Microbe of the Month: ESBL-Producing Organisms

Extended-spectrum beta-lactamase (ESBL)-producing organisms are an increasing challenge for healthcare practitioners fighting healthcare-associated infections (HAIs). Escherichia coli, Klebsiella pneumoniae, and Klebsiella oxytoca are the most common ESBL-producing pathogens.

ESBL-producing organisms are generally resistant to many classes of antibiotics, including aminoglycosides and fluoroquinolones; ESBL-producing organisms are able to attack newer cephems and monobactams as well as narrow-spectrum cephalosporins and antigram-negative penicillins.

They are associated with increased mortality and are difficult to detect and treat. The widespread use of extended-spectrum, third-generation cephalosporins, introduced in the 1980s to treat antibiotic-resistant bacteria, is believed to be a major contributor to the emergence of ESBL-producing organisms.

A research team from The University of Texas (UT) Health Science Center at San Antonio, examining bacterial isolates obtained in hospital and non-hospital clinical settings between 2000 and 2006, has identified drug-resistant strains of E. coli and Klebsiella bacteria in more than 50 blood, urine and respiratory samples. These resistant strains, which resemble bacteria reported in Latin America, Asia and Europe, were thought to be rare in the United States. This is the first report of this phenomenon in the United States, the researchers said.

The resistant strains produce ESBLs, or enzymes that destroy penicillin or cephalosporin class drugs, thereby conferring resistance to those drugs, says James Jorgensen, PhD, professor of pathology, medicine, microbiology and clinical laboratory sciences at the Health Science Center. "These very common bacteria, when they produce these enzymes, are much harder to kill with antibiotics," Jorgensen says.

The fact that outpatients presented with the ESBLs caught the researchers’ attention. "This antibiotic resistance problem is likely to become widespread," says study co-author Jan Evans Patterson, MD, professor of medicine, infectious diseases and pathology at the UT Health Science Center. "It affects the way we will treat infections in the future. In the past, we were concerned with antibiotic resistance in the hospital primarily, but in this review many of the strains we detected were from the community. This tells us antibiotic resistance is spreading in the community, as well, and will affect how we choose antibiotics for outpatient infections."

If the trend continues, it may become difficult to select appropriate antibiotic therapy for urinary tract infections, for example. "The trend over the last decade has been to treat urinary infections empirically, to pick the drug that has worked," Jorgensen says. "Now it is important for physicians to culture the patient’s urine to be sure they have selected the right antibiotic. The top three drugs that are often prescribed may not be effective with these resistant bacteria."

Laboratories that perform urine cultures and susceptibility testing should check for the presence of ESBLs, something they don’t always do currently, Jorgensen advises.