Microbiology: Fundamentals of Microbial Growth (Ch 7)

An organism that harvests sunlight for energy and requires an external source of organic carbon in its metabolism is characterized as a _________. A. photoautotroph B. photoheterotroph C. chemoautotroph D. chemoheterotroph

B An organism that harvests sunlight for energy and requires an external source of organic carbon in its metabolism is characterized as a photoheterotroph. Organisms that use light energy are phototrophs. Heterotrophic organisms require an external source of organic carbon, such as sugars, lipids, and proteins. These cells then extract the carbon from these nutrients for use in various cell parts. Organisms that break down chemical compounds for energy are chemotrophs. Autotrophs are "self-feeding" organisms that use a process called carbon fixation to convert inorganic carbon into organic carbon.

Which of the following "germicides" should only be applied as a disinfectant? A. Alcohols B. Formaldehyde C. Halogens D. Peroxygens

B Formaldehyde acts as a high- or intermediate-level disinfectant based on its concentration. Formaldehyde is sold in both liquid and gas states, which makes it a versatile option for equipment cleaning. It works by reacting with proteins and nucleic acids and is also used to sterilize surgical instruments, endoscopes, dialyzers, and anesthesia and respiratory equipment. One major drawback of formaldehyde is that it generates irritating fumes and is a suspected carcinogen; it can only be used as a disinfectant. Alcohols, halogens, and peroxygens all can be applied as disinfectants and antiseptics.

Which of the following techniques is the best way to directly count viable cells? A. Direct microscopic counting of only motile cells B. Measuring the turbidity of a solution C. Measuring the consumption of oxygen D. Plate counts

D A method to directly enumerate a population is the viable plate count. Samples from a liquid broth culture are serially diluted and either spread on solid agar (spread plate method) or poured into a petri plate with melted agar media (pour plate method). After an incubation period, the visible colonies (derived from one single viable cell) can be counted. Direct microscopic counting of cells is a common method; it is easy and requires minimal reagents and equipment. The disadvantages are that dead cells are not always easily differentiated from living cells, and a fairly dense culture suspension is needed. Also, motile cells can be difficult to accurately count. One of the fastest and easiest ways to indirectly measure cell numbers is to measure the cloudiness, or turbidity, of a liquid culture. The more cells in the sample, the more turbid it is. A spectrophotometer measures either how much light can pass through the liquid or how much light is absorbed. The downside to this type of measurement is that both living and dead cells account for turbidity in the growth medium. While measuring the consumption of oxygen would be possible, it would only work for cultures that respire oxygen (aerobies, facultative anaerobes, and microaerophiles). Some prokaryotes that are anaerobes or aerotolerant anaerobes do not respire oxygen.

A sample obtained from a patient's throat was inoculated on blood agar. After 24 hours, there was a clear yellow zone covering the area of growth. The medium used was _________ and the organism is said to be _________. A. selective; beta hemolytic B. enriched; gamma hemolytic C. differential; alpha hemolytic D. differential; beta hemolytic

D Blood agar is an example of a differential medium, and the organism is said to be beta hemolytic. Blood agar contains sheep red blood cells that serve as both a nutrient and a differentiating indicator. Some pathogenic bacteria lyse red blood cells for nutrients by releasing an enzyme called hemolysin. When grown on blood agar, pathogens that make hemolysin lyse red blood cells in the media and generate a yellow zone around colonies. Bacteria that can do this are said to be beta hemolytic. Selective media single out bacteria that have specific properties. This is accomplished by including ingredients in the media that foster the growth of certain bacteria while suppressing the growth of others, like ampicillin. Blood agar is an example of an enriched medium, as it contains the complex ingredient blood. Gamma-hemolytic organisms do not lyse red blood cells, and the medium would remain red. Blood agar is an example of a differential medium, as it contains red blood cells that serve as both a nutrient and a differentiating indicator. Alpha-hemolytic organisms do not lyse red blood cells and instead just oxidize hemoglobin, the oxygen-carrying component of blood. Bacteria with this property turn blood agar a green color.

Which of the following chemicals is applied as a gas to sterilize equipment and disinfect surfaces? A. Ethylene oxide B. Hydrogen peroxide C. Isopropanol D. Phenol

A Ethylene oxide, a colorless gas, is a good sterilization method. It primarily works by damaging proteins and nucleic acids. Sterilization efficacy is influenced by temperature, exposure time, humidity, and gas concentration. Ethylene oxide is often applied to implant devices, such as pacemakers, that contain electronic parts or plastic components, but it can also be used on tissue, such as heart valves being prepared for transplantation. Hydrogen peroxide, isopropanol, and phenol are all usable as antiseptics and disinfectants but are applied to surfaces in liquid form.

The enzymes superoxide dismutase and catalase work together to convert reactive superoxide ions back to _________. A. molecular oxygen B. hydrogen peroxide C. ozone D. unreactive oxide ions

A The enzymes superoxide dismutase and catalase work together to convert reactive superoxide ions back to molecular oxygen. Superoxide dismutase converts reactive superoxide ions to hydrogen peroxide, and then catalase converts hydrogen peroxide to water and oxygen.

A patient's preliminary diagnosis, as well as the stage and site of infection, affect how and when a representative sample is obtained for microbiological analysis. Which would be the most appropriate tools to collect samples from a patient infected with Clamydia trachomatis? A. Swab with a plastic or wire shaft and a non-cotton tip B. Cotton-tipped swab with plastic stick C. Cotton-tipped swab with wooden stick D. Non-cotton-tipped swab with wooden or plastic stick

A A swab with a plastic or wire shaft and a non-cotton tip should be used to collect samples from a patient suspected to be infected with Chlamydia trachomatis. Wood and cotton interfere with the bacterium's isolation. Once a sample is collected, it may have to be immediately refrigerated or placed on dry ice to ship to a laboratory. If samples are improperly collected, left at the wrong temperature, and/or transport to the lab is delayed, then lab data is very likely to be compromised, which can lead to the wrong diagnosis and improper treatment.

You would like to show the presence of Gram-negative lactose fermenters in your unknown pure culture. Which of the following media would be most appropriate? A. Eosine methylene blue (EMB) agar B. Mannitol salt agar (MSA) C. Blood agar D. Thioglycolate

A Eosin methylene blue agar (EMB) would be the most appropriate medium for the task. It contains the dyes eosin and methylene blue, which limit Gram-positive bacterial growth while allowing Gram-negative bacteria to grow. It differentiates among Gram-negative species based on their ability to ferment the sugar lactose. Mannitol salt agar (MSA) is selective due to its high salt content (5%). Staphylococcus species, which are Gram-positive bacteria that tolerate high salt are commonly found on our skin. MSA differentiates organisms based on their ability to ferment a sugar called mannitol. Blood agar is a differential medium that contains sheep red blood cells (or other blood samples) that serve as both a nutrient and a differentiating indicator. The semisolid thioglycolate medium is used to generate an oxygen gradient ranging from an oxygen-rich environment at the very top of the media to an anaerobic environment at the bottom.

The type of medical equipment that regularly comes into contact with sterile body sites or the vascular system and must therefore be sterilized is classified as _________. A. critical equipment B. semicritical equipment C. noncritical equipment D. Most medical equipment does not have to be decontaminated.

A Most medical equipment requires regular decontamination, and it is classified in three tiers. Critical equipment comes into contact with sterile body sites or the vascular system and must therefore be sterilized. Examples include surgical tools, implants, and urinary and cardiac catheters. Semicritical equipment comes in contact with mucous membranes or non-intact skin and should be free of bacteria, fungi, and viruses, but low numbers of endospores are not a threat. Tubing for endoscopes or anesthesia or respiratory therapy is an example of semicritical equipment. Noncritical equipment, such as stethoscopes, blood pressure cuffs, and most surfaces in patient rooms, only contact patients' intact skin and therefore require less stringent disinfection than semicritical and critical equipment.

Which of the following microbes would most likely be able to infect a deep wound on a patient? A. Neutralophilic mesophiles B. Acidophilic thermophiles C. Alkaliphilic thermophiles D. Phototrophic mesophiles

A Neutralophilic mesophiles would most likely infect a deep wound. Neutralophiles grow best in a pH range of 5-8. They make up the majority of microorganisms, especially pathogens, known today. Acidophilic and alkaliphilic thermophiles and phototrophic mesophiles would not likely infect a human. The pH of human blood is around 7.4, and normal body temperature is 37°C. Acidophiles can grow at pH 1 (or less) to pH 5. Alkaliphiles are microbes that grow in the basic pH range of 9-11. Thermophiles prefer warm temperatures of roughly 40-75°C. Phototrophs use light energy and are not pathogenic.

Although Staphylococcus aureus can grow at 7% NaCl, it grows best at NaCl levels less than 1%. This organism is a __________. A. facultative halophile B. halophile C. neutralophile D. mesophile

A Organisms such as Staphylococcus aureus that tolerate higher solute concentrations but may not grow especially well in them are called facultative halophiles. These bacteria that thrive on our skin despite salty sweat can withstand solute concentrations of up to 15%. Some organisms thrive in high-salt environments and are called halophiles. These microbes tolerate environments that are up to 35% salt and can thrive in places like the Dead Sea and the Great Salt Lake of Utah. Neutralophiles are organisms that grow best in a pH range of 5-8. They make up the majority of microorganisms known today, especially pathogens. The term mesophile refers to the specific temperature need of an organism. Mesophiles prefer moderate temperatures and tend to grow best around 10-50°C.

Your initial inoculum was 200 cells. How many cells would be present after 2 hours if the generation time was 30 minutes? A. 3,200 cells B. 800 cells C. 400 cells D. 1,600 cells

A Prokaryotic cells replicate by binary fission; one parent cell will divide and produce two identical daughter cells. Therefore, with each generation the number of cells will double. Your initial inoculum was 200 cells. After one generation, 400 cells would be present, after two generations 800 cells, after three generations 1,600 cells, and after four generations, 3,200 cells woud be present in your culture.

You inoculated a pure culture of unknown bacteria into a tube containing thioglycollate medium to study their metabolic relationship with oxygen. The culture was incubated for an appropriate length of time at the optimal growth temperature of the organism. When you examined the culture, you observed that although the organism grew throughout the medium, the growth was denser at the surface. What would you conclude about the oxygen requirements for this unknown species? A. The unknown is a facultative anaerobe. B. The unknown is an obligate aerobe. C. The unknown is an obligate anaerobe. D. The unknown is a microaerophile.

A The unknown is a facultative anaerobe. Facultative anaerobes have enzymes to detoxify reactive oxygen species (ROS), and they can switch between using oxygen for metabolism or employing fermentation as an anaerobic form of metabolism. The top layer of the medium contains oxygen; the bottom of the medium is anaerobic. You would find most growth in the top layer because oxygen respiration yields the most ATP, thus the dense growth. An obligate aerobe has an absolute dependence on oxygen for cellular processes and will die unless oxygen is abundant. An obligate anaerobe would only be found in the anoxic portion of the medium. Obligate anaerobes do not use oxygen in their metabolism, and they tend to die in aerobic environments because they can't eliminate ROS. Microaerophiles use only small amounts of atmospheric oxygen and live in low-oxygen settings where they can limit their exposure to ROS while still meeting their oxygen needs. A microaerophile would not grow at the very top of the tube

Which of the following infectious agents are the most difficult to control with standard autoclaving methods? A. Protozoans B. Prions C. Endospore-forming bacteria D. Enveloped viruses

B Infectious proteins called prions can withstand standard autoclaving times and temperatures. They can, however, be eliminated through a combination of chemical treatments and increased temperature and pressure during autoclaving. Different stages of a protozoan's life cycle can resist certain control methods, though they can be removed easily with boiling and autoclaving. Additional methods used are filtration, application of carbon dioxide, and ozone treatments, as well as exposure to ultraviolet light. Endospore-forming bacteria can be removed by autoclaving. Other methods include using hydrogen peroxide vapor at high heat or applying specific disinfectants termed sporicides that usually contain bleach. The envelopes of enveloped viruses contain lipids that are sensitive to heat, drying, and detergents, which makes most enveloped viruses easier to destroy than naked (nonenveloped) viruses.

Listeria monocytogenes can grow slowly in your refrigerator, with its optimum temperature range between 30 to 37°C. This organism is characterized as a __________. A. psychrophile B. psychrotroph C. mesophile D. thermophile

B Listeria monocytogenes is a psychrotroph. Psychrotrophs are cold-tolerant organisms that can grow at about 0-30°C. These organisms are commonly associated with foodborne illness because they grow at room temperature as well as in refrigerated and frozen foods. Listeria monocytogenes readily grow in refrigerated foods and can contaminate meat and fruit to cause food infections. Psychrophiles can thrive between about −20°C and 10°C. These organisms tend to live in environments that are consistently cold. Mesophiles prefer moderate temperatures and tend to grow best around 10-50°C, a range that includes body temperature. Thermophiles prefer warm temperatures of roughly 40-75°C.

You are preparing a growth medium for an isolate that you obtained from a patient. You are adding yeast extract, glucose, peptones, malt extract, and ampicillin. Which kind of medium are you preparing? A. defined and selective B. complex and selective C. complex and differential D. defined and differential

B You are preparing a complex and selective medium. Complex media contain a mixture of organic and inorganic nutrients that are not fully defined. Instead, they contain more complex ingredients, like blood, milk proteins, or yeast extracts. These are chemically sophisticated mixtures for which we don't know the precise quantity of every vitamin or even the exact level of a given carbon nutrient. Selective media single out bacteria that have specific properties. This is accomplished by including ingredients in the media that allow for the growth of certain bacteria while suppressing the growth of others, like ampicillin. Defined media have a chemically defined or precisely known composition; each organic and inorganic component is completely known and quantified. A differential medium is a specialized medium that is formulated so that we can visually distinguish one microbe from another based on how they metabolize media components. The formulation of differential media makes bacterial colonies with different physiological properties from one another appear as different colors on the medium, or the medium itself may change color in response to the presence of certain bacteria.

_________ reduces the number of microbes on a given non-living surface of a sample. A. Sterilization B. Aseptic technique C. Disinfection D. Pasteurization

C Disinfection reduces the number of microbes on a given non-living surface of a sample. Disinfection is usually sufficient for most cosmetics, foods, surfaces, and medical equipment that doesn't contact internal body tissues. Sterilization eliminates all bacteria, viruses, and endospores. It is often required for drugs, objects used for invasive medical procedures, and for media and glassware used to culture microbes in a lab. Aseptic techniques prevent the introduction of contaminating microbes to a patient, a clinical sample, or others in the healthcare setting. Pasteurization is applied to dairy and other food products to eliminate pathogenic bacteria. Application of moderate heat well below the liquid's boiling point eliminates pathogens and reduces (though does not fully eliminate) harmless microbes that cause spoilage.

Prokaryotes reproduce via _________. A. mitosis B. biofilm formation C. binary fission D. endospore formation

C Prokaryotes reproduce via binary fission. For most prokaryotes, this involves dividing a single cell into two genetically identical cells. Mitosis is a form of asexual reproduction and is the most common way eukaryotic cells divide. Mitosis generates two genetically identical offspring from one parent cell. Biofilms form when planktonic microorganisms adhere to a surface. Some microbes produce a sticky, polysaccharide matrix that allows other microbes to adhere to the growing biofilm. Eventually a mixed microbial community develops. Bacterial endospores are the thick-walled, nongrowing structures that some bacteria like Bacillus and Clostridium species form as a way to survive unfavorable growth conditions.

You calculated the generation time of a strain of Pseudomonas aeruginosa as 40 minutes. How long would it take (in hours) for three generations to grow? A. 3 hours B. 120 hours C. 2 hours D. 0.21667 hrs (13 minutes)

C Since the generation time is 40 minutes, it would take 2 hours for three generations to grow. The best way to approach the problem is to multiply 40 minutes/generation with the number of generations. 40 minutes/generation times three generations equals 120 minutes. The problem is asking for hours; therefore 120 minutes divided by 60 minutes/hour equals 2 hours.

Which of the following methods would be best for sterilizing a sample of a heat-sensitive solution? A. UV radiation B. Autoclaving C. Freezing D. Filtration

D Heat-sensitive solutions are best sterilized by filtration. The application of heat would compromise the solution. Liquids can be sterilized using superfine membrane filters made from a variety of polymers, such as cellulose nitrate and polyester. Pore sizes can range from 0.1 μm to filter out bacteria and protists to 0.01 μm to remove viruses. UV light treatments cannot pass through packaging (such as flasks or plastic tubes containing glucose), and they are ineffective at decontaminating objects covered in organic material. Temperature and distance from the UV light sources can also affect decontamination efficacy. Autoclaving would not be an appropriate method for sterilizing a heat-sensitive solution. Only materials that withstand moisture, high heat, and pressure can be treated. Some plastics easily melt, and bottles may collapse. Certain media and solutions and drugs also can't be autoclaved without affecting their chemical integrity. Refrigeration (4°C) and freezing (0°C) do not eliminate microbes, but they slow the growth of many microbes associated with food spoilage and foodborne illnesses. In the laboratory, cold temperatures preserve specimen isolates and increase the shelf life of media and other reagents. Refrigeration also preserves clinical samples (blood, tissue biopsies, or swabs containing suspected pathogens) until they are delivered to the clinical lab for analysis.

You would like to sterilize bacterial growth media in a flask. You have about 1.5 hours before your patient's sample will be ready for inoculation. Which method would be best to sterilize the media within the timeframe? A. Dry heat B. Pasteurization C. Boiling for 15 minutes D. Autoclaving

D Most substances are rendered sterile within 20 minutes using standard autoclave settings, which exert a pressure of 15 pounds per square inch and steam heat at 121°C. This method would allow you to sterilize and cool your growth media in your available time. Incineration or hot-air ovens can also be used for sterilization or disinfection, but in this case would destroy the growth medium due to oxidation and/or dehydration. Dry heat sterilization takes much longer than autoclaving—about two hours in a dry heat oven at 170°C. Pasteurization is the application of moderate heat, well below the liquid's boiling point, to eliminate pathogens and reduce (though does not fully eliminate) harmless microbes that cause spoilage. It will not sterilize your growth medium. Boiling a solution for 15 minutes would not ensure that your growth medium is sterile.

When preparing growth media for Enterococcus durans, one must add amino acids, vitamins, and other growth factors in order for the organism to grow properly. This organism is said to be _________. A. autotrophic B. nitrogen fixing C. phototrophic D. fastidious

D Organisms that need multiple growth factors are said to be fastidious. When growing fastidious microbes in the laboratory, amino acids, vitamins, and/or nitrogenous bases must be supplied in the growth medium—this is in addition to supplying all the essential nutrients and an energy source. Autotrophs are "self-feeding" organisms that use a process called carbon fixation to convert inorganic carbon into organic carbon. Consequently, these organisms, which include photosynthetic microbes and plants, do not require an external source of organic carbon. Enterococcus durans is a chemotroph. Nitrogen-fixing cells get their nitrogen directly from the atmosphere in a process. This process is an important step in converting atmospheric nitrogen from a gas form to a nongaseous form, like ammonia, that can be used by other cellular life. Enterococcus durans does not fix nitrogen. Organisms that use light energy are phototrophs. Enterococcus durans is a chemotroph.

You would like to calculate the generation time of an unknown organism and set up a culture. During which of four distinct growth phases would it be best to collect data? A. Lag phase B. Stationary phase C. Death phase D. Log phase

D The logarithmic (log) phase is characterized by an upward-sloped line that results when the number of viable cells is plotted on a logarithmic scale as a function of time. This phase lasts as long as sufficient nutrients are available and metabolic wastes are not appreciably accumulating. The generation time can be calculated during this phase since population increase is favorable. Cells adjusting to their new environment is called the lag phase. During this phase, cells alter their gene expression in response to their new setting. For example, they may make new enzymes and transporter proteins so that they can take up and metabolize nutrients provided in their new environment. At some point during growth of bacteria in a culture, available nutrients are depleted and waste products accumulate. Over time, the population growth rate slows and eventually levels off as the number of cells dying matches the number of cells dividing. This is the stationary phase. At a critical point of waste buildup and decreasing nutrients, the cells begin to die. During the death phase, the rate of cell death is exponential and varies based on the starting factors and species being grown.