Controlling Microbial Growth

CONTROLLING MICROBIAL GROWTH

It is often necessary to control the growth of certain microorganisms, particularly those that grow on fomites, which are inanimate objects that can harbor organisms. Some organisms will be resistant to antimicrobial treatments, such as Clostridium botulinum, which survives canning procedures.

There are four biological safety levels as outlined by the CDC. They depend on the infectivity of the organism, the ease at which it is transmitted, the disease severity possible, and the type of work being performed by the organism. A BSL-1 organism does not cause disease in the healthy host and there is little risk in working with them. A BSL-2 organism can cause a moderate disease in workers but are indigenous to the area. Personal protective equipment is required. BSL-3 agents can be lethal and some are considered exotic to the area. These include things like the organism that causes HIV and tuberculosis. BSL-4 organisms are very dangerous and usually fatal with no available treatments, such as smallpox and Ebola.

Sterilization will kill cells, viruses, and endospores. Heat, filtration, pressure, or chemicals can kill these microbes. Aseptic technique is required to prevent contamination of surfaces that are considered sterile. Maintaining a sterile field in surgery helps to prevent sepsis in the surgical patient. Foods can be sterilized in commercial sterilization, such as those that get rid of botulism.

Disinfection gets rid of most organisms on a fomite surface using chemicals or heat. It does not create sterility but is usually fast and inexpensive to do. Chlorine bleach is an example of a disinfectant. Antiseptics are those antimicrobials that are safe for human tissues. Degerming involves handwashing in order to physically get most of the organisms off the hands. Sanitation will clean fomites in order to prevent disease transmission. It involves things like cleaning bathrooms and using a dishwasher.

There are physical and chemical methods of getting rid of microorganisms. Some methods are bactericidal or viricidal because they will kill the organism. Fungicides will kill fungi. Things that are fungistatic or bacteriostatic do not kill the organism but stops the growth of the organism. Bacteriostatic agents in plastics, for example, will inhibit

growth. Longer exposure times will kill more of the microorganisms. Low population volumes are killed easier than high populations of organisms.

Heat is the oldest method of microbial control. Cooking and canning will do this. There is a thermal death point, which is the lowest temperature that will kill the organism in a ten-minute exposure time. Endospore-formers are definitely more heat tolerant. The thermal death time is the time necessary to kill all microorganisms at a specific temperature, which might be boiling temperature or the temperature of an autoclave. Boiling will not kill endospores easily unless high pressures are involved. There is dryheat sterilization and moist-heat sterilization technique. Ovens will use dry heat, while boiling uses moist heat. Autoclaves are moist-heat sterilization techniques. It uses both pressure and steam to kill the organisms.

There are indicator tapes in autoclaves that will turn color when a certain temperature has been achieved but it doesn’t indicate how long the heat has been applied. A better indicator is using a suspension of endospores, which will tell if the heat has been applied long enough and high enough. G. stearothermophilus is the bacterium often used because it is very heat-stable. Diack tubes are glass ampules that will contain a melted pellet if the proper sterilization temperature has been reached.

Pasteurization is a food-based technique that kills organisms while maintaining normal food quality. The food is heated to reduce organisms but it is not considered sterile. Milk, honey, and apple juice are often treated with pasteurization. There are differences in pasteurization techniques. With HTST pasteurization, normal milk is exposed to a temperature of 72 degrees for 15 seconds. UHT pasteurization or ultra-hightemperature pasteurization is what’s done to store milk unrefrigerated for long periods of time.

Freezing and refrigeration will be bacteriostatic for organisms that are not psychrophiles. Growth of the microorganisms is curtailed but not necessarily permanently so. Freezing will kill certain organisms, however. Thawed foods are not protected and should be considered perishable. Dry ice and liquid nitrogen can easily keep bacteria out of the food product.