The Inoculum Effect in the Era of Multidrug Resistance: Minor Differences in Inoculum Have Dramatic Effect on MIC Determination.
The Inoculum Effect in the Era of Multidrug Resistance: Minor Differences in Inoculum Have Dramatic Effect on MIC Determination.
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DOI：
10.1128/aac.00433-18
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发表时间：
2018-08-01
影响因子：
4.9
通讯作者：
Kirby, James E
中科院分区：
文献类型：
--
作者：
Smith, Kenneth P;Kirby, James E
观察到的最低抑菌浓度可能取决于最初接种到检测中的细菌数量。这种现象被称为接种效应(IE)，在表达β-内酰胺酶的菌株中，这种现象通常对β-内酰胺类抗生素最为明显。临床和实验室标准协会(CLSI)推荐的接种量为5*105 cfu ml-1，可接受范围为2*105到8*105 cfu ml-1，IE测试通常使用的接种量是CLSI推荐接种量的100倍。因此，在根据CLSI指南执行的测试过程中，IE是否会影响麦克风仍是未知的。在此，我们利用喷墨打印技术对头孢吡肟、美罗培南和头孢他啶-阿维巴坦进行了IE测试。首先，我们确定在2倍(R2=0.99)或1.1倍(R2=0.98)的系列稀释中，喷墨分配体积与输送到微孔中的细菌数量有很好的相关性。然后，我们通过分配细菌细胞和抗生素的正交滴定来量化IE。对于头孢吡肟耐药和敏感的剂量依赖菌株，接种量增加2倍导致MIC增加1.6log2倍。对于碳青霉烯酶产生菌，接种量每减少2倍，美罗培南MIC降低1.26log2倍。在CLSI允许接种量范围的低端，美罗培南在测试耐药菌株集时观察到34.8%的微小错误率。头孢他啶-阿维巴坦不受明显的IE影响。我们的结果表明，在多药耐药的革兰氏阴性病原菌中，IE对美罗培南和头孢吡肟的作用足够明显，从而影响了标准实验室检测中的分类解释。
The observed MIC may depend on the number of bacteria initially inoculated into the assay. This phenomenon is termed the inoculum effect (IE) and is often most pronounced for beta-lactams in strains expressing beta-lactamase enzymes. The Clinical and Laboratory Standards Institute (CLSI)-recommended inoculum is 5 * 105 CFU ml-1 with an acceptable range of 2 * 105 to 8 * 105 CFU ml-1 IE testing is typically performed using an inoculum 100-fold greater than the CLSI-recommended inoculum. Therefore, it remains unknown whether the IE influences MICs during testing performed according to CLSI guidelines. Here, we utilized inkjet printing technology to test the IE on cefepime, meropenem, and ceftazidime-avibactam. First, we determined that the inkjet dispense volume correlated well with the number of bacteria delivered to microwells in 2-fold (R2 = 0.99) or 1.1-fold (R2 = 0.98) serial dilutions. We then quantified the IE by dispensing orthogonal titrations of bacterial cells and antibiotics. For cefepime-resistant and susceptible dose-dependent strains, a 2-fold increase in inoculum resulted in a 1.6 log2-fold increase in MIC. For carbapenemase-producing strains, each 2-fold reduction in inoculum resulted in a 1.26 log2-fold reduction in meropenem MIC. At the lower end of the CLSI-allowable inoculum range, minor error rates of 34.8% were observed for meropenem when testing a resistant-strain set. Ceftazidime-avibactam was not subject to an appreciable IE. Our results suggest that IE is sufficiently pronounced for meropenem and cefepime in multidrug-resistant Gram-negative pathogens to affect categorical interpretations during standard laboratory testing.