الفهرس Only 14 pages are availabe for public view
 |  | from | 176 |  |  |
| | | from | 176 | | |
Abstract
The most common cause of bacterial resistance to ß-lactam antibiotics is the production of ß-lactamases. ß-lactamases are enzymes that cause destruction of the ß-lactam ring rendering the ß-lactam antibiotic inactive. In order to understand the relationship of these enzymes to one another, two classification systems have been developed; Ambler (molecular) classification and Bush (functional) classification. Bush classified them into four groups according to substrate and inhibitor profiles, while Ambler classified them into four classes(A-D) based on the nucleotide and amino acid sequences .Classes A,C and D act by a serine based mechanism ,whereas class B or metallo –ß-lactamases need zinc for their action .
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.