Affiliation:
1. Institute of Medical Microbiology, Justus Liebig University, Giessen, Germany
2. German Center for Infection Research, Partner Site Giessen-Marburg-Langen, Giessen, Germany
Abstract
ABSTRACT
The plasmid-located colistin resistance gene
mcr-1
confers low-level resistance to colistin, a last-line antibiotic against multidrug-resistant Gram-negative bacteria. Current CLSI-EUCAST recommendations require the use of a broth microdilution (BMD) method with cation-adjusted Mueller-Hinton (CA-MH) medium for colistin susceptibility testing, but approximately 15% of all MCR-1 producers are classified as sensitive in that broth. Here we report on an improved calcium-enhanced Mueller-Hinton (CE-MH) medium that permits simple and reliable determination of
mcr-1
-containing
Enterobacteriaceae
. Colistin susceptibility testing was performed for 50
mcr-1
-containing
Escherichia coli
and
Klebsiella pneumoniae
isolates, 7 intrinsically polymyxin-resistant species,
K. pneumoniae
and
E. coli
isolates with acquired resistance to polymyxins due to
mgrB
and
pmrB
mutations, respectively, and 32
mcr-1
-negative, colistin-susceptible isolates of
Acinetobacter baumannii
,
Citrobacter freundii
,
Enterobacter cloacae
,
E. coli
,
K. pneumoniae
, and
Salmonella enterica
serovar Typhimurium. A comparison of the colistin MICs determined in CA-MH medium and those obtained in CE-MH medium was performed using both the BMD and strip-based susceptibility test formats. We validated the data using an isogenic IncX4 plasmid lacking
mcr-1
. Use of the CE-MH broth provides clear separation between resistant and susceptible isolates in both BMD and gradient diffusion assays; this is true for both
mcr-1
-containing
Enterobacteriaceae
isolates and those exhibiting either intrinsic or acquired colistin resistance. CE-MH medium is simple to prepare and overcomes current problems associated with BMD and strip-based colistin susceptibility testing, and use of the medium is easy to implement in routine diagnostic laboratories, even in resource-poor settings.
Funder
Bundesministerium für Bildung und Forschung
Publisher
American Society for Microbiology
Cited by
19 articles.
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