Control of Microbial Growth Methods are employed to destroy, remove, and/or inhibit the growth of microorganisms. This is either on inanimate objects or on body surfaces. Most of these approaches are non-selective meaning that they can negatively impact a diversity of microbial (and non-microbial) life forms. We can distinguish these microbial control methods into Physical and Chemical methods. “…simple hand washing with plain soap and water is considered the simple most important step in preventing the spread of many infectious diseases. Plain soap itself generally does not destroy many organisms; it simply aids in the mechanical removal of transient microbes, including most pathogens...”
Nosocomial Infections “Minimizing the numbers of microorganisms in a hospital is particularly important because of the danger of hospital-acquired, or nosocomial, infections. Hospitalized patients are often more susceptible to infectious agents because of their weakened condition. In addition, patients may be subject to invasive procedures such as surgery, which breaches the intact skin that would otherwise help prevent infection. Finally, pathogens are more likely to be found in hospitals because of the high concentration of patients with infectious disease. These patients may shed pathogens in their feces, urine, respiratory droplets, or other body secretions. Thus, hospitals must be scrupulous in their control of microorganisms. Nowhere is this more important than in the operating room, where instruments used in invasive procedures must be sterile to avoid introducing even normally benign microbes into deep body tissue where they could easily establish infection.”
Aseptic Technique “To work with pure cultures, all media and instruments that contact the culture must first be rendered sterile to avoid contaminating the culture with environmental bacteria.” “All materials used to grow microorganisms must again be treated before disposal to avoid contamination of workers and the environment.” “The use of specific methods to exclude contaminating microorganisms from an environment is called aseptic technique.” “Although all microbiology laboratory personnel must use these prudent measures, those who work with known disease-causing microbes must be even more diligent.”
Killing-Resistant Microorganisms Endospore formers (endospores): Bacillus spp. & Clostridium spp. are highly heat- and chemical-treatment resistant. Mycobacterium spp. are highly chemical- treatment resistant. Certain Pseudomonas spp. can actually grow in certain disinfectants (i.e., in quats and iodophores). Naked viruses (i.e., those lacking lipid envelopes) often are resistant to chemical disinfection.
Exponential Death “It takes more time to kill a large population of bacteria than it does to kill a small population, because only a fraction of organisms die during a given time interval.
More Terms & Concepts Decimal Reduction Time (D value) = the time it takes to kill 90% of a bacterial population (1 log reduction) under specific conditions. Note importance of temperature: higher temperatures can result in dramatically faster disinfection. Dirt and organic compounds can interfere with disinfection—if possible it consequently is important to clean items well before disinfecting or sterilizing (which also will reduced organism number). Obviously the above is not practical if one is disinfecting in order to avoid contacting an item. Critical Items penetrate into body tissues. Semicritical Items come into contact with but don’t penetrate mucous membranes. Non-Critical Items come into contact only with unbroken skin.
10 min boiling (at sea level). 121°C for 15 min to kill endospores (but must be penetrating, moist heat, and 15 min at 121°C)
Categories of Germicides “To perform properly, germicides must be used strictly according to the manufacturer’s directions, especially as they relate to dilution, temperature, and the amount of time they must be in contact with the object being treated.” “It is extremely important that the object be thoroughly cleaned and free of organic material before the germicidal procedure is begun.”
But don’t use scented chlorine bleach to purify water! Note that temperature can affect rate of water purification (i.e., cold water purifies slowly).
Categories of Germicides Iodine typically is used as a tincture (i.e., dissolved in alcohol). Today iodine is more often used as an iodophore which is a less-irritating, slow-release form of iodine. Watch out because Pseudomonas spp. can live and grow in some iodophores!
Tincture “Formulae: Fresh juice of Organic Habanero peppers, New Mexico Jalapeno, African Bird peppers and Hatch Chili peppers. Dosage: Five to thirty drops, three times daily. CAUTION ~ EXTREMELY HOT!! Therapeutic Action: Cayenne is the greatest herbal aid to circulation and should be used on a regular basis. The extract is very concentrated and gets into the bloodstream quickly and makes it a perfect first aid remedy for heart attacks, stroke, fainting, shock, dizziness, hemorrhage, internal and external bleeding. Use a few drops to 10 droppers full. It has saved many lives.
Irradiation: UV and Gamma Gamma radiation is penetrating and effective but not always practical (or legal) to use. UV radiation is not terribly penetrating but generally is safe (and legal) to employ. UV disinfection generally is limited to surfaces and shallow (and UV transparent) aqueous solutions (e.g., water). Neither is terribly effective against bacterial endospores. Contact with either is hazardous to human tissues.
Food Preservation Chemical preservatives. Low-temperature storage (freezing works better than above freezing). Removing water by submerging in highly osmotic solutions (usually high salt or high sugar). Dessication = drying. Lyophilization = freeze drying. All of above generally are better at preventing microbial growth than they are at outright killing microbes, so some care must be taken upon reversal of these food- preservation conditions. One also must consider that removing microorganisms from foods can make food more susceptible to subsequent microbial contamination. An important example of the latter is staphylococcal food poisoning which can result when Staphylococcus aureus is allowed to incubate in only moderately salt- preserved foods such as potato salad.