الفهرس 
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Abstract
Microbial resistance through extended spectrum ß-lactamases (ESBL) was first reported in the early 1980s in Europe and subsequently in the United States soon after the introduction of third –generation cephalosporins in clinical practice. Today, this resistance mechanism has emerged globally, and ESBL-producing Enterobacteriaceae are recognized worldwide as nosocomial pathogens of major importance .Extended –spectrum ß-lactamases are enzymes that mediate resistance to oxyimino-cephalosporins and aztreonam, they are most commonly recognized in Klebsiella spp. and E.coli but have also been detected in a variety of Enterobacteriaceae.
The Clinical Laboratory Standards Institute (CLSI) recommends a phenotypic confirmatory disk test for ESBL production that consists of measuring the zone diameters of cefotaxime and ceftazidime disks with or without clavulanic acid to determine whether enzymatic hydrolysis of the cephalosporin tested is inhibited in the presence of clavulanic acid. Once an ESBL producing strain is detected, it should be reported as “resistant” to all pencillins, cephalosporins, and aztreonam, even if they are susceptible.
The AmpC ß-lactamases (also termed class C or group 1) are typically encoded on the chromosome of many gram negative bacteria but also may be carried on plasmids. AmpC ß-lactamases, in contrast to
ESBLs, hydrolyze broad and extended cephalosporins (cephamycins as well as to oxymino-ß-lactams) but are not inhibited by ß-lactamase inhibitors such as clavulanic acid .and hence the presence of an ESBL can be masked by the expression of an AmpC. Thus, the coexistence of both pAmpCs and ESBLs in the same strain may result in false-negative tests for the detection of ESBLs .Genes encoding plasmid-mediated AmpC β-lactamases coexist on the same plasmid with genes encoding mechanisms of resistance to other classes of antibiotics, leaving clinicians with limited therapeutic options .
Therefore, there is a need for alternative method that can detects ESBLs and AmpCs alone and in combination by a simple disc diffusion method. The current study focused on the combined clavulanate disc method by CPD and CPM discs and comparing the results with the other methods as DDST, FOX/CTX induction test,combined boronic disc and E-test.
The study included 276 Enterobacterial isolates that exhibited resistance to third generation cephalosporins (CTX) & (CPD) and to cephamycins (FOX). Bacterial isolates were collected from different specimens processed at the main microbiology laboratory of Kasr-alini hospitals. Isolates were identified to species level by morphological, cultural and biochemical characteristics.
By screening tests ESBLs were suspected in 178/400 (44.5%) ,AmpC were suspected in 98/400 (24.5%). By DDST ESBLs were
confirmed in 173/276 (62.7%) while in combined clavulanate test ESBL alone were found in only 2 isolates while 253/276(91.7%) were found o be ESBL in presence of AmpC .
AmpCs were detected by the combined clavulanate method in 64/276 (23.2%) and by combined boronic method in 65/276 (23.5%) . There is no significant difference between both methods. Comparing these results to the gold standard E-test combined clavulanate method found to be a reliable method (sensitivity 100% &PPV 96%) in relation to the approximation method DDST (sensitivity 64.7% &specificity 83.3%).
Agreement tests were done between the related phenotypic confirmatory tests and revealed a moderate agreement between results of the DDST & combined disc for detection ESBL in presence of AmpC, very good agreement between results of the Approximation (FOX/CTX) induction test& combined CPD/FOX/ CPDCL induction test for inducible AmpC and very good agreement between results of the Combined FOX boronic & combined clavulanate disc methods for detection of AmpC.