Intracellular antimicrobial activity appearing as a relevant factor in antibiotic efficacy against an experimental foreign-body infection caused by Staphylococcus aureus

Objectives: The presence of bacterial biofilm, tolerance to antibiotics and dysfunctional activity of phagocytic cells are all related to difficulties in eradicating foreign-body infections. We aimed to quantify the presence of intracellular Staphylococcus aureus and to study the extent to which the intracellular activity of antibiotics might determine their efficacy against an experimental rat tissue-cage model of foreign-body infection. Methods: Using this model, animals were treated for 7 days with 100 mg/kg/day levofloxacin or 200 mg/kg/12 h cloxacillin, or were left untreated. Antibiotic efficacy was evaluated by means of bacterial counts from tissue-cage fluid (TCF); these counts were derived separately in total, intracellular and extracellular bacteria. The presence of intracellular bacteria was checked by electron microscopy. Population analysis was performed with surviving bacteria recovered at the end of levofloxacin therapy. Results: Among a total number of bacteria (mean log cfu/mL +/- SD) from TCF of 6.86 +/- 0.6, we identified 6.38 +/- 0.8 intracellular bacteria and 5.57 +/- 0.5 extracellular bacteria. Levofloxacin was more efficient than cloxacillin (P<0.05) against both intracellular and extracellular bacteria. The killing activity of levofloxacin against the intracellular population was higher than against the extracellular bacteria (P=0.1). The frequency of levofloxacin-resistant mutants among surviving bacteria at the end of levofloxacin therapy was similar to that for the wild-type strain. Conclusions: Intracellular bacteria accounted for the largest proportion of the total inoculum in this model of foreign-body infection. The intracellular activity of an antibiotic seems to be an additional relevant factor in the antibiotic response to these infections.