Beta-lactamase alteration of beta-lactam inhibitory zones

Antimicrobial resistance and prevalence of extended spectrum beta-lactamase among clinical isolates of gram-negative bacteria in Riyadh

The activity of ciprofloxacin, imipenem and 12 other commonly used antibiotics was evaluated against 106 documented clinical isolates from a medical Intensive Care Unit (ICU). The resistance rates to ceftriaxone, cefotaxime, aztreonam and ceftazidime were 42, 25, 24 and 21%, respectively. Apart from Pseudomonas aeruginosa, all isolates were sensitive to ciprofloxacin and imipenem. Complete cross resistance among tested beta-lactam groups was uniformly evident in Enterobacter cloacae, Citrobacter freundii and P. aeruginosa. On the other hand, penicillins and second generation cephalosporins showed cross resistance among Escherichia coli and Klebseilla pneumoniae isolates. Induction experiments indicate that 70 and 62% of P. aeruginosa and E. cloacae or C. freundii produce class I cephalosporinase, respectively. Among all tested isolates, plasmid mediated extended spectrum beta-lactamase (ESBL) was detected in one isolate of K. pneumoniae. The plasmid mediated beta-lactamase is transferable and inhibited by beta-lactamase inhibitors. The transconjugates not only expressed resistance to extended spectrum beta-lactams and aztreonam but also toward tested aminoglycoside antibiotics, with the exception of gentamicin. The obtained transconjugates conferred high level resistance to ceftazidime and aztreonam but considerably low resistance to ceftriaxone and cefotaxime. The isoelectric point for the extended-spectrum beta-lactamase is 8.2.

Clinical importance of inducible beta-lactamases in gram-negative bacteria

The clinical problems caused by inducible beta-lactamases in certain gram-negative bacteria are being recognized with increasing frequency. These problems include the rapid emergence of multiple beta-lactam resistance during therapy with many of the newer beta-lactam antibiotics. Such multiply resistant organisms are now spreading within the hospital and have become important nosocomial pathogens. This has been a particularly difficult problem for intensive care units, cystic fibrosis centers and burn units where there are clusters of patients who are highly susceptible to infections with organisms like Enterobacter spp., Serratia spp. and Pseudomonas aeruginosa, which possess inducible beta-lactamases. Only through an awareness of these problems, their cause, and restriction of the use of certain newer beta-lactam antibiotics can these problems be controlled.