Small Animal Dermatology, 3rd edition

CHAPTER 3 DIAGNOSTIC CULTURE AND IDENTIFICATION (BACTERIAL AND FUNGAL) 29

Examine directly under the microscope and/or gently press samples onto test

medium.

 Toothbrush (Figure 3.3):
Samples from large or poorly demarcated lesions may be collected with a sterile

(or new “in package”) toothbrush

Brush hair against the direction of growth to encourage removal of fragile

(infected) hair shafts

Gently stab the bristles into the test media – a large amount of debris does not

need to be transferred.

Colony Growth and Identification

 Monitor culture plates for color change and colony growth daily.

 Observe for growth up to 28 days.

 DTM color changes from yellow to redprior toorat the same time asmacroscopic

colony growth.

 Dermatophytes preferentially metabolize proteins in the medium, creating an alkaline

pH turning the yellow color of the DTM to red at the same time that the dermatophyte

colony appears; most other fungi metabolize carbohydrates in the media first, creating

an acidic environment with no color changebut over time,they will consume the

proteins and cause a red color change,

 Dermatophyte colonies are white, creamy, or lightly tan but not pigmented (Figure

3.4).

 Colonies may be cottony, wool-like, or powdery.

Fungal Identification

 Transfer colonies to a glass slide using clear acetate tape or a sterile loop.

 Lactophenol cotton blue stain is most often recommended to enhance the appearance

of hyphae and conidia, but any dark stain will suffice (Figure 3.5).

 Examine slides for hyphae, macroconidia, and/or microconidia for identification (Fig-

ure 3.6).

 Microsporum canis,Microsporum gypseum,andTrichophyton mentagrophytesare the

most common dermatophytes isolated from lesions of dogs;Microsporum canisis most

commonly isolated from cats.

 Colonies that cannot be identified should be submitted to a reference laboratory for

identification; consult with your laboratory prior to submitting samples.

 A multicenter study compared the interpretation skills of “ in-house” cultures by

clinicians (dermatologists and general practitioners) with reference mycology lab-

oratory results: specialists (dermatologists) demonstrated a 3% error rate while

general practitioners misdiagnosed 20% of the cases. This discrepancy may be

due to a lack of microscopic identification or erroneous medium color change

notation.