisms
Germicide/Biocide
A chemical agent that demonstrates killing power against various microbes
Antisepsis
Refers to the killing or removal of microbes on living tissues
Disinfection
Refers to the killing of microbes on inanimate objects or materials
Sterilization
Kills or removes all forms of life, including bacterial endospores
Static
Processes or chemical agents that inhibit bacterial growth but do not necessarily kill microbes
Sanitization
Usually used by the food industry. Reduces microbes on eating utensils to safe, acceptable levels for public health.
Pasteurization
A heating process that REDUCES the number of spoilage germs and eliminates pathogens in milk and other heat sensitive foods
Clean
"Clean" has a very restricted meaning in microbiology. In this context, clean refers to the removal of VISIBLE dirt and debris from tissues or objects. Clean does not equal sterile!
Return to top
Chemical Methods of Microbial Control
Properties of an ideal antimicrobial agent
* Fast-acting
* Acts against many microbes without harming tissues (selective toxicity)
* Penetrating power (improves if dirt and debris are first removed)
* Inexpensive
* Easy to prepare
* Chemically stable
* Inoffensive odor
* Not harmful to the environment
Any one agent is unlikely to possess all of the above qualities but it is helpful to assess an agent for as many of these characteristics as possible. Germicides are commonly used in hospitals, homes and elsewhere. Germicidal agents are also regularly used as preservatives of foods, cosmetics, vaccines and medical supplies.
Many antiseptics and disinfectants act at multiple sites and targets on microbial cells and therefore do not show a great deal of selective toxicity as do many antibiotics.
There is also broad spectrum in susceptibility to germicides among different microbes:
*
Microbial susceptibility to Germicides in descending order (from high susceptibility to lowest)
Examples
Comments
Enveloped or medium-sized viruses
o HIV*
o Hepatitis B virus
o Cytomegalovirus
o Herpes simplex virus
* Recent studies demonstrated viable HIV could be found in contaminated syringes for up to 4 weeks, emphasizing the need for "clean" needles
Nonspore-forming gram positive bacteria
o Staphylococci
o Streptococci
usually very susceptible
Nonlipid (nonenveloped) viruses
o Poliovirus
o Rhinovirus
Fungi
o Trichophyton
o Candida
o Cryptococcus
Spores may be resistant to germicides
Nonspore-forming gram negative
bacteria
o Proteus spp
o Pseudomonas aeruginosa*
* Pseudomonads can grow in some disinfectants
Protozoan cysts
o Giardia
o Cryptosporidium
Both organsisms are highly resistant to standard levels of water chlorination
Helminth ova
o Ascaris lumbricoides
Eggs may remain viable for many years, even in some preservatives
Mycobacteria
o Mycobacterium tuberculosis
Bacterial endospores
o Bacillus spp
o Clostridium spp
Prions
o Transmissible spongiform encephalopathies
Thought to be the most resistant of all agents
Return to top
A Summary of Mechanisms of Inactivation by Biocides
Microbial Targets
Chemical(s)
Vegetative bacterium:
Cell wall
* Formaldehyde
* CRAs *
* Mercury
* Phenols
Cytoplasmic coagulation
* Chlorhexidine
* Glutaraldehyde
* Hexachlorophene
* Mercurial compounds
* Silver salts
* QACS*
Cell membrane: membrane potential or electron transport
* Hexachlorophene
* Phenols
* Parabens
* Weak acids used as food preservatives such as benzoic, sorbic and proprionic acids
Leakage of cell components
* Phenols
* Chlorhexidine
* Alcohols
* QACs
Nucleic acids
* Alkylating agents such as ethylene oxide gas
Bacterial endospores:
Spore core
* Glutaraldehyde
* Formaldehyde
Spore cortex
* CRAs
* Glutaraldehyde
* Nitrous acid
* Nitrates/nitrates act as food preservatives by preventing germination of endospores
Virus
Envelopes
* Alcohols
* CRAs
* QACs
* Chlorhexidine
Viral nucleic acid
* CRAs
Capsid
* Glutaraldehyde
* QACS
* CRAs
* Iodine
* Phenols
* Alcohols
Fungus
Cell membrane
* Chlorhexidine
* Alcohols
* QACS
Cell wall
* Glutaraldehyde
Nucleic acid
* Acridine dyes
Source: Russell et al. 1997. ASM News. 63 (9): 481-487.
*CRAs = Chlorine releasing agents
QACs = Quaternary ammonium compounds
Return to top
Physical Methods of microbial control
* Heat
* Filtration
* Radiation
* Refrigeration
* Desiccation
Heat Sterilization
Heating is the most frequently used means to destroy microbes, being both economical and easily controlled. Successful heat sterilization must consider the degree of heat resistance demonstrated by a microorganism. Death from heating is an exponential function and occurs more rapidly as temperature increases. The nature of heat is also important: moist heat penetrates better than dry heat.
Decimal reduction time (DRT) is a concept employed by the canning industry to determine heat sterilization and is defined as the time it takes for 90% of microbes will be killed at a particular temperature.
Moist heat
Boiling will kill most vegetative bacteria and viruses within 10 minutes. Bacterial endospores can survive boiling temperatures. Certain bacterial toxins such as Staphylococcal enterotoxin are also heat resistant.
Autoclaving
Uses steam heat under pressure to penetrate and kill microorganisms. Steam produced at 15 psi heats to 121 C and will kill endospores after 15 minutes. Denser materials or large objects will need to be autoclaved for longer periods.
The USDA recommends that all low-acid foods should be processed by pressure cooking. This is to prevent the persistence of endospores in foods such as canned tomatoes. Jellies and jams should also be stored with sealed lids and not wax seals in order to prevent growth of molds and toxin production.
Pasteurization
Pasteurization is named for a process developed by Louis Pasteur as he looked for ways to prevent wine spoilage. It is important to note that Pasteurization is not synonymous with sterilization. This process employs heat to destroy pathogens and reduce the number of spoilage microbes in foods. Before this process was developed, milk was a common source of diseases such as tuberculosis, typhoid fever and brucellosis. Today, pasteurization is primarily used to prolong the shelf-life of various foods.
Pasteurization employs the concept of equivalent treatments. As temperature increases less time is needed to kill a certain number of microbes that would take more time to kill at a lower temperature. Classical (bulk) pasteurization heated foods at 63 C for 30 minutes. Today, flash pasteurization or high temperature, short-time (HTST) methods are favored as they kill heat-resistant organisms more effectively and are less likely to alter the flavor of foods. The HTST methods involve continuos passage of foods past a heat exchanger. Pasteurization methods include:
* 72 C for 15 sec (HTST)
* 140 C for 15 sec (Ultra-High Temp)
* 149 C for 0.5 sec (UHT)
Dry heat sterilization
Dry heat takes more time to kill microbes as it does not penetrate as well in the absence of steam.. Common uses of dry heat sterilization are flaming of inoculating loops and the sterilization of glassware in hot air drying ovens.
Filtration
Filter sterilization is commonly employed for substances that can not tolerate heat. Membrane filters with pore sizes between 0.2-0.45 um are commonly used to remove particles from solutions that can't be autoclaved. Membrane filtration of beer eliminates spoilage germs and pasteurization is no longer needed. Filtered beer is permitted to be sold as "draft beer." Sub-micron filters are also being marketed for removal of protozoan cysts from drinking water.
Radiation
One of the most controversial areas of microbial control involves the use of radiation. The controversy largely results from a lack of understanding of the different types and uses of radiation.
The effects of types of radiation depend on three important factors:
* Time (of exposure)
* Distance (from the source)
* Shielding (how penetrating is the radiation?)
Irradiation of various food has been used in the U.S since the 1960's and has been used to sterilize foods such as herbs and spices.
Nonionizing radiation
Includes microwaves and ultra violet radiation. Microwaves are not particularly antimicrobial in and of themselves, rather the killing effect of microwaves are largely due to the heat that they generate. Microwaves are not recommended for cooking large volumes or thick cuts of meat as the heat may not penetrate the foodstuffs sufficiently.
UV radiation is of short wavelength, between 220 and 300 nm and is not very penetrating. UV can be stopped by glass, a sheet of paper, or the top layers of your skin! UV rays can kill exposed microbes by causing damage to their DNA. UV radiation is useful for the disinfection of exposed surfaces such as laboratory hoods. However, the usefulness of UV radiation is limited by the fact that certain microbes possess DNA repair mechanisms and can recover after exposure to this kind of radiation. In addition, UV light does not penetrate organisms well that are protected in mucus or debris.
Ionizing radiation
Includes gamma rays and X rays which are highly penetrating to cells and tissues and have potent antimicrobial effects. After colliding with a target, ionizing radiation generates ions and other reactive species from molecules including hydroxyl (free) OH- radicals. These free radicals can cause irreversible breaks in DNA, proteins and enzymes.
Radiation is currently used for sterilization by the medical supply and food industries. The FDA has approved irradiation for sterilization of surgical supplies, vaccines and drugs. Irradiation of spices and seasonings has grown over the use of ethylene oxide gas. This gas must be expelled from products before use, and is mutagenic (possible cancer-causing agent).
Many individuals and groups have voiced concern against the irradiation of foods. However, to date no study has demonstrated any long-term ill effects from this practice and the nutritional quality of the foods seem largely unaltered. The irradiation of foods can be likened to your bag going through an X ray machine at an airport. When you pick up your bag at the other end your baggage is not "glowing" with radiation and will do you no harm. You get a lot more exposure to natural forms of radiation (sunlight) than you do from your foodstuffs! You should be more concerned about your risk of skin cancer from sunbathing!
The USDA approved the irradiation of red meat this February. The USDA rule would permit, but not require, irradiation for refrigerated or frozen uncooked meat and some meat products. Irradiation is the only effective means known to eliminate E. coli 0157 from meat. This process can also eliminate the food pathogens, Listeria (likely to a 16 deaths in the US late 98/early 99) Campylobacter and Salmonella
Vegetarians look out! What about alfalfa sprouts?
Consumption of alfalfa sprouts carries a high risk of infection with Salmonella.
Over 20,000 cases of Salmonella were linked to eating alfalfa sprouts in North America in 1995. The fundamental problem was linked to the fact that there is no "kill step" used to eliminate the pathogens during the germination of the seeds. The CDC has advised people at high risk of complications from Salmonella (young, elderly, pregnant women, immune compromised) to avoid eating these sprouts.
Despite opposition, the only thing that may save the alfalfa sprout industry is to irradiate their plants!
Refrigeration
Refrigeration will slow down and inhibit the growth of most microbes but it will not kill them! Note: some spoilage germs and psychrophiles can continue to replicate at cooler temperatures. Organisms can be maintained viable at -80 C if suspended in glycerol or DMSO.
Desiccation
Desiccation of microbes is a very useful means of food preservation and to control the growth of spoilage germs and pathogens. Foods that have a high water activity are most subject to spoilage and typically must be refrigerated or frozen. Numerous foods are preserved by adding salt or sugar to decrease the water activity of the foods. This process creates hypertonic conditions and causes water to leave bacterial cells (plasmolyze). Salting of foods does not protect against all potential pathogens. Many fungi are halophilic as is Staphylococcus aureus, a common source of bacterial food poisoning.
Introduction
With the advent of the "Germ Theory" of Disease, the medical community gradually began to grow aware of the problem of nosocomial infections and that there was a need to practice "asepsis" in order to prevent contamination of wounds, dressings and surgical instruments. Ignatz Semmelweis (1816-1865) and Joseph Lister (1827-1912), are considered to be important pioneers for the promotion of "asepsis"; working in a germ free environment. Semmelweiss, an Hungarian obstetrician, recognized (and enforced) the need for hospital staff to frequently wash their hands and take other precautions to minimize transmitting infections to their patients. Semelweiss' interventions significantly reduced deaths due to "childbirth fever;" a serious streptococcal infection. Lister, a British physician, pioneered antiseptic surgery. He performed surgery while using a mist of phenolic compounds and endorsed the treatment of dressings with carbolic acid. Lister also promoted the heat sterilization of surgical instruments.
Today we take "aseptic" practices for granted, especially in a hospital environment. However, many thousands of people die each year in the U.S. and abroad of nosocomial (hospital-acquired) infections, many of which can be prevented!
We are also living in an excessively "germ phobic" culture. Many over-the-counter antimicrobial products are being marketed and sold but prolonged and frequent use of these products can be detrimental in several important ways:
Antibacterial skin lotions and "sanitizers" destroy beneficial normal skin microbes which help out-compete pathogens. Overuse of such agents can therefore create an opportunity for pathogens to cause infection.
Chemical pressure may "select" for the evolution of resistant microbes that are no longer susceptible to the activity of the antimicrobial agent.
Many of the "antimicrobial" products are more costly than equivalent brands.
"Antimicrobial" chemicals are being applied to commonly used products such as children's toys. These chemicals may be ingested by young children or cause sensitivity responses.
Although it is important and reasonable to expect a life free of infection and disease, the issues listed above stress that we should keep our expectations realistic. Microbes also inhabit and create important ecosystem: if we upset the balance of microbial ecology we can pay the price!
Terminology related to control of microorganisms
Germicide/Biocide
A chemical agent that demonstrates killing power against various microbes
Antisepsis
Refers to the killing or removal of microbes on living tissues
Disinfection
Refers to the killing of microbes on inanimate objects or materials
Sterilization
Kills or removes all forms of life, including bacterial endospores
Static
Processes or chemical agents that inhibit bacterial growth but do not necessarily kill microbes
Sanitization
Usually used by the food industry. Reduces microbes on eating utensils to safe, acceptable levels for public health.
Pasteurization
A heating process that REDUCES the number of spoilage germs and eliminates pathogens in milk and other heat sensitive foods
Clean
"Clean" has a very restricted meaning in microbiology. In this context, clean refers to the removal of VISIBLE dirt and debris from tissues or objects. Clean does not equal sterile!
Return to top
Chemical Methods of Microbial Control
Properties of an ideal antimicrobial agent
* Fast-acting
* Acts against many microbes without harming tissues (selective toxicity)
* Penetrating power (improves if dirt and debris are first removed)
* Inexpensive
* Easy to prepare
* Chemically stable
* Inoffensive odor
* Not harmful to the environment
Any one agent is unlikely to possess all of the above qualities but it is helpful to assess an agent for as many of these characteristics as possible. Germicides are commonly used in hospitals, homes and elsewhere. Germicidal agents are also regularly used as preservatives of foods, cosmetics, vaccines and medical supplies.
Many antiseptics and disinfectants act at multiple sites and targets on microbial cells and therefore do not show a great deal of selective toxicity as do many antibiotics.
There is also broad spectrum in susceptibility to germicides among different microbes:
*
Microbial susceptibility to Germicides in descending order (from high susceptibility to lowest)
Examples
Comments
Enveloped or medium-sized viruses
o HIV*
o Hepatitis B virus
o Cytomegalovirus
o Herpes simplex virus
* Recent studies demonstrated viable HIV could be found in contaminated syringes for up to 4 weeks, emphasizing the need for "clean" needles
Nonspore-forming gram positive bacteria
o Staphylococci
o Streptococci
usually very susceptible
Nonlipid (nonenveloped) viruses
o Poliovirus
o Rhinovirus
Fungi
o Trichophyton
o Candida
o Cryptococcus
Spores may be resistant to germicides
Nonspore-forming gram negative
bacteria
o Proteus spp
o Pseudomonas aeruginosa*
* Pseudomonads can grow in some disinfectants
Protozoan cysts
o Giardia
o Cryptosporidium
Both organsisms are highly resistant to standard levels of water chlorination
Helminth ova
o Ascaris lumbricoides
Eggs may remain viable for many years, even in some preservatives
Mycobacteria
o Mycobacterium tuberculosis
Bacterial endospores
o Bacillus spp
o Clostridium spp
Prions
o Transmissible spongiform encephalopathies
Thought to be the most resistant of all agents
Return to top
A Summary of Mechanisms of Inactivation by Biocides
Microbial Targets
Chemical(s)
Vegetative bacterium:
Cell wall
* Formaldehyde
* CRAs *
* Mercury
* Phenols
Cytoplasmic coagulation
* Chlorhexidine
* Glutaraldehyde
* Hexachlorophene
* Mercurial compounds
* Silver salts
* QACS*
Cell membrane: membrane potential or electron transport
* Hexachlorophene
* Phenols
* Parabens
* Weak acids used as food preservatives such as benzoic, sorbic and proprionic acids
Leakage of cell components
* Phenols
* Chlorhexidine
* Alcohols
* QACs
Nucleic acids
* Alkylating agents such as ethylene oxide gas
Bacterial endospores:
Spore core
* Glutaraldehyde
* Formaldehyde
Spore cortex
* CRAs
* Glutaraldehyde
* Nitrous acid
* Nitrates/nitrates act as food preservatives by preventing germination of endospores
Virus
Envelopes
* Alcohols
* CRAs
* QACs
* Chlorhexidine
Viral nucleic acid
* CRAs
Capsid
* Glutaraldehyde
* QACS
* CRAs
* Iodine
* Phenols
* Alcohols
Fungus
Cell membrane
* Chlorhexidine
* Alcohols
* QACS
Cell wall
* Glutaraldehyde
Nucleic acid
* Acridine dyes
Source: Russell et al. 1997. ASM News. 63 (9): 481-487.
*CRAs = Chlorine releasing agents
QACs = Quaternary ammonium compounds
Return to top
Physical Methods of microbial control
* Heat
* Filtration
* Radiation
* Refrigeration
* Desiccation
Heat Sterilization
Heating is the most frequently used means to destroy microbes, being both economical and easily controlled. Successful heat sterilization must consider the degree of heat resistance demonstrated by a microorganism. Death from heating is an exponential function and occurs more rapidly as temperature increases. The nature of heat is also important: moist heat penetrates better than dry heat.
Decimal reduction time (DRT) is a concept employed by the canning industry to determine heat sterilization and is defined as the time it takes for 90% of microbes will be killed at a particular temperature.
Moist heat
Boiling will kill most vegetative bacteria and viruses within 10 minutes. Bacterial endospores can survive boiling temperatures. Certain bacterial toxins such as Staphylococcal enterotoxin are also heat resistant.
Autoclaving
Uses steam heat under pressure to penetrate and kill microorganisms. Steam produced at 15 psi heats to 121 C and will kill endospores after 15 minutes. Denser materials or large objects will need to be autoclaved for longer periods.
The USDA recommends that all low-acid foods should be processed by pressure cooking. This is to prevent the persistence of endospores in foods such as canned tomatoes. Jellies and jams should also be stored with sealed lids and not wax seals in order to prevent growth of molds and toxin production.
Pasteurization
Pasteurization is named for a process developed by Louis Pasteur as he looked for ways to prevent wine spoilage. It is important to note that Pasteurization is not synonymous with sterilization. This process employs heat to destroy pathogens and reduce the number of spoilage microbes in foods. Before this process was developed, milk was a common source of diseases such as tuberculosis, typhoid fever and brucellosis. Today, pasteurization is primarily used to prolong the shelf-life of various foods.
Pasteurization employs the concept of equivalent treatments. As temperature increases less time is needed to kill a certain number of microbes that would take more time to kill at a lower temperature. Classical (bulk) pasteurization heated foods at 63 C for 30 minutes. Today, flash pasteurization or high temperature, short-time (HTST) methods are favored as they kill heat-resistant organisms more effectively and are less likely to alter the flavor of foods. The HTST methods involve continuos passage of foods past a heat exchanger. Pasteurization methods include:
* 72 C for 15 sec (HTST)
* 140 C for 15 sec (Ultra-High Temp)
* 149 C for 0.5 sec (UHT)
Dry heat sterilization
Dry heat takes more time to kill microbes as it does not penetrate as well in the absence of steam.. Common uses of dry heat sterilization are flaming of inoculating loops and the sterilization of glassware in hot air drying ovens.
Filtration
Filter sterilization is commonly employed for substances that can not tolerate heat. Membrane filters with pore sizes between 0.2-0.45 um are commonly used to remove particles from solutions that can't be autoclaved. Membrane filtration of beer eliminates spoilage germs and pasteurization is no longer needed. Filtered beer is permitted to be sold as "draft beer." Sub-micron filters are also being marketed for removal of protozoan cysts from drinking water.
Radiation
One of the most controversial areas of microbial control involves the use of radiation. The controversy largely results from a lack of understanding of the different types and uses of radiation.
The effects of types of radiation depend on three important factors:
* Time (of exposure)
* Distance (from the source)
* Shielding (how penetrating is the radiation?)
Irradiation of various food has been used in the U.S since the 1960's and has been used to sterilize foods such as herbs and spices.
Nonionizing radiation
Includes microwaves and ultra violet radiation. Microwaves are not particularly antimicrobial in and of themselves, rather the killing effect of microwaves are largely due to the heat that they generate. Microwaves are not recommended for cooking large volumes or thick cuts of meat as the heat may not penetrate the foodstuffs sufficiently.
UV radiation is of short wavelength, between 220 and 300 nm and is not very penetrating. UV can be stopped by glass, a sheet of paper, or the top layers of your skin! UV rays can kill exposed microbes by causing damage to their DNA. UV radiation is useful for the disinfection of exposed surfaces such as laboratory hoods. However, the usefulness of UV radiation is limited by the fact that certain microbes possess DNA repair mechanisms and can recover after exposure to this kind of radiation. In addition, UV light does not penetrate organisms well that are protected in mucus or debris.
Ionizing radiation
Includes gamma rays and X rays which are highly penetrating to cells and tissues and have potent antimicrobial effects. After colliding with a target, ionizing radiation generates ions and other reactive species from molecules including hydroxyl (free) OH- radicals. These free radicals can cause irreversible breaks in DNA, proteins and enzymes.
Radiation is currently used for sterilization by the medical supply and food industries. The FDA has approved irradiation for sterilization of surgical supplies, vaccines and drugs. Irradiation of spices and seasonings has grown over the use of ethylene oxide gas. This gas must be expelled from products before use, and is mutagenic (possible cancer-causing agent).
Many individuals and groups have voiced concern against the irradiation of foods. However, to date no study has demonstrated any long-term ill effects from this practice and the nutritional quality of the foods seem largely unaltered. The irradiation of foods can be likened to your bag going through an X ray machine at an airport. When you pick up your bag at the other end your baggage is not "glowing" with radiation and will do you no harm. You get a lot more exposure to natural forms of radiation (sunlight) than you do from your foodstuffs! You should be more concerned about your risk of skin cancer from sunbathing!
The USDA approved the irradiation of red meat this February. The USDA rule would permit, but not require, irradiation for refrigerated or frozen uncooked meat and some meat products. Irradiation is the only effective means known to eliminate E. coli 0157 from meat. This process can also eliminate the food pathogens, Listeria (likely to a 16 deaths in the US late 98/early 99) Campylobacter and Salmonella
Vegetarians look out! What about alfalfa sprouts?
Consumption of alfalfa sprouts carries a high risk of infection with Salmonella.
Over 20,000 cases of Salmonella were linked to eating alfalfa sprouts in North America in 1995. The fundamental problem was linked to the fact that there is no "kill step" used to eliminate the pathogens during the germination of the seeds. The CDC has advised people at high risk of complications from Salmonella (young, elderly, pregnant women, immune compromised) to avoid eating these sprouts.
Despite opposition, the only thing that may save the alfalfa sprout industry is to irradiate their plants!
Refrigeration
Refrigeration will slow down and inhibit the growth of most microbes but it will not kill them! Note: some spoilage germs and psychrophiles can continue to replicate at cooler temperatures. Organisms can be maintained viable at -80 C if suspended in glycerol or DMSO.
Desiccation
Desiccation of microbes is a very useful means of food preservation and to control the growth of spoilage germs and pathogens. Foods that have a high water activity are most subject to spoilage and typically must be refrigerated or frozen. Numerous foods are preserved by adding salt or sugar to decrease the water activity of the foods. This process creates hypertonic conditions and causes water to leave bacterial cells (plasmolyze). Salting of foods does not protect against all potential pathogens. Many fungi are halophilic as is Staphylococcus aureus, a common source of bacterial food poisoning.
