Introduction:
Growth of microbial agents are controlled through the use of physical agents & chemical agents.
Basic Terminology:
Sterilization: process of destroying all living organisms & viruses, including endospores
Disinfection: elimination of microbes from inanimate objects / surfaces
Decontamination: treating objects or inanimate surfaces to make them safe to handle
Disinfectant: used to disinfect inanimate objects, generally too toxic to use on human tissues
Antiseptic: agent that kills / inhibits growth of microbes, safe to use on human tissue
Sanitizer: agent that reduces but may not eliminate microbes numbers to a safe level
Antibiotic: metabolic product of one organism that inhibits / kills another microbe
Chemotherapeutic antimicrobial agent: synthetic chemical to inhibit / kill other microbes
Cidal: action will kill microorganisms / viruses
Static: agent will inhibit growth of microorganism
Use of temperature:
Microorganisms have an optimum, minimum, and maximum temperature for growth. Temperatures below the minimum usually have a static action, while those above the maximum tend to have a cidal action. High temperatures can generally kill vegetative organisms from 50 to 70 degrees Celsius with moist heat because it is able to penetrate microbial cells. Bacterial endospores are very resistant to heat and may require extended exposure. Heat may denature proteins & melt lipids. Dry heat kills microbes through protein oxidation.
1. Autoclaving: uses steam under pressure, so the temperature is raised to 121 degrees Celsius under 15 psi. This is sufficient to kill endospores.
2. Boiling water: 100 degrees Celsius, generally kills vegetative cells after 10 minutes, but certain viruses such as hepatitis may survive up to 30 minutes, and endospores may even survive hours of boiling.
3. Hot air sterilization: ovens use high dry temperatures such at 171 degrees Celsius for 1 hour, 160 degrees Celsius for 2 hours, or 121 degrees Celsius for 16 or more hours, and generally used for glassware & instruments.
4. Incineration: used to destroy disposable materials by burning.
5. Pasteurization: mild heating to kill particular organisms or pathogens (used for milk), but does not kill all organisms, milk heated to 71.6 degrees Celsius for at least 15 seconds or 62.9 degrees Celsius for 30 minutes
6. Low temperature: inhibits microbial growth by slowing metabolism, 5 degrees Celsius slows growth for food for a few days, freezing at -10 degrees Celsius stops microbial growth but generally does not kill organisms.
Other methods to control microbial growth:
1. Dessication: this has a static effect. Microbial enzymes are inhibited by the lack of water. This is the method used in freeze-dried foods.
2. Osmotic pressure: this is a method using the concentration of water and dissolved materials to alter the natural environment of the organisms and inhibit growth. Water will move from the greater water and lower solute concentration to the area with lesser water and greater solute concentration. An environment where the solute is higher in the cell than outside the cell, the environment in the cell is hypotonic and water will flow into the cells. Rigid cell walls of bacteria and fungi prevent bursting of the cell. If the solute is higher outside the cell than inside the cell, the environment inside the cell is hypertonic, and water will flow outside the cell. When the solute is the same on both the inside and outside of the cell, the environment is isotonic. When the inside of the cell is hypertonic, water will flow out and the cell becomes dehydrated, inhibiting growth. This is the action of canning jams or preserves with a high sugar concentration. Mold tends to be more tolerant of hypertonic conditions and thus require sealing to exclude oxygen (molds are aerobic).
3. UV radiation: This includes wavelengths from 100 to 400nm. The cidal activity depends on the length of exposure and the wavelength used. The most cidal wavelengths are 260-270nm where it will be absorbed by nucleic acids. Mutations caused by UV radiation can leady to faulty protein synthesis, and if they are sufficient it may block metabolisms and kill the organism. UV light has poor penetrating power and is only effective for microbes on the surface. It may also damage the eyes, cause burns, and cause mutations in human cells.
4. Ionizing radiation: this includes x-rays and gamma rays, and have greater penetrating power. This type of radiation can disrupt DNA and proteins, and is used to sterilize pharmaceuticals, disposable supplies, and in certain foods.
5. Filtration: these may be used in cases where the filters may have small enough pores to allow organisms-free fluid to pass and prevent the passage of microbes. Filters have pores ranging from 25nm to 0.45micrometers. This technique is not effective for viruses which are able to fit through the pores.
