Chapter 07: Section 7.1-7.4 - The Control of Microbial Growth
The DRT for a particular bacterial species at 60°C is 30 minutes. How long would it take at this temperature to remove 99.9% of this bacterial population?
90 minutes DRT is the time, in minutes, in which 90% of a population of bacteria at a given temperature will be killed. After 30 minutes, 90% of the microbes would be killed; after 60 minutes another 90% would be killed (overall 99%); and finally after 90 minutes another 90% of microbes would be removed (99.9% of the overall population)
Decimal reduction time is the time in minutes in which __________ of the population at a given temperature will be killed.
90%
Which of the following is true of an antimicrobial substance with a microbial death rate of 90% after 1 minute?
After the first minute of application of the agent, 10% of the original microbes are still left on the surface.
Which of the following treatments achieves sterilization?
Autoclaving Sterilization is the removal or destruction of all living microorganisms. Heating (e.g. autoclaving) is the most common method used for killing microbes, including the most resistant forms, such as endospores.
Which term refers to an agent that inhibits bacterial growth?
Bacteriostatic A bacteriostatic agent inhibits bacterial growth. The suffix "-stat" or "-stasis," means to "stop" or to "steady," as in bacteriostasis. Once a bacteriostatic agent is removed, growth might resume.
All of the following methods are used for food preservation EXCEPT __________.
Direct flaming Direct flaming is NOT a method used for food preservation. Desiccation (removal of water), osmotic pressure (addition of salts and sugars), pasteurization (application of heat) allow for food preservations.
Which of the following are NOT major targets for action of antimicrobials?
Flagella Flagella are NOT major targets for action of antimicrobials. A microorganism's plasma membrane, located just inside the cell wall, is the target of many microbial control agents. This membrane actively regulates the passage of nutrients into the cell and the elimination of wastes from the cell. Damage to the lipids or proteins of the plasma membrane by antimicrobial agents causes cellular contents to leak into the surrounding medium and interferes with the growth of the cell. In addition, damage to nucleic acids by heat, radiation, or chemicals is frequently lethal to the cell; the cell can no longer replicate, nor can it carry out normal metabolic functions such as the synthesis of enzymes.
Operating rooms may receive ventilation from air that has passed through __________ filters to remove microorganisms.
HEPA
You contaminate the kitchen counters with Salmonella enterica in chicken blood and expose it to a variety of treatments to study the survival of the bacteria on kitchen surfaces. The following DRT values were obtained: 2.0 minutes at 52°C; 3.7 minutes at pH 2.6; 13.3 minutes with 10 mM hydrogen peroxide; and > 35 days without treatment. Which treatment is most effective against S. enterica?
Heat (52ºC) The application of heat (52ºC) would be the most effective treatment against Salmonella enterica. It took the shortest amount of time to reduce the population of this organism by 90% (decimal reduction time).
Which one of the following physical methods of microbial control is bactericidal?
Incineration Incineration is a physical method of microbial control. It is bacteriocidal. Incineration is an example of dry heat sterilization. It kills by oxidation effects. A simple analogy is the slow charring of paper in a heated oven, even when the temperature remains below the ignition point of paper. It is a very effective method to sterilize paper cups, contaminated dressings, animal carcasses, bags, and wipes.
Which of the following types of radiation is the LEAST effective control method against microorganisms?
Microwaves Microwaves do NOT have much direct effect on microorganisms and is the least effective control method against microorganisms. Bacteria can readily be isolated from the interior of recently operated microwave ovens. Moisture-containing foods are heated by microwave action, and the heat will kill most vegetative pathogens. Solid foods heat unevenly because of the uneven distribution of moisture. For this reason, pork cooked in a microwave oven has been responsible for outbreaks of trichinellosis.
Which type of radiation is LEAST effective in killing microbes?
Microwaves Microwaves do NOT have much direct effect on microorganisms, and bacteria can readily be isolated from the interior of recently operated microwave ovens. Moisture-containing foods are heated by microwave action, and the heat will kill most vegetative pathogens. Solid foods heat unevenly because of the uneven distribution of moisture. For this reason, pork cooked in a microwave oven has been responsible for outbreaks of trichinellosis.
You are preparing a medium for growing fastidious bacteria and must add several heat-labile solutions of growth factors. Which of the following is an appropriate strategy for preparing and sterilizing this medium?
Prepare and autoclave the medium before adding the growth factors. You should prepare and autoclave the medium before adding the growth factors. Autoclaving is the preferred method of sterilization in health care environments, unless the material, such as growth factors, to be sterilized can be damaged by heat or moisture. These solutions are commonly filter sterilized in order to achieve sterilization.
Which of these conditions is likely to interfere with the effectiveness of an antiseptic or disinfectant?
Presence of biofilms The presence of biofilms are difficult for antiseptics or disinfectants to reach effectively. A biofilm embeds microbes within a self-produced extra-cellular matrix and also allows microbes to adhere to a surface in a more efficient manner.
Which of the following is LEAST likely to be damaged by exposure to gamma radiation?
Prions Gamma rays are emitted by certain radioactive elements such as cobalt. Electron beams are produced by accelerating electrons to high energies in special machines. Gamma rays penetrate deeply but may require hours to sterilize large masses; high-energy electron beams have much lower penetrating power but usually require only a few seconds of exposure. Prions are infectious proteins that have posed a challenge to their removal. They are LEAST likely to be damaged by exposure to gamma radiation.
Which of the following correctly describes antisepsis?
Removal of vegetative microorganisms from living tissue Disinfection might make use of chemicals, ultraviolet radiation, boiling water, or steam. In practice, the term is most commonly applied to the use of a chemical (a disinfectant) to treat an inert surface or substance. When this treatment is directed at living tissue to eliminate all microorganisms, it is called antisepsis, and the chemical is then called an antiseptic.
You discover a compound that you suspect reduces spoilage in fruit. You treat apples with your compound and inoculate them with Botrytis, a common spoilage fungus. After 7 days, the dry weight of fungi is 5 mg on the treated apples and 10 mg on the untreated apples. You conclude from these results that __________.
The compound is fungistatic
Which of these is NOT a characteristic of the autoclave?
The requirement for long (hours) exposure times The following is NOT a characteristic of the autoclave: the requirement for long (hours) exposure times. Autoclaving is the preferred method of sterilization in health care environments, unless the material to be sterilized can be damaged by heat or moisture. The higher the pressure in the autoclave, the higher the temperature. It does not require as long of exposure times as other methods. Sterilization in an autoclave is most effective when the organisms either are contacted by the steam directly or are contained in a small volume of aqueous (primarily water) liquid. Under these conditions, steam at a pressure of about 15 psi (121°C) will kill all organisms (but prions) and their endospores in about 15 minutes.
While waiting for your meal at a restaurant, you read the information on the creamer containers at your table and notice that they do not need to be refrigerated. You are surprised, because dairy products typically must be kept cold to prevent spoilage. How were these products treated to eliminate microbial growth and prevent spoilage?
Ultra-high-temperature (UHT) treatment Milk can also be sterilized—something quite different from pasteurization—by ultra-high temperature (UHT) treatments. It can then be stored for several months without refrigeration. In the United States, UHT is used on the small containers of coffee creamers found in restaurants. During this process, the liquid milk (or juice) is sprayed through a nozzle into a pressurized chamber filled with high-temperature steam. A small volume of fluid sprayed into an atmosphere of high-temperature steam exposes a relatively large surface area on the fluid droplets to heating by the steam; sterilizing temperatures are reached almost instantaneously. After reaching a temperature of 140°C for 4 seconds, the fluid is rapidly cooled in a vacuum chamber. The milk or juice is then packaged in a pre-sterilized, airtight container.
For control of microbial growth on a surface, which of these conditions is LEAST likely to interfere with the effectiveness of an antiseptic or disinfectant?
Warm environmental conditions Warm environmental conditions are LEAST likely interfere with the effectiveness of an antiseptic or disinfectant. By reading the label, we can learn a great deal about an antiseptic's or disinfectant's properties. Certain conditions that might affect the efficacy of antiseptics and disinfectants are the concentration, pH of the medium, the type of organism being treated, and the time of exposure.
Pasteurization was first used by Pasteur to control spoilage of __________.
Wine In the early days of microbiology, Louis Pasteur found a practical method of preventing the spoilage of beer and wine. Pasteur used mild heating, which was sufficient to kill the organisms that caused the particular spoilage problem without seriously damaging the taste of the product.
Sterilization is the __________.
destruction of all microbial life Sterilization is the removal or destruction of all microbial life. Heating is the most common method used for killing microbes, including the most resistant forms, such as endospores. A sterilizing agent is called a sterilant.
A chemical that reduces spoilage in fruit by destroying fungi but does NOT appear to affect other microbes would be called a __________.
fungicidal agent A chemical that reduces spoilage in fruit by destroying fungi but does NOT appear to affect other microbes would be called a fungicidal agent. Fungicides kill fungi.
An antiseptic is used to remove microbes from __________.
skin before an injection An antiseptic is used to remove microbes from skin before an injection. Control directed at destroying harmful microorganisms is called disinfection. It usually refers to the destruction of vegetative (non-endospore-forming) pathogens, which is not the same thing as complete sterility. When this treatment is directed at living tissue, it is called antisepsis, and the chemical is then called an antiseptic.
Pasteurized milk in an unopened container spoils in the refrigerator. A sample reveals the presence of microorganisms. The most likely explanation is __________.
the microbes that survived pasteurization were able to grow at 4ºC
__________ bacteria may survive pasteurization.
thermoduric Thermoduric bacteria may survive pasteurization. Pasteur used mild heating, which was sufficient to kill the organisms that caused spoilage in beer and wine without seriously damaging the taste of the product. The same principle was later applied to milk to produce what we now call pasteurized milk. The intent of pasteurization of milk was to eliminate pathogenic microbes. It also lowers microbial numbers, which prolongs milk's good quality under refrigeration. Many relatively heat-resistant, thermoduric, and endospore-forming bacteria may survive pasteurization.
Packages of milk and coffee creamers may be stored without refrigeration if they have been sterilized by __________.
ultra-high-temperature (UHT) treatment Milk can also be sterilized by ultra-high-temperature (UHT) treatments. It can then be stored for several months without refrigeration UHT-treated milk is widely sold in Europe; in the United States, UHT is sometimes used on the small containers of coffee creamers found in restaurants. A small volume of fluid sprayed into an atmosphere of high-temperature steam exposes a relatively large surface area on the fluid droplets to heating by the steam; sterilizing temperatures are reached almost instantaneously. After reaching a temperature of 140°C for 4 seconds, the fluid is rapidly cooled in a vacuum chamber. The milk or juice is then packaged in a pre-sterilized, airtight container.
