Intracellular Activity of Antibiotics against Staphylococcus aureus in a Mouse Peritonitis Model

Abstract

Antibiotic treatment of Staphylococcus aureus infections is often problematic due to the slow response to therapy and the high frequency of infection recurrence. The intracellular persistence of staphylococci has been recognized and could offer a good explanation for these treatment difficulties. Knowledge of the interplay between intracellular antibiotic activity and the overall outcome of infection is therefore important. Several intracellular in vitro models have been developed, but few experimental animal models have been published. The mouse peritonitis/sepsis model was used as the basic in vivo model exploring a quantitative ex vivo extraand intracellular differentiation assay. The intracellular presence of S. aureus was documented by electron microscopy. Five antibiotics, dicloxacillin, cefuroxime, gentamicin, azithromycin, and rifampin (rifampicin), were tested in the new in vivo model; and the model was able to distinguish between their extra-and intracellular effects. The intracellular effects of the five antibiotics could be ranked as follows as the mean change in the log 10 number of CFU/ml (⌬log 10 CFU/ml) between treated and untreated mice after 4 h of treatment: dicloxacillin (3.70 ⌬log 10 CFU/ml) > cefuroxime (3.56 ⌬log 10 CFU/ml) > rifampin (1.86 ⌬log 10 CFU/ml) > gentamicin (0.61 ⌬log 10 CFU/ml) > azithromycin (0.21 ⌬log 10 CFU/ml). We could also show that the important factors during testing of intracellular activity in vivo are the size, number, and frequency of doses; the time of exposure; and the timing between the start of infection and treatment. A poor correlation between the intracellular accumulation of the antibiotics and the actual intracellular effect was found. This stresses the importance of performing experimental studies, like those with the new in vivo model described here, to measure actual intracellular activity instead of making predictions based on cellular pharmacokinetic and MICs.

Activity of sitafloxacin against extracellular and intracellular Staphylococcus aureus in vitro and in vivo: comparison with levofloxacin and moxifloxacin

“…9,10 However, the intracellular location of the bacteria might explain the slow response to antibiotic as they might be protected from the effects of antibiotics. 11 In general, intracellular antimicrobial activity is markedly impaired compared with the activity seen in broth or the extracellular milieu, [12][13][14] although we know of situations in which the opposite is true. Antibiotic treatments should therefore be optimized not only toward the extracellular forms of S. aureus but also toward the intracellular forms.…”

Physiology and Pathology of Multidrug-Resistant Bacteria: Antibodies- and Vaccines-Based Pathogen-Specific Targeting

“…Meanwhile, intracellular persistence allows bacteria to escape from antibiotic and immune atack. Indeed, most of the current antibiotics are less eicient in intracellular S. aureus killing, which may partly explain the poor response to treatment and the high frequency of recurrence in clinical practice [99,100].…”

Intra- and Extracellular Activities of Dicloxacillin and Linezolid against a Clinical Staphylococcus aureus Strain with a Small-Colony-Variant Phenotype in an In Vitro Model of THP-1 Macrophages and an In Vivo Mouse Peritonitis Model

“…The mouse peritonitis model, performed as described previously, was used for all in vivo infection studies (17,41). In short, outbred female NMRI mice (Harlan Netherlands, Horst, Netherlands) were used for all studies.…”

“…By using a recently described mouse peritonitis intracellular S. aureus model (41), the intra-and extracellular presence of the bacteria could be distinguished, and the leukocyte responses for both phenotypes could be determined. Furthermore, kidneys were sampled to detect the metastatic spread of bacteria.…”

“…It has further been shown that the intracellular milieu induces and/or selects for the SCV phenotype (51). The intracellular persistence of bac-teria complicates the antibiotic treatment of staphylococcal infections (41,48,49), as most antibiotics have an impaired intracellular activity against this organism (3,11,41) and especially against SCVs (33).…”