Carbapenem-Resistant Bacteria Hiding in Less Common Species, 2014–2017

Infections with carbapenemase-producing carbapenem-resistant Enterobacteriaceae (CP-CRE) carry high death rates. The genes for carbapenemases — enzymes that destroy carbapenem antibiotics — often sit on plasmids that pass between bacterial strains, so they can spread resistance quickly.

The Enterobacteriaceae family has approximately 50 recognized genera, but surveillance has focused on the three most often behind clinical infections: Klebsiella, Enterobacter and Escherichia coli. Less common genera have generally not been tested for carbapenemases, partly because some are naturally resistant to the carbapenem imipenem and others carry their own chromosomal carbapenemases. But they can also carry the plasmid kind. This report looked at what testing at CDC and in Minnesota turned up.

CDC’s reference testing

CDC tests submitted CRE for five major carbapenemases: KPC, NDM, VIM, IMP and OXA-48-type. From January 1, 2014 to May 25, 2017, it identified 1,039 CP-CRE, 63 (6.1%) of them from less common genera, submitted by 23 states — most from Iowa (10) and Pennsylvania (7).

The most common combination was KPC-producing Citrobacter (27, 42.9%). Others included Proteus mirabilis (11, mostly KPC), Providencia (10, mostly IMP) and Morganella (3).

Non-KPC carbapenemases have historically been rare in the United States and often linked to health care abroad. Of 28 patients with non-KPC CP-CRE from less common genera, CDC had details for 20, with a median age of 62.5 (range 2 months to 79 years). Of the 18 with a travel history:

• 10 had not traveled outside the United States in the previous year — five of them from a single cluster
• 6 had been hospitalized abroad: three in India, and one each in the Philippines, Romania and Spain

Minnesota’s statewide surveillance

The Minnesota Department of Health began testing all CRE species in Hennepin and Ramsey counties in 2012 and statewide on January 1, 2016. From January 1, 2014 to September 30, 2017:

• 149 (12%) of 1,241 CRE submitted produced a carbapenemase — the same share for common and less common genera
• 20 (13.4%) of those 149, all from different patients, were from less common genera
• the most frequent were IMP-producing Providencia rettgeri (7, 35%) and KPC-producing Citrobacter freundii (6, 30%)

The 20 Minnesota patients had a median age of 56.5 (range 14–75); 15 (75%) were in the hospital when cultured, and no clusters were found. Only two had been hospitalized abroad in the previous year, one each in Kenya and Kuwait.

Why it matters

Less common genera make up a small but potentially important share of CP-CRE — how large is unclear without systematic testing. Many carried carbapenemases other than KPC, often in patients who had not traveled abroad, pointing to acquisition within the United States.

Clinicians should know that CRE from less common genera can carry carbapenemases, and consider asking state public health laboratories for carbapenemase testing to guide infection control. CRE surveillance covering a broader range of genera was being piloted in 10 states.

Sources

Based on Walters MS, Witwer M, Lee YK, et al., "Notes from the Field: Carbapenemase-Producing Carbapenem-Resistant Enterobacteriaceae from Less Common Enterobacteriaceae Genera — United States, 2014–2017," MMWR Vol. 67, No. 23, CDC; a work of the United States government in the public domain.