What organisms are commonly classified as Carbapenem-Resistant Enterobacteriaceae (CRE)?

Common Organisms Classified as CRE

Klebsiella pneumoniae is the most commonly encountered CRE species in the United States, followed by Enterobacter cloacae complex and Escherichia coli. 1, 2

Primary CRE Species

The carbapenem-resistant Enterobacteriaceae family consists of several key organisms with distinct epidemiological patterns:

Most Common Species

• Klebsiella pneumoniae represents the predominant CRE organism, accounting for approximately 53-77% of all CRE isolates in surveillance studies 1, 3, 2, 4

  • Carbapenem-resistant K. pneumoniae (CRKP) is the species most frequently recovered in U.S. healthcare settings 1
  • The prevalence increased dramatically from <1% of all Klebsiella isolates in 2000 to 8% by 2007 1
  • Sequence type ST258 is the predominant clone among carbapenemase-producing K. pneumoniae in the United States 2

• Enterobacter cloacae complex is the second most common CRE species, representing 18-31% of CRE isolates 3, 2

  • ST171 is the predominant sequence type for this organism 2
  • IMP carbapenemases are frequently detected in E. cloacae (69% in some studies) 4

• Escherichia coli accounts for 12-14% of CRE isolates 3, 2

  • ST131 is the predominant sequence type among carbapenemase-producing E. coli 2
  • NDM and IMP carbapenemases are more frequent in E. coli compared to K. pneumoniae (53% in some studies) 4

Geographic and Epidemiological Variations

The distribution of CRE species varies by acquisition setting and geography:

• Healthcare-acquired CRE predominantly involves K. pneumoniae and is associated with intra-abdominal infections, prior surgery, and indwelling devices 3

• Community-acquired CRE more commonly affects elderly females and presents with urinary tract infections, though K. pneumoniae remains the most common pathogen 3

• Regional patterns show that carbapenemase-producing K. pneumoniae, E. cloacae complex, and E. coli occurrence varies significantly by U.S. region 2

Clinical Surveillance Focus

Current CDC and HICPAC guidelines specifically target Klebsiella species and E. coli for active surveillance cultures because these organisms represent the majority of CRE encountered in the United States and can be detected without molecular methods. 1

• Surveillance methodology is designed to detect carbapenem resistance or carbapenemase production specifically in Klebsiella spp. and E. coli 1
• This focused approach facilitates laboratory testing while capturing the most clinically significant CRE organisms 1

Resistance Mechanisms by Species

The carbapenemase enzyme distribution differs among CRE species:

• K. pneumoniae: KPC (Klebsiella pneumoniae carbapenemase) predominates, accounting for 87% of carbapenemase genes in this species 4, 5
• E. coli: NDM and IMP carbapenemases are more prevalent (53%) 4
• E. cloacae: IMP carbapenemases dominate (69%) 4

Overall, among all CRE isolates tested, 73-89% harbor carbapenemase genes, with KPC variants predominating (70-97% of carbapenemase-positive isolates) 2, 4