The primary test to identify Pseudomonas aeruginosa in the laboratory is

RRB Nsg. Superintendent -29 April-2025 (shift-2nd)

Microbiology

Easy

Appeared in: RRB Nsg. Superintendent -29 April-2025 (shift-2nd)

Explanation

The oxidase test is a primary, rapid biochemical test used for the presumptive identification of Pseudomonas aeruginosa.
This test detects the presence of the enzyme cytochrome c oxidase, which is produced by P. aeruginosa.
A positive result (a deep purple color) is crucial as it differentiates Pseudomonas from the large family of Enterobacteriaceae (like E. coli and Klebsiella), which are oxidase-negative.
This differentiation is a key initial step in the laboratory workflow for identifying gram-negative rods.

Option B: While P. aeruginosa is capable of reducing nitrate, this is not the primary screening test used for its identification because many other facultative anaerobic bacteria also give a positive result.
Option C: The coagulase test is used exclusively for the identification of gram-positive cocci, specifically to differentiate Staphylococcus aureus from other Staphylococcus species.
Option D: P. aeruginosa is catalase-positive, but so are most other aerobic and facultative anaerobic bacteria, including Enterobacteriaceae. Therefore, the test lacks the specificity needed to be a primary differentiator for Pseudomonas.

Visual 1: Diagram - A diagram illustrating the oxidase test procedure, showing a bacterial colony being smeared on a filter paper with oxidase reagent, resulting in a positive (purple) and a negative (no color change) reaction.
Visual 2: Image - A culture plate of Pseudomonas aeruginosa on nutrient agar, highlighting the characteristic blue-green pyocyanin pigment and the metallic sheen of the colonies.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Biochemical identification of Pseudomonas aeruginosa as background academic context rather than a clinical decision trigger.
Rapid and accurate identification of P. aeruginosa is critical in healthcare as it is a major opportunistic pathogen, frequently causing hospital-acquired infections (nosocomial infections) like ventilator-associated pneumonia, burn wound infections, and urinary tract infections.
This organism is notorious for its intrinsic resistance to many antibiotics and its ability to acquire further resistance, making prompt identification essential for guiding effective antimicrobial therapy and preventing treatment failure.
What if? If the organism in question was a gram-positive coccus from a wound swab, the primary test would not be oxidase. Instead, a nurse should anticipate a catalase test to differentiate staphylococci from streptococci, followed by a coagulase test if staphylococci are suspected.

How to Approach the Question

Step 1: Identify the core subject of the question, which is the laboratory identification of a specific bacterium: Pseudomonas aeruginosa.
Step 2: Note the keyword 'primary test'. This asks for the most important initial or screening test used to differentiate this organism from others.
Step 3: Recall the key characteristics of P. aeruginosa. It is a gram-negative rod, a non-lactose fermenter, and famously oxidase-positive.
Step 4: Evaluate the given options based on this knowledge. The oxidase test directly aligns with a primary differentiating feature of Pseudomonas.
Step 5: Analyze the distractors. Recognize that the coagulase test is for staphylococci (a different bacterial category) and the catalase test is too general (positive for many bacteria) to be a primary identifier in this context.
Step 6: Conclude that the oxidase test is the most specific and rapid primary test among the choices for presumptively identifying P. aeruginosa.

Concept Tested & Keywords

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