Bedenić B, Boras A
Zavod za mikrobiologiju, SNZ A. Stampar Rockefellerova 4, 10000 Zagreb.
Lijec Vjesn. 2001 Nov-Dec;123(11-12):293-6.
The plasmid-mediated extended-spectrum beta-lactamases (ESBL) confer resistance to oxymino-cephalosporins, such as cefotaxime, ceftazidime, and ceftriaxone and to monobactams such as aztreonam. It is well known fact that ESBL producing bacteria exhibit a pronounced inoculum effect against broad spectrum cephalosporins like ceftazidime, cefotaxime, ceftriaxone and cefoperazone. The aim of this investigation was to determine the effect of inoculum size on the sensitivity and specificity of double-disk synergy test (DDST) which is the test most frequently used for detection of ESBLs, in comparison with other two methods (determination of ceftazidime MIC with and without clavulanate and inhibitor potentiated disk-diffusion test) which are seldom used in clinical laboratories. The experiments were performed on a set of K. pneumoniae strains with previously characterized beta-lactamases which comprise: 10 SHV-5 beta-lactamase producing K. pneumoniae, 20 SHV-2 + 1 SHV 2a beta-lactamase producing K. pneumoniae, 7 SHV-12 beta-lactamase producing K. pneumoniae, 39 putative SHV ESBL producing K. pneumoniae and 26 K. pneumoniae isolates highly susceptible to ceftazidime according to Kirby-Bauer disk-diffusion method and thus considered to be ESBL negative. According to the results of this investigation, increase in inoculum size affected more significantly the sensitivity of DDST than of other two methods. The sensitivity of the DDST was lower when a higher inoculum size of 10(8) CFU/ml was applied, in distinction from other two methods (MIC determination and inhibitor potentiated disk-diffusion test) which retained high sensitivity regardless of the density of bacterial suspension. On the other hand, DDST displayed higher specificity compared to other two methods regardless of the inoculum size. This investigation found that DDST is a reliable method but it is important to standardize the inoculum size.
质粒介导的超广谱β-内酰胺酶（ESBL）可使细菌对氧亚氨基头孢菌素（如头孢噻肟、头孢他啶和头孢曲松）以及单环β-内酰胺类抗生素（如氨曲南）产生耐药性。众所周知，产ESBL的细菌对头孢他啶、头孢噻肟、头孢曲松和头孢哌酮等广谱头孢菌素表现出明显的接种量效应。本研究的目的是确定接种量大小对双纸片协同试验（DDST）的敏感性和特异性的影响，DDST是检测ESBL最常用的试验，与临床实验室很少使用的其他两种方法（测定头孢他啶在有或无克拉维酸时的最低抑菌浓度以及抑制剂增强纸片扩散试验）进行比较。实验是在一组先前已鉴定β-内酰胺酶的肺炎克雷伯菌菌株上进行的，这些菌株包括：10株产SHV-5β-内酰胺酶的肺炎克雷伯菌、20株产SHV-2 + 1株产SHV 2aβ-内酰胺酶的肺炎克雷伯菌、7株产SHV-12β-内酰胺酶的肺炎克雷伯菌、39株推测产SHV ESBL的肺炎克雷伯菌以及26株根据 Kirby-Bauer 纸片扩散法对头孢他啶高度敏感因而被认为ESBL阴性的肺炎克雷伯菌分离株。根据本研究结果，接种量的增加对DDST敏感性的影响比对其他两种方法的影响更为显著。当接种量为10⁸CFU/ml时，DDST的敏感性较低，这与其他两种方法（最低抑菌浓度测定和抑制剂增强纸片扩散试验）不同，后两种方法无论细菌悬液密度如何均保持高敏感性。另一方面，无论接种量大小，DDST与其他两种方法相比均表现出更高的特异性。本研究发现DDST是一种可靠的方法，但标准化接种量大小很重要。
