Citrate utilization test (Simmons’ citrate): Which observation(s) indicate a positive result showing the organism can use citrate as the sole carbon source on Simmons’ citrate agar?

Introduction / Context:
Simmons’ citrate test differentiates Enterobacterales by assessing the ability to transport and metabolize citrate when it is the sole carbon source, with ammonium salts as nitrogen source and bromothymol blue as the pH indicator.

Given Data / Assumptions:

• Positive organisms alkalinize the medium via ammonium utilization.
• Indicator turns from green (neutral) to blue (alkaline).
• Growth may be evident even before strong color change.

Concept / Approach:
A true positive is evidenced by growth and usually a blue colour shift as alkaline metabolites accumulate. Some weak positives show scant growth with delayed colour change; both observations are acceptable indicators when properly incubated and controlled.

Step-by-Step Solution:
Recognize that growth demonstrates citrate uptake/utilization. Note that alkalinization shifts bromothymol blue to blue. Accept either hallmark as positive; best practice is to see both growth and colour change. Select the combined option.

Verification / Alternative check:
Typical positives: Enterobacter, Klebsiella; typical negatives: Escherichia coli. QC confirms media performance.

Why Other Options Are Wrong:
Colour alone without growth can be artifact; gas in Durham tubes is not part of this test.

Common Pitfalls:
Inoculating too heavily (carries nutrients), or reading too early; incubate up to 48 hours to detect slow positives.

Final Answer:
Both (a) and (b).