الفهرس 
BETA LACTAM ANTIBIOTICSOnly 14 pages are availabe for public view
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Abstract
The most common form of bacterial resistance to ß-lactam antibiotics is the production of beta lactamases which are enzymes that hydrolytically inactivate the beta lactam ring by breaking the ring open deactivating the molecule’s antibacterial properties of penicillins, cephalosporins and related drugs.
Microbial resistance through extended spectrum ß-lactamases ESBL is best defined as any β-lactamase enzyme, ordinarily acquired and not inherent to a species, that can rapidly hydrolyse, or confer resistance to, oxyiminocephalosporins (third and fourth generation ) such as ceftazidime, cefotaxime and cefepime as well as aztreonam.
Enterobacteriaceae-producing extended spectrum ß-lactamase enzymes are a clinical threat and have been associated with increased mortality in patients with severe infections. The prevalence of ESBL producers is rapidly increasing worldwide.
ESBL- producing organisms pose unique challenges to clinical microbiologists, clinicians, infection control professionals and antibacterial-discovery scientists as they are often encoded by genes located on large plasmids and these also carry genes for resistance to other antimicrobial agents, making the options for treatment extremely limited.
The Clinical Laboratory Standards Institute (CLSI) recommends that ESBL detection should be a two-step process which is labour-intensive and time consuming.
That is why several methods have been developed and commercialized such as chromogenic media, allowing more specific and direct differentiation of microorganisms on the primary plates directly from clinical specimens. They not only minimize the need for further identification tests but also reduce the time required to report the results to the clinician to facilitate early initiation of antibiotic therapy.
Therefore our current study focused on the use of Chrom ID ESBL (Biomerieux, France) which is a ready made chromogenic medium designed specifically for the screening of ESBL and is enriched with a rich nutrient base including a variety of peptones and a mixture of antibiotics. This mixture includes Cefpodoxime, an antibiotic recognized as being the marker of choice for this resistance mechanism. Also two chromogenic substrates aided by a natural substrate enabled the direct identification of the most frequently found ESBL Enterobacteria by different colors:
Escherichia coli: pink to burgundy coloration
Klebsiella, Enterobacter, Serratia, Citrobacter (KESC): green to browny-green coloration
Proteeae (Proteus, Providencia, Morganella): dark to light brown coloration.
And this allowed their detection in mixed culture and shorter time to report results.
Our study compared ChromID ESBL with a home made screening media which is Mac Conkey supplemented with 2 mg/liter ceftazidime.
The results obtained by both media were confirmed by CLSI 2008 modified disc diffusion synergy test.
Our results revealed that ChromID ESBL showed a specificity of 86.5%, a sensitivity of 88.9% and an overall efficacy of 87.1% with a high negative predictive value of 95.7% which allowed not only easy discrimination of the different colonies by color but also a quick and easy confirmation of the absence of ESBL-producers in clinical specimens.
As regards, MCKC, it showed lower sensitivity of 66.7% but a higher specificity of 90.4% and an overall efficacy of 84.3%.
Our results showed that Chrom ID ESBL is a reliable culture media for screening of multi resistant organisms ESBLs directly from clinical specimens.
Though it is slightly more expensive but it is a rapid way of screening for ESBL producing enterobacteriaceae, allowing lower time taken by conventional cultural media and limiting further confirmation. Thus appropriate antibiotic can be given early enough to prevent nosocomial and community acquired infections by ESBL producing enterobacteriacea.
It is recommended to use these methods in clinical microbiology laboratory on routine basis.