Diminished clearance of S aureus after peritoneal infection in mice that lack prothrombin. (A) Determination of the number of CFU in peritoneal lavage fluid harvested from fII^loxand control mice 1 hour after the intraperitoneal injection of 1 × 10⁹ CFU of bacteria. Values are mean plus or minus SEM. Whereas a modest trend toward decreased bacterial clearance was observed in fII^lox/lox animals in the absence of Cre recombination, the more profound diminution of circulating prothrombin associated with hepatic induction of Cre in Mx⁺fII^lox/lox mice (5 days after poly I:C treatment) resulted in a dramatic impairment in the ability to clear peritoneal S aureus (*P < .008 vs wild-type; **P < .02 vs wild-type). Representative cytospins of peritoneal lavage fluid collected 1 hour after infection from a Cre-induced Mx⁺fII^lox/lox animal (5 days after poly I:C treatment) (B) and a wild-type mouse challenged in parallel (C). Note that cytospin preparations from animals lacking prothrombin (B) were characterized by both abundant clusters of free S aureus and bacteria-engorged and disrupted phagocytes, whereas parallel preparations from control mice were largely clear of bacteria. Bars represent 50 μm.