9.E: Microbial Growth (Exercises)

9.1: How Microbes Grow

The bacterial cell cycle involves the formation of new cells through the replication of DNA and partitioning of cellular components into two daughter cells. In prokaryotes, reproduction is always asexual, although extensive genetic recombination in the form of horizontal gene transfer takes place, as will be explored in a different chapter. Most bacteria have a single circular chromosome; however, some exceptions exist.

Multiple Choice

Which of the following methods would be used to measure the concentration of bacterial contamination in processed peanut butter?

1. turbidity measurement
2. total plate count
3. dry weight measurement
4. direct counting of bacteria on a calibrated slide under the microscope

Answer

B

In which phase would you expect to observe the most endospores in a Bacillus cell culture?

1. death phase
2. lag phase
3. log phase
4. log, lag, and death phases would all have roughly the same number of endospores.

Answer

A

During which phase would penicillin, an antibiotic that inhibits cell-wall synthesis, be most effective?

1. death phase
2. lag phase
3. log phase
4. stationary phase

Answer

C

Which of the following is the best definition of generation time in a bacterium?

1. the length of time it takes to reach the log phase
2. the length of time it takes for a population of cells to double
3. the time it takes to reach stationary phase
4. the length of time of the exponential phase

Answer

B

What is the function of the Z ring in binary fission?

1. It controls the replication of DNA.
2. It forms a contractile ring at the septum.
3. It separates the newly synthesized DNA molecules.
4. It mediates the addition of new peptidoglycan subunits.

Answer

B

If a culture starts with 50 cells, how many cells will be present after five generations with no cell death?

1. 200
2. 400
3. 1600
4. 3200

Answer

C

Filamentous cyanobacteria often divide by which of the following?

1. budding
2. mitosis
3. fragmentation
4. formation of endospores

Answer

C

Which is a reason for antimicrobial resistance being higher in a biofilm than in free-floating bacterial cells?

1. The EPS allows faster diffusion of chemicals in the biofilm.
2. Cells are more metabolically active at the base of a biofilm.
3. Cells are metabolically inactive at the base of a biofilm.
4. The structure of a biofilm favors the survival of antibiotic resistant cells.

Answer

C

Quorum sensing is used by bacterial cells to determine which of the following?

1. the size of the population
2. the availability of nutrients
3. the speed of water flow
4. the density of the population

Answer

D

Which of the following statements about autoinducers is incorrect?

1. They bind directly to DNA to activate transcription.
2. They can activate the cell that secreted them.
3. N-acylated homoserine lactones are autoinducers in gram-negative cells.
4. Autoinducers may stimulate the production of virulence factors.

Answer

A

Fill in the Blank

Direct count of total cells can be performed using a ________ or a ________.

Answer

hemocytometer, Petroff-Hausser counting chamber

The ________ method allows direct count of total cells growing on solid medium.

Answer

plate count

A statistical estimate of the number of live cells in a liquid is usually done by ________.

Answer

most probable number

For this indirect method of estimating the growth of a culture, you measure ________ using a spectrophotometer.

Answer

turbidity

Active growth of a culture may be estimated indirectly by measuring the following products of cell metabolism: ________ or ________.

Answer

ATP, acid from fermentation

Matching

Match the definition with the name of the growth phase in the growth curve.

___Number of dying cells is higher than the number of cells dividing	A. Lag phase
___Number of new cells equal to number of dying cells	B. Log phase
___New enzymes to use available nutrients are induced	C. Stationary phase
___Binary fission is occurring at maximum rate	D. Death phase

Answer

D, C, A, B

Short Answer

Why is it important to measure the transmission of light through a control tube with only broth in it when making turbidity measures of bacterial cultures?

In terms of counting cells, what does a plating method accomplish that an electronic cell counting method does not?

Order the following stages of the development of a biofilm from the earliest to the last step.

1. secretion of EPS
2. reversible attachment
3. dispersal
4. formation of water channels
5. irreversible attachment

Infections among hospitalized patients are often related to the presence of a medical device in the patient. Which conditions favor the formation of biofilms on in-dwelling catheters and prostheses?

Critical Thinking

A patient in the hospital has an intravenous catheter inserted to allow for the delivery of medications, fluids, and electrolytes. Four days after the catheter is inserted, the patient develops a fever and an infection in the skin around the catheter. Blood cultures reveal that the patient has a blood-borne infection. Tests in the clinical laboratory identify the blood-borne pathogen as Staphylococcus epidermidis, and antibiotic susceptibility tests are performed to provide doctors with essential information for selecting the best drug for treatment of the infection. Antibacterial chemotherapy is initiated and delivered through the intravenous catheter that was originally inserted into the patient. Within 7 days, the skin infection is gone, blood cultures are negative for S. epidermidis, and the antibacterial chemotherapy is discontinued. However, 2 days after discontinuing the antibacterial chemotherapy, the patient develops another fever and skin infection and the blood cultures are positive for the same strain of S. epidermidis that had been isolated the previous week. This time, doctors remove the intravenous catheter and administer oral antibiotics, which successfully treat both the skin and blood-borne infection caused by S. epidermidis. Furthermore, the infection does not return after discontinuing the oral antibacterial chemotherapy. What are some possible reasons why intravenous chemotherapy failed to completely cure the patient despite laboratory tests showing the bacterial strain was susceptible to the prescribed antibiotic? Why might the second round of antibiotic therapy have been more successful? Justify your answers.

Why are autoinducers small molecules?

Refer to Figure B.1 in Appendix B. If the results from a pond water sample were recorded as 3, 2, 1, what would be the MPN of bacteria in 100 mL of pond water?

Refer to Figure 9.1.14. Why does turbidity lose reliability at high cell concentrations when the culture reaches the stationary phase?

9.2: Oxygen Requirements for Microbial Growth

Ask most people “What are the major requirements for life?” and the answers are likely to include water and oxygen. Few would argue about the need for water, but what about oxygen? Can there be life without oxygen? The answer is that molecular oxygen is not always needed. The earliest signs of life are dated to a period when conditions on earth were highly reducing and free oxygen gas was essentially nonexistent.

Multiple Choice

An inoculated thioglycolate medium culture tube shows dense growth at the surface and turbidity throughout the rest of the tube. What is your conclusion?

1. The organisms die in the presence of oxygen
2. The organisms are facultative anaerobes.
3. The organisms should be grown in an anaerobic chamber.
4. The organisms are obligate aerobes.

Answer

B

An inoculated thioglycolate medium culture tube is clear throughout the tube except for dense growth at the bottom of the tube. What is your conclusion?

1. The organisms are obligate anaerobes.
2. The organisms are facultative anaerobes.
3. The organisms are aerotolerant.
4. The organisms are obligate aerobes.

Answer

A

Pseudomonas aeruginosa is a common pathogen that infects the airways of patients with cystic fibrosis. It does not grow in the absence of oxygen. The bacterium is probably which of the following?

1. an aerotolerant anaerobe
2. an obligate aerobe
3. an obligate anaerobe
4. a facultative anaerobe

Answer

B

Streptococcus mutans is a major cause of cavities. It resides in the gum pockets, does not have catalase activity, and can be grown outside of an anaerobic chamber. The bacterium is probably which of the following?

1. a facultative anaerobe
2. an obligate aerobe
3. an obligate anaerobe
4. an aerotolerant anaerobe

Answer

D

Why do the instructions for the growth of Neisseria gonorrheae recommend a CO₂-enriched atmosphere?

1. It uses CO₂ as a final electron acceptor in respiration.
2. It is an obligate anaerobe.
3. It is a capnophile.
4. It fixes CO₂ through photosynthesis.

Answer

C

Matching

Four tubes are illustrated with cultures grown in a medium that slows oxygen diffusion. Match the culture tube with the correct type of bacteria from the following list: facultative anaerobe, obligate anaerobe, microaerophile, aerotolerant anaerobe, obligate aerobe.

Answer

(a) obligate anaerobe, (b) obligate aerobe, (c) microaerophile, (d) facultative anaerobe

Short Answer

Why are some obligate anaerobes able to grow in tissues (e.g., gum pockets) that are not completely free of oxygen?

Why should Haemophilus influenzae be grown in a candle jar?

In terms of oxygen requirements, what type of organism would most likely be responsible for a foodborne illness associated with canned foods?

Critical Thinking

A microbiology instructor prepares cultures for a gram-staining practical laboratory by inoculating growth medium with a gram-positive coccus (nonmotile) and a gram-negative rod (motile). The goal is to demonstrate staining of a mixed culture. The flask is incubated at 35 °C for 24 hours without aeration. A sample is stained and reveals only gram-negative rods. Both cultures are known facultative anaerobes. Give a likely reason for success of the gram-negative rod. Assume that the cultures have comparable intrinsic growth rates.

9.3: The Effects of pH on Microbial Growth

Bacteria are generally neutrophiles. They grow best at neutral pH close to 7.0. Acidophiles grow optimally at a pH near 3.0. Alkaliphiles are organisms that grow optimally between a pH of 8 and 10.5. Extreme acidophiles and alkaliphiles grow slowly or not at all near neutral pH. Microorganisms grow best at their optimum growth pH. Growth occurs slowly or not at all below the minimum growth pH and above the maximum growth pH.

Multiple Choice

Bacteria that grow in mine drainage at pH 1–2 are probably which of the following?

1. alkaliphiles
2. acidophiles
3. neutrophiles
4. obligate anaerobes

Answer

B

Bacteria isolated from Lake Natron, where the water pH is close to 10, are which of the following?

1. alkaliphiles
2. facultative anaerobes
3. neutrophiles
4. obligate anaerobes

Answer

A

In which environment are you most likely to encounter an acidophile?

1. human blood at pH 7.2
2. a hot vent at pH 1.5
3. human intestine at pH 8.5
4. milk at pH 6.5

Answer

B

Fill in the Blank

A bacterium that thrives in a soda lake where the average pH is 10.5 can be classified as a(n) ________.

Answer

alkaliphile

Lactobacillus acidophilus grows best at pH 4.5. It is considered a(n) ________.

Answer

acidophile

Short Answer

Which macromolecule in the cell is most sensitive to changes in pH?

Which metabolic process in the bacterial cell is particularly challenging at high pH?

Critical Thinking

People who use proton pumps inhibitors or antacids are more prone to infections of the gastrointestinal tract. Can you explain the observation in light of what you have learned?

9.4: Temperature and Microbial Growth

Microorganisms thrive at a wide range of temperatures; they have colonized different natural environments and have adapted to extreme temperatures. Both extreme cold and hot temperatures require evolutionary adjustments to macromolecules and biological processes. Psychrophiles grow best in the temperature range of 0–15 °C whereas psychrotrophs thrive between 4 °C and 25 °C. Mesophiles grow best at moderate temperatures in the range of 20 °C to about 45 °C. Pathogens are usually mesophiles.

Multiple Choice

A soup container was forgotten in the refrigerator and shows contamination. The contaminants are probably which of the following?

1. thermophiles
2. acidophiles
3. mesophiles
4. psychrotrophs

Answer

D

Bacteria isolated from a hot tub at 39 °C are probably which of the following?

1. thermophiles
2. psychrotrophs
3. mesophiles
4. hyperthermophiles

Answer

C

In which environment are you most likely to encounter a hyperthermophile?

1. hot tub
2. warm ocean water in Florida
3. hydrothermal vent at the bottom of the ocean
4. human body

Answer

C

Which of the following environments would harbor psychrophiles?

1. mountain lake with a water temperature of 12 °C
2. contaminated plates left in a 35 °C incubator
3. yogurt cultured at room temperature
4. salt pond in the desert with a daytime temperature of 34 °C

Answer

A

Matching

Match the type of bacterium with its environment. Each choice may be used once, more than once, or not at all. Put the appropriate letter beside the environment.

___psychotroph	A. water heater set at 50 °C
___mesophile	B. hydrothermal vent
___thermophile	C. deep ocean waters
___hyperthermophile	D. human pathogen
___psychrophile	E. soil bacteria in temperate forest

Answer

C, D, E, B, A

Short Answer

How are hyperthermophile’s proteins adapted to the high temperatures of their environment?

Why would NASA be funding microbiology research in Antarctica?

Critical Thinking

The bacterium that causes Hansen’s disease (leprosy), Mycobacterium leprae, infects mostly the extremities of the body: hands, feet, and nose. Can you make an educated guess as to its optimum temperature of growth?

Refer to Figure 9.4.2. Some hyperthermophiles can survive autoclaving temperatures. Are they a concern in health care?

9.5: Other Environmental Conditions that Affect Growth

Microorganisms interact with their environment along more dimensions than pH, temperature, and free oxygen levels, although these factors require significant adaptations. We also find microorganisms adapted to varying levels of salinity, barometric pressure, humidity, and light.

Multiple Choice

Which of the following is the reason jams and dried meats often do not require refrigeration to prevent spoilage?

1. low pH
2. toxic alkaline chemicals
3. naturally occurring antibiotics
4. low water activity

Answer

D

Bacteria living in salt marshes are most likely which of the following?

1. acidophiles
2. barophiles
3. halotolerant
4. thermophiles

Answer

C

Fill in the Blank

A bacterium that thrives in the Great Salt Lake but not in fresh water is probably a ________.

Answer

halophile

Bacteria isolated from the bottom of the ocean need high atmospheric pressures to survive. They are ________.

Answer

barophiles

Staphylococcus aureus can be grown on multipurpose growth medium or on mannitol salt agar that contains 7.5% NaCl. The bacterium is ________.

Answer

halotolerant

Short Answer

Fish sauce is a salty condiment produced using fermentation. What type of organism is likely responsible for the fermentation of the fish sauce?

9.6: Media Used for Bacterial Growth

The study of microorganisms is greatly facilitated if we are able to culture them, that is, to keep reproducing populations alive under laboratory conditions. Culturing many microorganisms is challenging because of highly specific nutritional and environmental requirements and the diversity of these requirements among different species.

Multiple Choice

EMB agar is a medium used in the identification and isolation of pathogenic bacteria. It contains digested meat proteins as a source of organic nutrients. Two indicator dyes, eosin and methylene blue, inhibit the growth of gram-positive bacteria and distinguish between lactose fermenting and nonlactose fermenting organisms. Lactose fermenters form metallic green or deep purple colonies, whereas the nonlactose fermenters form completely colorless colonies. EMB agar is an example of which of the following?

1. a selective medium only
2. a differential medium only
3. a selective medium and a chemically defined medium
4. a selective medium, a differential medium, and a complex medium

Answer

D

Haemophilus influenzae must be grown on chocolate agar, which is blood agar treated with heat to release growth factors in the medium. H. influenzae is described as ________.

1. an acidophile
2. a thermophile
3. an obligate anaerobe
4. fastidious

Answer

D

Fill in the Blank

Blood agar contains many unspecified nutrients, supports the growth of a large number of bacteria, and allows differentiation of bacteria according to hemolysis (breakdown of blood). The medium is ________ and ________.

Answer

complex, differential

Rogosa agar contains yeast extract. The pH is adjusted to 5.2 and discourages the growth of many microorganisms; however, all the colonies look similar. The medium is ________ and ________.

Answer

complex, selective

Short Answer

What is the major difference between an enrichment culture and a selective culture?

Critical Thinking

Haemophilus, influenzae grows best at 35–37 °C with ~5% CO₂ (or in a candle-jar) and requires hemin (X factor) and nicotinamide-adenine-dinucleotide (NAD, also known as V factor) for growth.¹ Using the vocabulary learned in this chapter, describe H. influenzae.

Footnotes

1. 1 Centers for Disease Control and Prevention, World Health Organization. “CDC Laboratory Methods for the Diagnosis of Meningitis Caused by Neisseria meningitidis, Streptococcus pneumoniae, and Haemophilus influenza. WHO Manual, 2nd edition.” 2011. http://www.cdc.gov/meningitis/lab-ma...ull-manual.pdf