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1.  ramR Mutations in Clinical Isolates of Klebsiella pneumoniae with Reduced Susceptibility to Tigecycline▿  
Five Klebsiella pneumoniae isolates with reduced susceptibility to tigecycline (MIC, 2 μg/ml) were analyzed. A gene homologous to ramR of Salmonella enterica was identified in Klebsiella pneumoniae. Sequencing of ramR in the nonsusceptible Klebsiella strains revealed deletions, insertions, and point mutations. Transformation of mutants with wild-type ramR genes, but not with mutant ramR genes, restored susceptibility to tigecycline and repressed overexpression of ramA and acrB. Thus, this study reveals a molecular mechanism for tigecycline resistance in Klebsiella pneumoniae.
doi:10.1128/AAC.00085-10
PMCID: PMC2876394  PMID: 20350947
2.  Combined ramR Mutation and Presence of a Tn1721-Associated tet(A) Variant in a Clinical Isolate of Salmonella enterica Serovar Hadar Resistant to Tigecycline ▿  
A Salmonella enterica serovar Hadar strain resistant to tigecycline (MIC, 16 μg/ml) was isolated. Molecular characterization revealed the presence of a plasmid-borne tet(A) variant associated with Tn1721 mediating a rise of the MIC for tigecycline when transferred to Escherichia coli. Additionally, a truncating mutation in ramR was detected. Transformation with wild-type ramR but not with the mutated ramR lowered the MIC for tigecycline. Characterization of this Salmonella isolate implicates ramR in resistance to tigecycline.
doi:10.1128/AAC.00993-09
PMCID: PMC2826014  PMID: 20028818

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