BACTERIAL CONCENTRATIONS IN PUS AND INFECTED PERITONEAL-FLUID - IMPLICATIONS FOR BACTERICIDAL ACTIVITY OF ANTIBIOTICS

Little is known about how many bacteria are present at an infectious f ocus at the onset of antibiotic therapy. The number of cfu was determi ned in pus and infected peritoneal fluids obtained from 41 patients. P athogens were detected in 71% of specimens. There were high concentrat ions of bacteria in culture-positive samples, in both soft-tissue and peritoneal infections, averaging 2 x 10(8) cfu/mL. These concentration s were much higher than the standard inoculum size used in in-vitro su sceptibility tests, 5 x 10(5) cfu/mL. The impact of this discrepancy o n antibacterial efficacy was studied with amikacin, ciprofloxacin, imi penem and piperacillin against Escherichia coil and Staphylococcus aur eus. The inhibitory and bactericidal activities of amikacin and ciprof loxacin determined with high inocula were two to four times lower than with standard inocula, whereas the activity of piperacillin was dimin ished at least 128-fold. Similar activity was observed with these drug s in Mueller-Hinton broth and peritoneal fluid. The bactericidal activ ity of imipenem was reduced in peritoneal fluid. Thus, conditions prev ailing at the infection site may compromise antibiotic activity determ ined in vitro.