Impact of Burden on Granulocyte Clearance of Bacteria in a Mouse Thigh Infection Model

We wished to delineate granulocytes' impact on the clearance of different bacterial burdens of Pseudomonas aeruginosa and Staphylococcus aureus in a granulocyte-replete mouse thigh infection model. A mouse thigh model was employed. Bacterial challenges from 10(5) to 3 x 10(7) CFU (S. aureus) and from 3 x 10(4) to 3 x 10(8) CFU (P. aeruginosa) were injected into murine posterior thighs. Organism quantitation was at baseline, 2 h (Pseudomonas only), and 24 h. A Michaelis-Menten population model was fit to the data for each organism. Breakpoints for microbial containment by granulocytes were identified. Bacterial burdens exceeding that breakpoint value resulted in organism multiplication. The Michaelis-Menten model fit the data well. For P. aeruginosa, the observed-predicted plot had a regression equation that explained over 98% of the variance (P << 0.001). For S. aureus, this relationship explained greater than 94% of the variance (P << 0.001). Maximal growth rate constants, maximal population burdens, and the bacterial loads at which granulocytes killed if half-saturated were not different. The kill rate constant for P. aeruginosa was almost 10 times that of S. aureus. Bacterial kill by granulocytes is saturable. No difference between saturation points of different isolates was seen. A higher bacterial burden means an increasing reliance on chemotherapy to drive bacterial clearance.