Chapter 7 Essay Questions

Explain how radiation kills cells

Finally, let's talk about radiation, another useful tool for lowering or eliminating numbers of microbes on a material. Two types of radiation can kill microorganisms, ionizing radiation and non-ionizing radiation. The principle effect of ionizing radiation is the ionization of water, which forms highly reactive hydroxyl radicals that damage cellular components, mainly DNA. It is used to sterilize spices and certain meats and vegetables. Ionizing radiation includes gamma rays, X rays, and high-energy electron beams. High-energy electron beams are used for the sterilization of pharmaceuticals and disposable dental and medical supplies. As a protection against bioterrorism, the postal service often uses electron beams to sterilize certain classes of mail. Nonionizing radiation, such as ultraviolet light, damages the DNA of exposed cells. This type of radiation is used to control microbes in the air, such as in hospital rooms, nurseries, and cafeterias.

Clinical Case: A school epidemic (pg 178)

It is 9:00 a.m. on a Wednesday morning, and amy Garza, the school nurse at Westview elementary School in rockville, Maryland, has been on the phone since she came in to work at 7:00 a.m. So far this morning, she has received reports of students unable to attend school today because of some sort of gastrointestinal ailment. they all have the same symptoms: nausea and vomiting, diarrhea, and a low-grade fever. as amy picks up the phone to call the principal to give her an update, she receives her eighth call of the day. Keith Jackson, a first-grade teacher who has been out sick since Monday, calls to tell amy that his physician sent his stool sample to the laboratory for testing. the results came back positive for norovirus. What is norovirus? read on to find out

Review the types of disinfectants highlighted in lecture: phenol and phenolics (derivatives), biguanides (chlorohexidine), halogens (iodine and chlorine), alcohols, heavy metals, surface-active agents, quaternary ammonium compounds, aldehydes, gaseous chemosterilizers (ethylene oxide gas) peroxygens (oxydizing agents).

Let's start with phenols. Phenol was first used by Joseph Lister to control surgical infections in the operating room. However, it had irritating properties and eventually a better alternative came along, namely the phenolics. Phenolics exert their action by damaging plasma membranes. The cell wall of mycobacterium, the cause of tuberculosis, is rich in lipids, which makes them susceptible to phenolics. A useful property of phenolics as disinfectants is that they remain active in the presence of organic materials such as pus, saliva, and feces. A very important phenolic, O-phenylphenol is the main ingredient in the product Lysol. Bisphenols are also structurally related to phenol. One widely used bisphenol is triclosan. Next time you use an antibacterial soap, examine the ingredients and you will probably find the word triclosan. Triclosan is effective against most bacteria and fungi. However, Pseudomonas aeruginosa, a gram negative bacteria is very resistant to triclosan as well as other disinfectants and antibiotics. Another example of a bisphenol is pHisoHex, which is used for hospital microbial control procedures. Chlorhexidine is a member of the biguanide group. It is used often in hospitals as a surgical scrub when combined with alcohol or a detergent. A new product, Avagard, combines chlorhexidine and ethanol. Avagard is low in toxicity and active on skin for about 6 hours. The third group we want to discuss is the halogens, which include iodine and chlorine. Iodine is available as a tincture - that is, in solution in alcohol. When iodine is combined with an organic molecule, it is called an iodophor and has the advantage over iodine of not staining and being less irritating. Iodine is effective against all kinds of bacteria, many endospores, various fungi, and some viruses. The most commonly used iodophor is Betadine. It is mainly used for skin disinfection and wound treatment. If you enjoy camping, you may also be familiar with iodine tablets or iodine-treated resins filters for water treatment. Chlorine, is a widely used disinfectant either as a gas or as a liquid. It is used extensively for disinfecting municipal drinking water, water in swimming pools and sewage. Another chlorine compound, sodium hypochlorite, is found in the household disinfectant bleach. Our fourth group is the alcohols. Alcohols have the advantage of acting and then evaporating rapidly and leaving no residue. When the skin is swabbed before an injection, most of the microbial control activity comes from the motion of wiping away dirt, skin oils, and microbes. If I asked you which concentration of alcohol would you predict would be the most effective, 100 percent or 70 percent, what would you answer? Well you may have thought the more concentrated 100 percent alcohol is best. But actually the 70 percent alcohol solution is preferred. Why? Well it seems that alcohols work by denaturing proteins and to denature proteins, you need water. Finally, let's discuss the aldehydes, such as formaldehyde. When this gas is an aqueous solution, it is called formalin and is used to preserve biological specimens. Glutaraldehyde is a chemical relative of formaldehyde used to disinfect hospital instruments. It is one of the few liquid disinfectants that can be considered a sterilizing agent. Both glutaraldehyde and formaldehyde are used by morticians for embalming.

table 7.8 (page 196) for a summary of the chemical control methods covered in lecture.

We will now discuss chemical agents. We use chemical agents to control microbes on living tissue and inanimate objects. Few chemical agents achieve sterility. Most of them reduce microbial populations to safe levels or remove vegetative forms of pathogens from objects. A common dilemma is often which chemical to choose. No single disinfectant is appropriate for all circumstances. In this review, we will cover phenol and some related chemicals, biguanides, halogens, alcohol, and aldehydes. I encourage you to read more about these and other chemical agents in your text. Your textbook also discusses the different ways to evaluate chemicals, such as the disk-diffusion method and the use-dilution test. You may be scheduled to perform some of these methods in the laboratory component of your course.

Describe how filtration, low temperature, high pressure, desiccation, and osmotic pressure suppress microbial growth

When we filter something, we pour the sample through a filter of known pore size. Bacteria are too large to pass through the filter and are retained on it. We can then recover the sterilized sample from the lower chamber. Filtration is the preferred method for sterilizing a heat-sensitive liquid. Filtration also works to clean air. You may be familiar with HEPA filters, which remove many microorganisms from the air. The third physical method we will discuss is low temperatures. The main effect of cold treatment is to slow microbial growth. Refrigeration has a bacteriostatic effect on most microbes so they cannot reproduce or make toxins. This means that the microbes survive but cannot grow in the refrigerator. However, psychrotrophs do grow fairly well at refrigerator temperatures and can cause food spoilage. Most pathogenic bacteria are mesophilic and will not grow at refrigerator temperatures. High pressure is another physical technique for controlling microbial growth. High pressure applied to liquid suspensions is transferred instantly and evenly throughout the sample. If the pressure is high enough, it alters the molecular structures of proteins and carbohydrates, rapidly destroying vegetative cells. You may have purchased fruit juices preserved by high-pressure treatments. An advantage of this method is that it preserves the flavors, colors, and nutrient values of these products. Osmotic pressure is our fifth physical method of control. Osmotic pressure is the concept behind the use of high concentrations of salts and sugars to cause water to leave cells. This is an excellent way to preserve foods. For example, we use concentrated salt solutions to cure meats and thick sugar solutions to preserve fruits. Generally, molds and yeasts resist osmotic pressures better than bacteria.

Compare the effectiveness of moist heat (boiling, autoclaving, pasteurization) and dry heat

The first treatment we will discuss is heat. Heat can be either moist or dry. Moist heat kills cells by denaturing their proteins. The three processes that use moist heat are boiling, using an autoclave, or pasteurization. Boiling water can kill vegetative forms of bacterial pathogens, but does not kill all viruses and endospores and so it is not a reliable sterilization procedure. Temperatures must be above 100 degrees Celsius in order to reliably kill all forms of life. These high temperatures can be achieved by steam under pressure in the autoclave. Autoclaving is the best way to sterilize, unless the material to be sterilized is heat or moisture sensitive. By increasing the pressure, the autoclave allows the water to reach higher than usual temperatures before evaporation takes place. The third method that uses moist heat to control microbial growth is pasteurization. The purpose of pasteurization is to eliminate pathogenic microbes and reduce microbial numbers. This gives the product a longer shelf life. Pasteurization of milk can be achieved by exposing it to 72 degrees Celsius for 15 seconds. Note that when milk is pasteurized it is not made sterile. How do you know this? Well, think about the sell-by date on a carton of milk. The dairy industry knows how long it will take for the small number of live bacteria still in the milk to cause it to sour, even if you never open it. Many products other than milk are also pasteurized, such as ice cream, yogurt, apple juice, and beer. Milk can also be sterilized using a form of pasteurization called ultra-high-temperature treatment or UHT. UHT is used on the small containers of coffee creamers found in restaurants. UHT exposes the milk to 140°C for 5 seconds. These little creamers do not need refrigeration, because they are sterile. Dry heat kills cells by oxidation effects. In lab, have you had a chance to sterilize an inoculating loop? Well, that's an example of dry heat sterilization. Sometimes garbage is incinerated to dispose of contaminated waste. Another form of dry heat is the hot-air oven. Generally, a temperature of about 170 degrees Celsius is maintained for about 2 hours.

Table 76.5 (page 186) for a summary of the physical control methods covered in lecture and table 7.8.

These methods are heat, filtration, low temperatures, high pressure, osmotic pressure and radiation. Heat can be either moist or dry. Moist heat kills cells by denaturing their proteins. The three processes that use moist heat are boiling, using an autoclave, or pasteurization. Boiling water can kill vegetative forms of bacterial pathogens, but does not kill all viruses and endospores and so it is not a reliable sterilization procedure. Temperatures must be above 100 degrees Celsius in order to reliably kill all forms of life. These high temperatures can be achieved by steam under pressure in the autoclave. Autoclaving is the best way to sterilize, unless the material to be sterilized is heat or moisture sensitive. By increasing the pressure, the autoclave allows the water to reach higher than usual temperatures before evaporation takes place. The third method that uses moist heat to control microbial growth is pasteurization. The purpose of pasteurization is to eliminate pathogenic microbes and reduce microbial numbers. This gives the product a longer shelf life. Pasteurization of milk can be achieved by exposing it to 72 degrees Celsius for 15 seconds. Note that when milk is pasteurized it is not made sterile. How do you know this? Well, think about the sell-by date on a carton of milk. The dairy industry knows how long it will take for the small number of live bacteria still in the milk to cause it to sour, even if you never open it. Many products other than milk are also pasteurized, such as ice cream, yogurt, apple juice, and beer.