Sabouraud Agar Culture Test Results - Histoplasmosis Fungal Infection

How to Perform an Agar Slant Culture Test for Fungal Infections

Sabouraud Agar Culture Test Results - Histoplasmosis Fungal Infection
Agar Slant Cultures of Histoplasmosis

The Sabouraud Agar Slant Culture test is used when fungal overgrowth of the oral tissues is suspected, and is used to confirm the diagnosis of a fungal infection.  Other adjunctive methods for the diagnosis of oral candidiasis include exfoliative cytology using PAS-stain or KOH and biopsy.

Related Reading: Infectious Lesions of the Oral Cavity: Histoplasmosis & Mucormycosis

What is in the Sabouraud agar media?

The agar media contains three key ingredients: dextrose, peptones (pancreatic digest of casein), and peptic digest of animal tissues.

Dextrose is added as the energy and carbon source, and peptones are a subunit of casein which is a protein found in milk and is a high quality source of amino acids.  Peptic digests of animal tissues provide a nutritious source for the growth of fungi and yeasts.

The pH of the agar is adjusted to approximately 5.6 and it usually contains cycloheximide and chloramphenicol to prevent overgrowth of accompanying fungi and inhibit bacterial growth.

Download the Diagnosing Vesicular Ulcerative Conditions checklist to learn about diagnosing Histoplasmosis, Mucormycosis, and other conditions affection the oral cavity.

Download the Checklist

Contraindications & Disadvantages

Before you start the procedure, here are a few things to keep in mind about the contraindications and disadvantages:

  1. It does not give information of which species are involved
  2. It does not provide quantitative information
  3. It may give false positive result

How to Perform an Agar Slant Culture Test

Prior to the procedure, introduce yourself to the patient, explain the purpose of the examination, obtain consent, and be sure to meet infectious control standards.

  1. Check expiration date of the Sabouraud Agar Slant
  2. Identify the area of suspected fungal infection
  3. Use a sterile cotton swab (Q-tip) and gently rub over the lesional tissue in attempt to collect the surface deposits
  4. Swab the Q-tip on to the agar for a couple of streaks
  5. Fasten the cap on slant tube, label it and keep it in an appropriate condition

Incubation

  1. Make sure the cap is fastened tight on the slant tube
  2. Put the tube in a test tube rack in a room with low/no light and a temperature of 25-37 °C
  3. After 2-7 days, examine the sample tube for fungal growth on the surface of the agar

Related Reading: A Dentist’s Guide: Oral Pathology of Vesicular Ulcerative Conditions

For Prosthesis

  1. Check expiration date of the Sabouraud Agar Slant
  2. Identify the area of suspected fungal infection
  3. Swab the first streak on the bottom-half of the agar slants with the oral sample
  4. Take a new Q-tip and moisturize it with water (sterile water is preferable)
  5. Rub the Q-tip against the prosthesis where it sits against the oral tissue
  6. Swab on to the top-half of the agar slants with prosthesis sample
  7. Indicate on the label on the tube which side is from oral tissue or prosthesis
  8. Fasten the cap on slant tube, label it and keep it an appropriate condition

Learn More:  USC’s Oral Pathology and Radiology Online Postgraduate Certificate Program

How to Interpret a Positive Result

A positive culture will show creamy white colonies. A negative or positive result must be read at the 2 to 3-day time point; the agar slant culture test is invalid beyond 3 days.  Keep the slant for at least two weeks before determining that the culture is negative.

Potential Causes of a Negative Result

  • There is no candida in the collected sample and the lesion is not due to fungal infection
  • The specimen was not collected properly
  • Antifungal treatment had been used prior to the collection of the specimen
  • The procedures were incorrect
  • The organism grew very slowly
  • Sample was taken from a hyperplastic candidiasis lesion

Download the Diagnosing Vesicular Ulcerative Conditions checklist to learn about diagnosing Histoplasmosis, Mucormycosis, and other conditions affection the oral cavity.

Download the Checklist

 

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About Parish Sedghizadeh, DDS, MS

Associate Professor of Dentistry. Director of the USC Center for Biofilms. Division of Periodontology, Dental Hygiene & Diagnostic Sciences. Diplomat, American Board of Oral & Maxillofacial Pathology.