Question

Which one of the following media is most suitable for identifying Neisseria gonorrhoeae in a cervical swab? A. Sheep blood agar B. Chocolate agar C. MacConkey agar D. Thayer-Martin medium E. Hektoen enteric agar

Short answer

The most suitable medium for identifying Neisseria gonorrhoeae in a cervical swab is 'D. Thayer-Martin medium'

Step-by-step solution

Understand the Organism

Neisseria gonorrhoeae is Gram-negative obligate human pathogen that causes gonorrhea. It grows best at 35-37°C with ~5% CO2 (or in a candle-jar), and they also need a medium rich in nutrients.

Review the Culture Media Options

Each of the listed media serves specific bacteriologic principles. \n - 'Sheep blood agar' is a nonspecific medium allowing the growth of most non-fastidious organisms. \n - 'Chocolate agar' is a non-selective, enriched medium used primarily to isolate Haemophilus and Neisseria species.\n - 'MacConkey agar' is a selective and differential medium designed to isolate and differentiate enterics based on their ability to ferment lactose. \n - 'Thayer-Martin (or VPN) medium' which is a chocolate agar designed to isolate Neisseria gonorrhoeae.\n - 'Hektoen enteric agar' is a selective medium for Gram-negative organisms and helps to differentiate between bacteria that ferment lactose and those that do not.

Analyze each Media’s Suitability for Neisseria gonorrhoeae

Given that Neisseria gonorrhoeae is a nutritionally fastidious bacterium, it would not thrive in sheep blood agar or MacConkey agar, and Hektoen enteric agar is too specialized for enteric bacteria. Though it could grow on chocolate agar, Thayer-Martin medium (a variant of chocolate agar) is specifically designed for this organism. Cultures are normally incubated in a moist atmosphere enriched with 3-10% \( CO_{2} \) (carbon dioxide), conditions that Thayer-Martin medium is designed to provide.

Key concepts

Neisseria gonorrhoeae identification

Identifying Neisseria gonorrhoeae can be particularly challenging due to its fastidious nature and its resemblance to other Neisseria species. Some main points in its identification include:

• Gram Staining: Neisseria gonorrhoeae is a Gram-negative diplococcus, meaning its cells appear as pairs and stain red or pink with Gram stain due to the thin peptidoglycan layer that does not retain the crystal violet dye.
• Oxidation Tests: It oxidizes only glucose, distinguishing it from other Neisseria species like Neisseria meningitidis, which oxidizes both glucose and maltose.
• Enriched Growth Requirements: The bacterium grows best at around 35-37°C with increased CO_{2} levels, often achieved using a candle jar or a CO_{2} incubator.

Cervical swabs from patients are generally cultured on selective media, as this organism requires enriched conditions and grows slowly.

Thayer-Martin medium

Thayer-Martin medium is a highly selective culture media tailored to grow Neisseria gonorrhoeae while inhibiting the growth of most other bacteria. This selectivity is crucial for isolating the organism directly from clinical specimens such as swabs.

• Composition: It is a modification of chocolate agar that includes several antibiotics like vancomycin to inhibit Gram-positive organisms, colistin to inhibit other Gram-negative organisms, and trimethoprim that helps suppress proteus species. These additions help reduce the normal flora, allowing Neisseria gonorrhoeae to thrive.
• Applications: This medium not only supports the growth of Neisseria gonorrhoeae but also Neisseria meningitidis. It can detect low numbers of the pathogen, making it vital for clinical diagnostics.
• Handling: Due to its composition, regular cycles of quality control are practiced to ensure the antibiotics within the medium have not lost potency, and incubations are typically performed under increased CO_{2} conditions.

Culturing fastidious organisms

Culturing fastidious organisms like Neisseria gonorrhoeae requires specific, enriched culture media and precise environmental conditions to support their growth. Let's break down what makes them fastidious and how to successfully culture them:

• Enrichment: Fastidious organisms need added nutrients like those found in chocolate agar, which provides hemoglobin and NADH from the lysed red blood cells, aiding in bacterial metabolism.
• Environmental Conditions: These organisms thrive in environments with higher humidity and increased CO_{2}. Specialized incubators or candle jars are often used to maintain these conditions.
• Antimicrobial Sensitivity: While growing within their selective media, antibiotics present prevent the growth of contaminating bacteria that might otherwise outcompete the target fastidious organism.

Attention to these delicate conditions is essential, as any deviation can result in suboptimal growth or complete culture failure of these demanding bacteria.