Distribution of Bacteria in Simplified Surrogate Extremities Shot With Small Caliber Projectiles

Experiments were conducted to determine bacteria distribution trends in wound cavities of simplified surrogate extremities shot using small caliber projectiles. Two different shapes of targets, cylindrical and square, were used in this study. Cylindrical targets are more representative of an extremity but create difficulties while conducting tests due to inconsistent cavity lengths and optical distortions. Square targets, which are not as susceptible to the problems mentioned above, could be used in place of cylindrical ones if their shape does not significantly affect the distribution of bacteria within the wound cavity. Surface contamination of the targets in the experiments was represented using a circular piece of filter paper moistened with a solution with a known amount of Escherichia coli strain K-12. The projectiles used were 11.43-mm (0.45-in) caliber round nose projectiles shot from a commercially available air rifle. The permanent cavities were extracted from the targets and sliced into small, evenly spaced segments and the area surrounding the permanent cavities was removed with a biopsy punch. The radial tears that were made by the formation of the temporary cavity and surround the permanent cavity were removed using a scalpel. The permanent cavity and radial tears for each section were processed and plated on agar plates. Commercial software was used to count the number of colony forming units on each plate and the percentage of the total bacterial colony count per segment was determined. High speed video and motion analysis software was used to qualitatively and quantitatively compare the temporary cavities in the cylindrical and square targets. The data from the experiments showed that the bacteria distribution trends for the cylindrical and square targets were similar even though the maximum openings of the temporary cavity at the entrance and exit locations were higher for the cylindrical ones. For both target shapes, the bacterium was evenly distributed between the permanent cavity and the radial tears in the middle sections of the “wound tracks.” In addition, significantly higher amounts of bacterium were found in the entrance and exit segments compared with the rest of the segments in the “wound tracks”.