Chapter 4 - Microbiology
You are working in a clinical laboratory in a hospital setting. You're handed a throat swab from a patient. You are told specifically that the physician is only interested in the presence and type of Gram positive cells. Identification isn't the main goal here—just a first step to work towards determining what Gram positive cells might be there. What might you do first to go about working towards this goal?
Perform a Gram stain.
The scientist that contributed most to the development of pure culture techniques was
Robert Koch.
A medium that inhibits the growth of organisms other than the one being sought is called a(n)
Selective medium.
A physician sends a stool sample to your lab, and wants to know if there are lactose fermenting microbes in the sample. How might you determine if these microbes are present or not from this mixed-microbe specimen?
Streak the sample for isolation on a MacConkey agar plate (which contains lactose and a pH indicator that turns pink when acid byproducts are present).
MacConkey agar is
a selective and differential medium.
Organisms that are indifferent to the presence of oxygen and do not use it are
aerotolerant anaerobes.
The solidifying agent used most successfully in bacterial nutrient media is
agar.
In the cultivation of microaerophilic and anaerobic bacteria,
atmospheric oxygen in a(n) candle/anaerobe jar is converted to water.
Bacteria on fish caught in the Arctic Ocean would
be psychrophiles AND would continue to grow while the fish is in the refrigerator.
Prokaryotic cells divide by a process known as
binary fission
Products that limit pH changes are often incorporated into media and are referred to as
buffers.
In the growth curve of a bacteria population, the bacteria are rapidly increasing in number in the
exponential (log) phase.
Agar replaced gelatin as the gelling (solidifying) agent for media because
fewer bacteria can break down agar compared with gelatin.
Medically important bacteria are often
grown on agar containing blood AND grown at 37°C.
Small organic molecules that must be provided to bacteria in order for them to grow are called
growth factors.
Agar
has chemical and physical properties that make it almost ideal for solidifying media.
Organisms that use organic molecules as their source of carbon are called
heterotrophs.
Trace elements
include zinc, copper, and manganese AND may be needed for enzyme function.
Late log phase of the bacterial growth curve
is characterized by the production of secondary metabolites.
When doing experiments with bacteria,
it is best to use bacteria from the same stage of growth.
The prefix photo- indicates that an organism will make use of ______ for energy purposes.
light
In a rapidly multiplying bacterial population, cell numbers increase
logarithmically.
Carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur may be considered
major elements.
In nature, bacteria
may adhere to surfaces by means of pili and slime layers.
A hot tub (with a water temperature of 104°F or 40°C) would most likely contain
mesophiles.
The lag phase of the bacterial growth curve is marked by
metabolically active cells.
Organisms that require gaseous oxygen for metabolism are referred to as
obligate aerobes.
Bacteria may be stored for several months
on a slant in the refrigerator, frozen in glycerol solution, AND freeze-dried.
Shake tubes are used to determine the
oxygen requirements for bacterial growth.
The optimum pH for growth of most species of bacteria is ______.
pH 7
Candle jars are usually used to
provide an atmosphere with CO2.
All the bacterial cells that result from the replication of a single original bacterial organism are referred to as a
pure culture.
Peptone
refers to a hydrolysate of proteins used in growth media.
The simplest technique for isolating bacteria in growth media is referred to as the
streak-plate method.
Organisms may derive energy from
sunlight AND metabolizing chemical compounds.
The enzymes that deal with toxic oxygen-containing molecules is/are
superoxide dismutase AND catalase.
Chemoheterotrophs
use preformed organic molecules as a carbon source AND as an energy source.
You are determining the number of colony forming units (CFUs) for Susan, who has a urinary tract infection (UTI). You added 1 mL of Susan's urine sample to 99 mL of diluent, and spread 0.1 mL of that dilution on a culture medium. After appropriate incubation, you count 143 colonies on the plate. What was the concentration of bacteria in the original urine sample?
1.43 x 105 cells/ mL
A pure culture in exponential growth phase has a bacterial concentration of 6.4 x 10⁸ cells/ml. If the bacterium has a generation time of 1 h, how long ago was the cell concentration 8.0 x 10⁷ cells/ml?
3 h
A urine sample with more than 100,000 organisms is considered indicative of infection. A urine sample containing 5,000 bacteria, with a generation time of 30 minutes, sits for 3 hours before finally being assayed. How many bacteria will then be present within the sample?
320,000
The optimal temperature for most human pathogens might be expected to range from
35-40°C.
You take absorbance readings on a spectrophotometer across a 6-hour culture of E. coli cells growing in tryptic soy broth (TSB). Your absorbance readings clearly indicate a lag phase, a log phase, and a stationary phase. You come back in and take readings at 8, 10, 12, 14, and 16 hours, but the absorbance number remains the same. Shouldn't it start coming down as the closed batch culture enters death phase? What's the most likely thing that is happening?
A spectrophotometer can measure absorbance as an indicator of cell number, but can't discriminate between a live cell and a dead one. The presence of dead cells keeps the absorbance high even into the death phase.
A halophilic, aerobic mesophile has a generation time of 20 minutes. You place 1x104 of these organisms on an appropriate solid medium, and incubate the plate for 100 minutes. Select the FALSE statement.
After incubation, a colony on the plate contains 5 x 104 cells.
Which of the statements about biofilms is true?
Biofilms may protect organisms against harmful chemicals.
During which phase of the bacterial growth curve does the total number of viable (live) cells decline?
Death.
During which phase of growth are bacteria most susceptible to antibiotics?
Exponential (log)
Differential media only allow certain bacteria to grow.
False
In microbiology, growth usually refers to an increase in size of the bacteria.
False
One would expect most strict anaerobic organisms to have superoxide dismutase.
False
There are five stages of growth in an open system of culture.
False
Freezing is an effective means of destroying bacteria.
False.
Clostridium paradoxum grows optimally at 55°C, pH 9.3; it will not grow in the presence of O2. Please select the TRUE statement about this organism.
It is an obligate anaerobe.
What would true about an organism called Bacillus fastidiosus?
It might be expected to be very strict in its growth requirements.
During which phase of the bacterial growth curve does a bacterial population become much more resistant to harmful conditions?
Late log phase.
Which of the following is (are) obligate aerobes?
Micrococcus luteus (italicized)
A microbe is discovered growing near a deep sea thermal vent. When researchers bring a sample up to the surface and try to grow it in a lab at room temperature in a normal incubator, they are unsuccessful. Why?
- The pressure isn't the same at sea level as it is on the ocean floor and the microbe may require a very specific pressure. - Oxygen concentrations are very different between the two environments—it's possible the microbe is a strict anaerobe and is poisoned by the air (oxygen) in the lab. - Salt concentrations might be different in the media the researchers are attempting to use and the salt water the microbe is used to living in. This might be causing osmotic pressure differences that the microbe can't tolerate. - The temperature is probably different—the thermal vent would be very hot, while these researchers are trying to grow this microbe at room temperature.
A single bacterial cell may multiply to form a visible colony.
True
It is assumed that every colony observed on a streak plate arose from a single bacterium.
True
Prokaryotes are the only organisms able to use atmospheric nitrogen as a nitrogen source.
True
Secondary metabolites may be antibiotics.
True
You are in charge of water quality for your city's water treatment plant. Of the methods at your disposal, which will be the most efficient and cheapest method of determining the number of viable bacteria in the water coming out of your plant?
Using membrane filtration followed by placing the membrane in a growth medium for colony counts after incubation.
