The effect of bacterial trapping by fibrin on the efficacy of systemic antibiotics in experimental peritonitis.

Sixty Sprague-Dawley rats were inoculated with 10(9) Escherichia coli either suspended in 2 milliliters of normal saline solution or incorporated in a 2 milliliter 0.4 per cent fibrin clot. One hour after inoculation, one-half of the rats in each group received gentamicin, 12.5 milligrams per kilogram, intramuscularly. Escherichia coli suspended in normal saline solution was uniformly lethal. Incorporation of the bacteria and fibrin resulted in a survival of 13 of 15 rats (p = less than 0.002), but in abscess formation in all survivors. Treatment with gentamicin nearly abolished the mortality of Escherichia coli suspended in normal saline solution (14 of 15 rats survived, p = less than 0.002) but did not prevent abscess formation when bacteria were incorporated into fibrin. The gentamicin level exceeded the minimal inhibitory concentration (0.98 microgram per milliliter) for the strain of Escherichia coli used in serum (15.84 micrograms per milliliter after one hour) and peritoneal fluid (14.75 micrograms per milliliter after one hour) but never reached inhibitory levels in the fibrin clot (0.75 microgram per milliliter after one hour). We conclude that entrapment of bacteria by fibrin abolishes systemic sepsis but also protects bacteria against the action of systemic antibiotics and favors abscess formation.