Wound swabs versus biopsies to detect methicillin resistant Staphylococcus aureus in experimental equine wounds.

Abstract

OBJECTIVE: To compare: (1) the load and diversity of cultivatable bacterial species isolated from tissue biopsies with cultures from surface swabs, and (2) the ability of each technique to detect methicillin-resistant Staphylococcus aureus (MRSA) in a model of MRSA-infected equine wounds. STUDY DESIGN: Experimental in vivo study. ANIMALS: Three light-breed adult horses. METHODS: Four 2.5&#x2009;&#xd7;&#x2009;2.5&#xa0;cm full-thickness skin wounds were created on the dorsolateral aspect of each forelimb. Five days later, each wound was inoculated with a pure culture of MRSA (ATCC 43300). One hundred microlitres of 0, 5&#x2009;&#xd7;&#x2009;10, 5&#x2009;&#xd7;&#x2009;10or 5&#x2009;&#xd7;&#x2009;10colony forming units (CFU)/ml was used to inoculate each wound. Surface swabs (Levine technique) and tissue biopsy samples (3&#x2009;mm punch biopsy) were obtained at 2, 7, 14, and 21&#x2009;days after inoculation. Quantitative aerobic culture was performed using routine clinical techniques. RESULTS: A similar bacterial profile was identified from the culture of each wound-sampling technique and there was moderate correlation (R&#xa0;=&#xa0;0.49, P&#x2009;<&#x2009;.001) between the bacterial bioburdens. Agreement was fair (&#x3ba;&#xa0;=&#xa0;0.31; 95% CI, 0.129-0.505) between the sampling techniques in identification of MRSA. Methicillin-resistant Staphylococcus aureus was isolated more frequently (P&#xa0;=&#xa0;.016) from cultures of tissue biopsies (79%; 76/96) than from surface swabs (62%; 60/96). CONCLUSION: Bacterial load and diversity did not differ between sampling techniques but MRSA was detected more often from the cultures of tissue biopsies. CLINICAL SIGNIFICANCE: Tissue biopsy should be preferred to culture swab in wounds where MRSA is suspected.