Notes

Question 1

A dichotomous key

Answer 1

Is a flowchart that can be used to identify an organism by making a series of choices until you arrive at one possibility. Each level of the key typically offers two choices for a bacterial characteristic.

Question 2

The techniques for identifying unknown bacteria can be summarized in four key steps:

Answer 2

Staining the unknown for initial characterization by microscopy.
Using a dichotomous key strategy to systematically rule out other organisms.
Testing the organism for key biochemical traits.
Accumulating data and characteristics of the unknown until a positive identification can be made.

Question 3

Gram staining is the most common initial test of unknown identification, as it separates organisms into two broad classes, gram-positive and gram-negative, based on the cell wall structure. It also shows the size, shape, and arrangement of bacterial cells, which is helpful in further characterizing the identity of the unknown organism

Question 4

After quick-characterization by staining, specialized growth media can be used to indicate further biochemical characteristics of an unknown organism. These tests use visual outcomes, such as colors, patterns, and changes in appearance. The results can be interpreted to then assign a characteristic to the unknown organism. For example, a color change may happen when an unknown organism is incubated in a mixed sugar broth. The color could indicate which sugar was (or was not) used by the bacterium for energy.

Question 5

Most bacteria grow well between 20–40°C and are commonly incubated at 37°C (human body temperature).

Question 6

There are many ways to perform unknown identifications and arrive at conclusions! It can involve different test choices that provide equivalent results or dichotomous key steps (often dependent on availability of testing reagents, costs, instructor preferences, etc).

Question 7

Quantitation by dilution and colony counting techniques not only grow microorganisms, but are most useful when quantification (counting) is necessary, such as when looking at organisms causing disease states, determining the number of bacteria required to infect a host (known as an infectious dose), or when assessing purity of a sample.

Question 8

The SPC technique can be described as having four key steps:
Perform a serial dilution series of a bacterial culture into a set of blank liquid media to reduce numbers.
Transfer an aliquot of prepared dilutions to solid agar media.
Spread the transferred samples across the agar surface using a spreader (often called a “hockey stick”).
Incubate agar plate, allowing for growth of colonies and further visual counting