Baudoux Pierre, Bles Nathalie, Lemaire Sandrine, Mingeot-Leclercq Marie-Paule, Tulkens Paul M, Van Bambeke Françoise
Unité de Pharmacologie Cellulaire et Moléculaire, Université Catholique de Louvain, Brussels, Belgium.
J Antimicrob Chemother. 2007 Feb;59(2):246-53. doi: 10.1093/jac/dkl489. Epub 2007 Jan 12.
Staphylococcus aureus survives in acid media, including phagolysosomes. Conflicting in vitro/in vivo data exist on its susceptibility to antibiotics in such environments.
Oxacillin and gentamicin activities against methicillin-susceptible S. aureus ATCC 25923 were compared extracellularly (broth; different pH) and assessed intracellularly (THP-1 macrophages), using a pharmacological approach (antibiotic concentrations: 0.01-1000 x MIC). Antibiotic cellular contents were determined by microbiological assay.
MICs and MBCs increased 72-fold for gentamicin, and decreased 8-fold for oxacillin between pH 7.4 and 5.0. Plots of log(10) colony-forming unit changes at 24 h versus log(10) of antibiotic concentration followed sigmoidal shapes, allowing calculation of EC(50) (relative potency) and apparent E(max) (relative efficacy) in all conditions. In broth, the EC(50) of gentamicin rose 316-fold and that of oxacillin decreased 15-fold with unchanged apparent E(max) [-5 log (limit of detection)] between pH 7.4 and 5. Intracellularly, EC(50)s were similar to those observed extracellularly at pH 7.4, but E(max) values were much lower (-1 log) for both antibiotics. Calculations based on the assumed pH in phagolysosomes (5.4) and on local accumulation of antibiotics (gentamicin, 23-fold; oxacillin, 0.05-fold) suggest that the contrasting effects of acid pH on relative potencies of gentamicin and oxacillin could be almost exactly compensated for by differences in accumulation.
The weak activity of gentamicin and oxacillin towards intraphagocytic S. aureus compared with extracellular forms is not related to an overall decrease of their relative potencies but to impaired efficacy, suggesting the need for new approaches to improve the eradication of intracellular S. aureus.
金黄色葡萄球菌能在酸性介质中存活，包括吞噬溶酶体。关于其在此类环境中对抗生素的敏感性，体外/体内数据存在冲突。
采用药理学方法（抗生素浓度：0.01 - 1000×最低抑菌浓度），比较了苯唑西林和庆大霉素对甲氧西林敏感的金黄色葡萄球菌ATCC 25923在细胞外（肉汤；不同pH值）的活性，并评估了其在细胞内（THP - 1巨噬细胞）的活性。通过微生物学测定法确定抗生素的细胞内含量。
在pH值7.4和5.0之间，庆大霉素的最低抑菌浓度（MIC）和最低杀菌浓度（MBC）增加了72倍，而苯唑西林则降低了8倍。24小时时对数（10）菌落形成单位变化与抗生素浓度对数（10）的关系图呈S形，从而可以计算出所有条件下的半数效应浓度（EC50）（相对效价）和表观最大效应（E(max)）（相对疗效）。在肉汤中，pH值从7.4降至5时，庆大霉素的EC50增加了316倍，苯唑西林的EC50降低了15倍，表观E(max)不变[-5 log（检测限）]。在细胞内，EC50与在pH值7.4时细胞外观察到的相似，但两种抗生素的E(max)值都低得多（-1 log）。根据吞噬溶酶体中假定的pH值（5.4）以及抗生素的局部积累情况（庆大霉素，23倍；苯唑西林，0.05倍）进行的计算表明，酸性pH值对庆大霉素和苯唑西林相对效价的相反影响几乎可以通过积累差异得到完全补偿。
与细胞外形式相比，庆大霉素和苯唑西林对吞噬细胞内金黄色葡萄球菌的活性较弱，这与它们相对效价的总体降低无关，而是与疗效受损有关，这表明需要新的方法来改善细胞内金黄色葡萄球菌的根除。
