A quantitative model for subcutaneous abscess formation in mice

The purpose of this experiment was to develop an experimental model of s.c. encapsulated abscesses in which abscess formation could be assessed by quantitative measurements. Inocula were composed of bacterial broth cultures, autoclaved mouse caecal contents or both in combination. These inocula were injected s.c. on the flank in 2 strains of mice. Large encapsulated abscesses formed in all recipients by Day 4 when the inoculum contained either B. fragilis or S. aureus combined with caecal contents. Bacterial concentrations per ml of pus at Day 6 were 10 ^10.1± ^0.02 for B. fragilis and 10 ^8.4± ^0.1 for S. aureus. Spontaneous external drainage began by 10-15 days, and complete healing of the lesion occurred by 4-6 weeks. The typical histological pattern consisted of a collagen capsule surrounding a rim of neutrophils, enclosing a central area of necrotic cells and fibre from the inoculum. The cross-sectional areas of the capsule, the neutrophil band and the entire abscess were measured in a reproducible manner by planimetry, and abscess volumes were calculated. Values for these measurements varied with different inocula and different times after inoculation, but were highly consistent for a specified time and inoculum.