Department of Fundamental Microbiology, University of Lausanne, Lausanne, Switzerland.
J Antimicrob Chemother. 2012 Mar;67(3):652-60. doi: 10.1093/jac/dkr521. Epub 2011 Dec 13.
Laboratory detection of vancomycin-intermediate Staphylococcus aureus (VISA) and their heterogeneous VISA (hVISA) precursors is difficult. Thus, it is possible that vancomycin failures against supposedly vancomycin-susceptible S. aureus are due to undiagnosed VISA or hVISA. We tested this hypothesis in experimental endocarditis.
Rats with aortic valve infection due to the vancomycin-susceptible (MIC 2 mg/L), methicillin-resistant S. aureus M1V2 were treated for 2 days with doses of vancomycin that mimicked the pharmacokinetics seen in humans following intravenous administration of 1 g of the drug every 12 h. Half of the treated animals were killed 8 h after treatment arrest and half 3 days thereafter. Population analyses were done directly on vegetation homogenates or after one subculture in drug-free medium to mimic standard diagnostic procedures.
Vancomycin cured 14 of 26 animals (54%; P<0.05 versus controls) after 2 days of treatment. When vegetation homogenates were plated directly on vancomycin-containing plates, 6 of 13 rats killed 8 h after treatment arrest had positive cultures, 1 of which harboured hVISA. Likewise, 6 of 13 rats killed 3 days thereafter had positive valve cultures, 5 of which harboured hVISA. However, one subculture of vegetations in drug-free broth was enough to revert all the hVISA phenotypes to the susceptible pattern of the parent. Thus, vancomycin selected for hVISA during therapy of experimental endocarditis due to vancomycin-susceptible S. aureus. These hVISA were associated with vancomycin failure. The hVISA phenotype persisted in vivo, even after vancomycin arrest, but was missed in vitro after a single passage of the vegetation homogenate on drug-free medium.
hVISA might escape detection in clinical samples if they are subcultured before susceptibility tests.
检测耐万古霉素中间金黄色葡萄球菌（VISA）及其异质性 VISA（hVISA）前体较为困难。因此，对于原本认为对万古霉素敏感的金黄色葡萄球菌，出现万古霉素治疗失败的情况，有可能是因为存在未诊断的 VISA 或 hVISA。我们在实验性心内膜炎中对此假说进行了验证。
对因耐甲氧西林金黄色葡萄球菌 M1V2（MIC 为 2 毫克/升，对万古霉素敏感）引起主动脉瓣感染的大鼠，用模拟静脉注射 1 克万古霉素、每 12 小时 1 次的人体药代动力学的剂量，进行 2 天的治疗。治疗停止后 8 小时处死一半治疗动物，3 天后处死另一半。直接对病灶匀浆进行群体分析，或在无药培养基中进行一次传代培养，以模拟标准诊断程序。
2 天的治疗后，13 例中有 14 例（54%；P<0.05 与对照组相比）治愈。在治疗停止 8 小时处死的 13 只大鼠中，有 6 只的病灶匀浆直接接种于含万古霉素的平板上，其培养结果为阳性，其中 1 例为 hVISA。同样，3 天后处死的 13 只大鼠中有 6 只的瓣膜培养结果为阳性，其中 5 例为 hVISA。然而，病灶在无药肉汤中进行一次传代培养足以使所有 hVISA 表型恢复为亲代的敏感模式。因此，在耐万古霉素敏感金黄色葡萄球菌引起的实验性心内膜炎的治疗过程中，万古霉素选择出了 hVISA。这些 hVISA 与万古霉素治疗失败有关。即使在万古霉素停止治疗后，hVISA 表型仍在体内持续存在，但在将病灶匀浆直接接种于无药培养基上进行一次传代培养后，在体外检测不到。
如果在进行药敏试验前对临床样本进行传代培养，hVISA 可能会漏检。
