Histological and observation evidence to help determine the clinical relevance of statistically significant reductions in Staphylococcus aureus counts in a pre-clinical trial of extremity war wound dressings

A randomised controlled pre-clinical trial utilising an existing extremity war wound model compared the efficacy of saline soaked gauze to commercial dressings. The Flexor Carpi Ulnaris of anaesthetised New Zealand rabbits was exposed to high-energy trauma using computer-controlled jig and inoculated with 106 Staphylococcus aureus 3 hours prior to application of dressing. After 7 days the animals were culled. Quantitative microbiological assessment of post-mortem specimens demonstrated statistically significantly reduced S aureus counts in groups treated with iodine or silver based dressings (2-way ANOVA p< 0.05). Clinical observations and haematology were performed during the study. Histopathological assessment of post-mortem muscle specimens included image analysis of digitally scanned haematoxylin and eosin stained tissue sections and subjective semi-quantitative assessment of pathology severity using light microscopy to grade muscle injury and lymph node activation. Tissue samples were also examined using scanning electron microscopy to determine the presence of bacteria and biofilm formation within the injured muscle. Non-parametric data were compared using Kruskal-Wallis. There were no bacteraemias, significantly raised white cell counts, abscesses, purulent discharge or evidence of contralateral axillary lymph node activation. All injured muscle specimens showed evidence of haemorrhage, inflammatory cell infiltration and fibrosis. All ipsilateral axillary lymph nodes were activated. There were no significant differences in the amount of muscle loss, size of the activated lymph nodes or in subjective semi-quantitative scoring criteria for muscle injury or lymph node activation. There was no evidence of bacterial penetration or biofilm formation. This study demonstrated statistically significant reductions in Staphylococcus aureus counts associated with iodine and silver dressings, and no evidence that these dressings cause harm. This was a time-limited study which was primarily powered to detect reduction in bacterial counts; however, there was no significant variation in secondary outcome measures of local or systemic infection over 7 days.