[Mechanisms of resistance in Enterobacteriaceae towards beta-lactamase antibiotics]

Edita Susić
Acta Medica Croatica: C̆asopis Hravatske Akademije Medicinskih Znanosti 2004, 58 (4): 307-12
Except for Salmonella spp., all Enterobacteriaceae produce intrinsic chromosomal encoded beta-lactamases which, beside their physiologic role in cell-wall synthesis and natural beta-lactam protection, are responsible for intrinsic resistance of individual species among Enterobacteriaceae. E. coli and Shigella spp. produce a small amount of AmpC beta-lactamases and are susceptible to ampicillin and other beta-lactam antibiotic agents. Enterobacter spp, C. freundii, Serratia spp., M. morganii, P. stuarti and P. rettgeri produce small amounts of inducible AmpC beta-lactamases which are not inhibited by beta-lactamases inhibitor, causing intrinsic resistance to ampicillin, co-amoxiclav and first-generation cephalosporins. K. pneumoniae produces small amounts of SHV-1 beta-lactamases, and K. oxytoca chromosomal K1 beta-lactamase, causing resistance to ampicillin, carbencillin, ticarcillin and attenuated zone of inhibition to piperacillin, compared to piperacillin with tazobactam. They are susceptible to beta-lactamase inhibitors. Whereas P. mirabilis shows a minor chromosomal expression of beta-lactamases, P. vulgaris produces chromosomal beta-lactamases of class A (cefuroximases), causing resistance to ampicillin, ticarcillin, and first- and second-generation cephalosporins. Antibiotics have caused the appearance of acquired or secondary beta-lactamases, with the sole function of protecting bacteria from antibiotics. The production of broad-spectrum beta-lactamases (TEM-1, TEM-2, SHV-1, OXA-1) results in resistance to ampicillin, ticarcillin, first-generation cephalosporins and piperacillin. A high level of beta-lactamases leads to resistance to their inhibitors. The plasmid-mediated extended-spectrum beta-lactamases (ESBLs) are of increasing concern. Most are mutants of classic TEM- and SHV-beta-lactamases types. Unlike these parent enzymes, ESBLs hydrolyze oxymino-cephalosporins such as cefuroxime, cefotaxime, ceftriaxone, ceftizoxime, ceftazidime, cefpirome and cefepime, aztreonam, as well as penicillins and other cephalosporins, except for cephamycin (cefoxitin and cefotetan). They are inhibited by beta-lactamase inhibitors. AmpC beta-lactamases are chromosomal and inducible in most Enterobacter spp., C. freundii, Serratia spp., M. morganii and Providentia spp. They are resistant to almost all penicillins and cephalosporins, to beta-lactamase inhibitors and aztreonam, and are susceptible to cefepime and carbapenems as well. Plasmid-mediated AmpC beta-lactamases have arisen through the transfer of chromosomal genes for the inducible AmpC beta-lactamase onto plasmids. All plasmid-mediated AmpC beta-lactamases have similar substrate profiles to the parental enzymes from which they appear to be derived. With one exception, plasmid-mediated AmpCs differ from chromosomal AmpCs in being uninducible. The National Committee for Clinical Laboratory Standards (NCCLS) has issued recommendations for ESBL screening and confirmation for isolates of E. coli, K. pneumoniae and K. oxytoca. No NCCLS recommendations exist for ESBLs detection and reporting for other organisms or for detecting plasmid-mediated AmpC beta-lactamases. High-level expression of AmpC may prevent recognition of an ESBL in species that produce a chromosomally encoded inducible AmpC beta-lactamase. AmpC-inducible species (e. g. Enterobacter spp. and C. freundii) can be recognized by cefoxitin/cefotaxime disk antagonism tests. Since clinical laboratories are first to encounter bacteria with new forms of antibiotic resistance, they need appropriate tools to recognize these bacteria, including trained staff with sufficient time and equipment to follow up important observations. Because bacterial pathogenes are constantly changing, training must be an ongoing process.

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