Denver Cluster of New Superbug Largest Yet

The CDC has reported the largest U.S. outbreak yet of bacteria that contain a worrisome antibiotic resistance gene.

A Denver hospital identified three patients with carbapenem-resistant Klebsiella pneumonia infection, and five others who were colonized with the strain but had no symptoms, the agency reported in the Feb. 15 issue of Morbidity and Mortality Weekly Report.

The isolates produced the New Delhi metallo-beta-lactamase (NDM) gene, first identified in 2010 in Great Britain, mainly among people who had travelled to India or Pakistan for medical reasons.

The gene codes for a beta-lactamase enzyme that confers resistance to antibiotics such as penicillin. But it's regarded as worrisome because the gene is usually found on a plasmid -- a piece of DNA that can leap from species to species -- in combination with up to 14 other resistance genes.

It is easily found in surface and drinking water in India, researchers have found, and is becoming more common in Europe and North America.

But the CDC noted that – aside from the new cluster -- only 16 NDM-positive isolates have been found in the U.S. since 2009, all in clusters with 2 or fewer cases. Fourteen of the isolates were from patients who had received medical care in South Asia.

How the NDM-producing bacteria were introduced to the Denver hospital remains unclear, the CDC reported.

The outbreak was first reported Aug. 16, 2012, when the Colorado Department of Public Health and Environment was notified that respiratory specimens from 2 patients included carbapenem-resistant K. pneumonia.

The isolates produced the NDM gene, the CDC reported.

A review of microbiology records found a third patient, also with NDM-producing carbapenem-resistant K. pneumonia in a respiratory specimen.

Active surveillance in September found the remaining five patients asymptomatically colonized with the bacteria. Pulsed field gel electrophoresis showed that all the isolates were closely related.

The eight patients, ages 23 to 75, had been admitted to 11 different units (some to more than one) for between 12 and 83 days before the resistant bacteria was identified.

All the isolates were resistant to all antibiotics except tigecycline (Tygacil), but colistin (polymyxin E) minimum inhibitory concentrations for six of the specimens were low – no more than 2 mcg/mL -- suggesting it might also be useful for treatment, the CDC reported.

Epidemiologic analysis suggested the bacteria had been transmitted several times, with three hospital units picked out as likely transmission sites. But acquisition of the bacteria by some patients was not explained by direct overlap, hinting that undetected patients without symptoms were involved.  MedPageToday