Enterobacteriaceae and beta-lactams: Wild susceptibility patterns

Four susceptibility patterns of wild types of enterobacteria against old beta-lactams including aminopenicillins, carboxypenicillins and first-generation cephalosporins were individualized during the 1980s : susceptible, penicillinase low level, cephalosporinase and a combination of penicillinase and cephalosporinase. Such indirect detection of a mechanism of resistance allowed an interpretative reading for this class of antibiotics. At the present time, seven susceptibility patterns were proposed for this family of gram negative bacilli. Nevertheless, an analysis of results in terms of MICs and diameters of inhibition zone sizes of the main bacterial species of enterobacteria, mainly obtained from the databank of European Committee on Antimicrobial Susceptibility Testing (EUCAST), compared to that observed when overproducing strains were isolated in vivo and in vitro and to the type of beta-lactamase identified and their amino acid sequences conducted to a proposal of five susceptibility patterns. The fifth wild type individualized in several enterobacteria since 2005 is related to the synthesis of various chromosomal extended-spectrum beta-lactamases (ESBL) which hydrolyze many beta-lactams including oxyimino-cephalosporins such as ceftriaxone or cefotaxime. Their expression in a wild strain is characteristic and conducted to our interest for their role as progenitors of the transferable CTM-M types. Otherwise, a medical biologist must consider the possibility of selection of a mutant with a chromosomal overproduced beta-lactamase. But within the same beta-lactam susceptibility pattern such as for Klebsiella pneumoniae and K. oxytoca or Citrobacter amalonaticus, the spectrum of inactivation will be highly variable according to the type of enzyme overproduced. Finally, a nice synergy observed between clavulanic acid and cefotaxime or ceftriaxone or even aztreonam does not mean anytime a transferable ESBL. In some cases according to the result of enterobacterial identification, the epidemiological impact will be very low, because without multidrug resistance (MDR). (C) 2011 Elsevier Masson SAS. All rights reserved.