Just Ask the Expert: How can I obtain optimal results from skin bacterial cultures?

Dr. Bloom welcomes dermatology questions from veterinarians and veterinary technicians.

Click here to submit your question, or send an e-mail to vm@advanstar.com with the subject line "Dermatology questions."

Please review the recommended protocol for successful bacterial culture of skin and, in particular, skin biopsies. Should we perform a direct culture technique or place the biopsy sample in a sterile tube with saline solution or culture media?

A. Samples from a pustule or intact nodule should be used for bacterial culture of the skin in veterinary patients, if possible. To collect a sample from a pustule, first, rinse the lesion with alcohol. Allow the alcohol to dry, and then use a sterile 26-ga needle to open the pustule. Next, carefully touch the tip of a Mini-Tip Culturette (BD) to the purulent material. In patients with superficial bacterial pyoderma, if an intact pustule is not available, you can obtain a sample from an intact papule in the same manner as you would for a pustule.

Paul Bloom

If you can't identify a papule, sample the edge of an epidermal collarette. Just lift the edge of the collarette with a sterile needle, and rub a Mini-Tip Culturette (the swab's tip is smaller and allows more precise sampling than a standard culturette does) under the edge. If none of the collarettes have an edge, you can run your dry swab across the surface of the collarette three or four times (don't prepare the site, but you may gently clip hair with scissors to expose the collarette, if needed).1 Using a Mini-Tip Culturette to obtain a sample deep within a draining tract is also useful in patients with deep pyoderma.

For all of these methods, place the swab in a transport medium, and submit it to the laboratory.

Finally, submitting a crust is also reliable as a culture sample.2 Place the crust into a standard bacterial transport media.

For bacterial culture of a nodule, submit a tissue sample. Surgically prepare the site, and use sterile technique to collect the sample with a 6- or 8-mm punch biopsy. Place the sample in a plain red top tube (not a serum separator tube) along with a few drops of non-bacteriostatic sterile saline solution. Submit the sample, and request a culture from macerated tissue. In patients with nodules or deep pyoderma, be sure to request a culture for aerobic and anaerobic bacteria, atypical mycobacterium, and fungi (e.g.Sporothrix species). At the lab, the sample is removed sterilely and crushed, and the macerated sample is used to inoculate the culture media.

Paul Bloom, DVM, DACVD, DABVP

Allergy, Skin and Ear Clinic for Pets

31205 Five Mile

Livonia, MI 48154

REFERENCES

1. White SD, Brown AE, Chapman PL, et al. Evaluation of aerobic bacteriologic culture of epidermal collarette specimens in dogs with superficial pyoderma. J Am Vet Med Assoc 2005;226(6):904-908.

2. Vaughan DF, Lemarie SL. Comparison of culture and susceptibility results of superficial vs. biopsy specimens in dogs with superficial pyoderma. Vet Dermatol 2008:19(2):106-113.