Establishment of an experimental model of a Staphylococcus aureus abscess in mice by use of dextran and gelatin microcarriers.

Staphylococcus aureus UC 9271 mixed with dextran or gelatin microcarrier beads and injected subcutaneously into mice resulted in the formation of reproducible, sustained abscesses with as few as 2 x 10(3) cfu. Without microcarrier beads, 4 x 10(7) cfu were required to produce an abscess. The abscesses that developed with microcarriers attained a diameter of up to 1.5 cm and persisted for several days before discharging through the skin. The pH of the abscesses fell from 7.1 to 6.6 within 24 h. Histological and microscopic examination of the abscesses revealed an influx of phagocytic cells, mostly polymorphonuclear leucocytes, within 1-2 h after injection. Cell debris accumulated and the abscess became encapsulated 24-48 h after infection. Enzymatic digestion of the abscess contents allowed analysis of the host and bacterial cell populations and treatment with lysostaphin permitted differentiation between phagocytosed and free bacterial populations of S. aureus. Phagocytosed but viable S. aureus comprised c. 50% of the total bacterial population after 24 h; however, by 96 h the phagocytosed population was only 1-5% of the total population, primarily because of an increase in extracellular bacterial numbers. Prevention of abscess formation by antibiotic treatment based upon the minimal inhibitory concentration (MIC) of an antibiotic for S. aureus was not always predictable. Tetracycline did not prevent abscess formation even though it possessed a low MIC for S. aureus; methicillin had a borderline MIC value but was quite active. However, the MIC values were quite predictive of antibiotic cures in a systemic-lethal S. aureus infection in mice.