BioSkill Pro_Sem-4

You notice poor or no bacterial growth on your EMB agar plates for a culture that is expected to grow well and produce characteristic colony colors. Which of the following factors is most likely responsible for the poor growth, and what adjustment should you consider to obtain accurate results?

• Adjust the pH of the EMB agar media to the optimal range for the specific bacteria being cultured.
• Reduce the incubation time for the culture on the EMB agar plates
• Use a higher concentration of antibiotics in the EMB agar media
• Increase the incubation temperature of the EMB agar plates

You perform a Citrate Utilization test and find that the slant remains green with significant bacterial growth. What is the most likely interpretation of this result?

• The culture is negative for citrate utilization but positive for other carbon sources

• The culture is positive for citrate utilization

• The culture is negative for citrate utilization and cannot grow on the medium

• The pH indicator in the medium has failed

In the Voges-Proskauer test, a culture produces a copper color after adding Barritt's reagents and waiting 30 minutes. What is the most likely explanation for this result?

• The culture is positive for acetoin production

• The reagents were added in the incorrect order

• The culture is negative for acetoin production

• The incubation time was insufficient

You perform a Methyl Red test on a bacterial culture expected to produce a red color, but after adding the Methyl Red indicator, the broth remains yellow. Which of the following could be the most likely cause of the unexpected result, and how should you correct it?

• The medium was not incubated long enough; extend the incubation period to 48 hours.

• The wrong indicator was added; ensure Methyl Red indicator was used.

• The inoculum was too small; increase the amount of bacterial culture added to the broth.

• The incubation temperature was too high; incubate at room temperature instead of 37°C.

You obtain positive gas and acid production in tubes inoculated with a water sample in the presumptive phase of the MPN test. However, subsequent confirmatory tests reveal no coliform bacteria present. What is the most likely explanation for this discrepancy?

• Insufficient incubation time in BGLB broth

• Contamination during sample handling

• Use of improper dilution technique

• Presence of non-coliform bacteria that can ferment lactose

You perform an MPN test on a water sample using BGLB broth. After incubation, you observe gas production in all three tubes of the highest dilution series (10^-1). What is the most appropriate action to confirm these results?

• Repeat the test using a different broth medium

• Assume the sample has a high coliform count

• Perform confirmation tests such as Gram staining

• Repeat the test with higher dilutions

During the isolation of plasmid DNA from E. coli cells using the alkaline lysis method, which of the following steps is critical for selectively denaturing chromosomal DNA while keeping plasmid DNA intact?

• Resuspension of the bacterial pellet in Tris-EDTA buffer

• Addition of a solution containing sodium hydroxide (NaOH) and sodium dodecyl sulfate (SDS)

• Neutralization with potassium acetate

• Precipitation of plasmid DNA using isopropanol

During a restriction digestion experiment, you find that the expected DNA fragments are not appearing on the gel, suggesting incomplete digestion. Which of the following adjustments is most likely to improve the completeness of the digestion?

• Increase the incubation time to ensure complete digestion

• Reduce the amount of DNA in the reaction to prevent enzyme inhibition

• Perform the digestion at room temperature instead of the recommended temperature

• Use a higher concentration of agarose in the gel for better separation of fragments

After performing a transformation using the CaCl2 method, you observe very few colonies on the selective plate. Which of the following adjustments is most likely to improve the transformation efficiency?

• Increase the concentration of CaCl2 solution used during the preparation of competent cells

• Decrease the amount of plasmid DNA added to the competent cells

• Incubate the cells at room temperature instead of on ice after adding plasmid DNA

• Skip the heat shock step and proceed directly to incubation with SOC medium

During the isolation of plasmid DNA from E. coli using the alkaline lysis method, you observe that the final plasmid DNA preparation contains a significant amount of chromosomal DNA contamination. Which step is most likely to have been performed incorrectly, leading to this contamination?

• Incomplete resuspension of the bacterial pellet in the resuspension buffer

• Insufficient mixing during the lysis step with NaOH and SDS

• Inadequate neutralization with potassium acetate

• Incomplete precipitation of plasmid DNA with isopropanol