Debridement Increases Survival in a Mouse Model of Subcutaneous Anthrax

Abstract

Anthrax is caused by infection with Bacillus anthracis, a spore-forming Gram-positive bacterium. A major virulence factor for B. anthracis is an immunomodulatory tripartite exotoxin that has been reported to alter immune cell chemotaxis and activation. It has been proposed that B. anthracis infections initiate through entry of spores into the regional draining lymph nodes where they germinate, grow, and disseminate systemically via the efferent lymphatics. If this model holds true, it would be predicted that surgical removal of infected tissues, debridement, would have little effect on the systemic dissemination of bacteria. This model was tested through the development of a mouse debridement model. It was found that removal of the site of subcutaneous infection in the ear increased the likelihood of survival and reduced the quantity of spores in the draining cervical lymph nodes (cLN). At the time of debridement 12 hours post-injection measurable levels of exotoxins were present in the ear, cLN, and serum, yet leukocytes within the cLN were activated; countering the concept that exotoxins inhibit the early inflammatory response to promote bacterial growth. We conclude that the initial entry of spores into the draining lymph node of cutaneous infections alone is not sufficient to cause systemic disease and that debridement should be considered as an adjunct to antibiotic therapy.

Opening moves: early events in anthrax pathogenesis and their consequences for dissemination

“…A luminescent B. anthracis 7702 strain, BIG23, was constructed by integrating the plasmid pIG6-19 into pX01, a kind gift from Michele Mock [261]. This strain contains the luxABCDE operon from Photorhabdus luminescens under the control of the PA promoter, leading to light expression in vivo.…”

“…Spores are capable of germination without prior engulfment by phagocytes and germinated within 15 minutes in a guinea pig model [79]. These infections appear to spread to lymphatics in a phagocyte independent manner, as debridement of the tissue prevents the infection from disseminating, despite the presence of spores in the draining cervical lymph nodes [261].…”

“…Given the high mortality of inhalational anthrax and its potential as a biological weapon, the majority of dissemination studies have focused on inhalation routes; however, some studies have investigated dissemination in subcutaneous and gastrointestinal B. anthracis infections [79,81,[260][261][262][263]. Regardless of the route of infection, B. anthracis has a common method of dissemination ( Figure 3) [264,265].…”

“…From these mucosal associated lymphoid tissues, the bacilli disseminate to the draining region lymph nodes and the access the blood stream from the efferent lymphatics [81,270]. Subcutaneous infections do not initiate in a mucosal associated lymphoid tissue, but appear to germinate and disseminate from the initial site of injection in the ear dermis [79,81,261]. Spores are capable of germination without prior engulfment by phagocytes and germinated within 15 minutes in a guinea pig model [79].…”

“…These BLI studies identified two novel sites of establishment and dissemination of infection [81,261,275,276]. In the upper respiratory passage, bacteria were first observed to replicate and disseminate from the nasal associated lymphoid tissue (NALT) [81,260,270,275].…”