Reviewed by:  Scott Curry, MD; Medical University of South Carolina

Active surveillance testing for multi drug-resistant organisms (MDROs) is most commonly done by direct detection from peri-rectal swab specimens plated directly on screening agars. A study by Sadek and colleagues highlights the methodological challenges in culture-based detection for these MDROs. Using stools from healthy volunteers spiked with known concentrations of VRE, CRE, ESBL, and colistin-R E. coli/Klebsiella, they compared the performance of direct plating onto chromogenic agar media to adding broth-enrichment steps before plating (with and without antibiotics). This represents a clinically significant methodological difference, as broth-pre-enrichment increases hands-on time, turn-around time, and cross-contamination opportunity while increasing the lab limit of detection in previous studies.  For each organism, broth enrichment resulted in 5-6 log10 increase in organism recovery; adding selective antibiotics to broth enrichment steps achieved an additional 0.5-2 log10 recovery beyond that, which was statistically significant for all organisms except VRE. There appears to be a compelling case for broth-enrichment methods, a version of which is currently recommended by the CDC, even though this involves an extra overnight incubation. While this was not a field study with actual patient samples, the absence of laboratory cross-contamination in the negative controls included in this validation was reassuring. In assessing the effectiveness of active surveillance testing strategies, the sensitivity of the screening methodology requires close attention in future intervention studies.

Highlighting a particular challenge in the detection of Gram-negative MDROs using nucleic acid detection methods, Girlich and colleagues describe a recurrent lack of detection of CRE from a young Senegalese patient with low-level carriage of CRE using a nucleic acid test commonly marketed for this purpose in the US, the Cepheid Xpert Carba-R, after testing 3 successive screening samples. Using culture-based backup methods, the patient's rectal swab was also pre-enriched in broth and had detection on a chromogenic medium of E. coli with blaOXA-181, which confers decreased susceptibility to ertapenem (but not to meropenem). blaOXA-181 is a variant of blaOXA-48, which is detected in the currently marketed test, and the investigators showed that the Xpert Carba-R showed a positive result when broth enrichment material was placed in the cartridge as opposed to direct inoculation of the Eswab diluent, which was negative. This case highlights the potential for false-negative testing for CRE carriage when direct molecular testing of rectal swabs without broth enrichment is performed for purposes of active surveillance.

References:

Sadek M, Poirel L, and Nordmann P. Diagn Microbiol Infect Dis. 2020 Jan;96(1):114919. PMID: 31679814 https://www.ncbi.nlm.nih.gov/pubmed/31679814 

Girlich D, Ouzani S et al. Successful use of culture and enrichment for the detection of OXA-181-producing Escherichia coli from rectal swab samples falsely categorized as negative by Xpert® Carba-R. Diagn Microbiol Infect Dis. 2020 Jan;96(1):114909. PMID: 31677960 https://www.ncbi.nlm.nih.gov/pubmed/31677960