Hidden Beta-Lactamases in the Enterobacteriaceae – Dropping the Extra Disks for Detection, Part II

Resistance to our beta-lactam and carbapenem antibiotics is becoming daunting for antimicrobial therapy for infections involving the Enterobacteriaceae. Similarly, laboratory testing to detect these resistance mechanisms is becoming more complex and perplexing for microbiology laboratories. Automated testing alone will not detect all of the resistance patterns that occur via beta-lactamases and carbapenemases. Failure to detect organisms with these enzymes can result in erroneous reports that would indicate an isolate is susceptible to beta-lactam and/or carbapenem antibiotics. In addition to the risk of compromised care of the patient, when pathogens with these enzymes go undetected, necessary infection control measures are precluded, thereby allowing the risk of these resistant organisms becoming endemic in a hospital environment. Supplemental testing, in addition to the routine susceptibility tests of isolates, has become necessary in order to detect the deluge of beta-lactamases and carbapenemases that clinical laboratories face today. Part II of this article reviews beta-lactam antibiotic resistance caused by AmpC cephalosporinases, class A carbapenemases, and the metallo-beta-lactamases, along with recommendations for their laboratory detection. Finally, a summary of suggested methods is proposed to enhance the laboratory detection of most of these beta-lactamase resistance mechanisms.