Tigecycline susceptibility in Klebsiella pneumoniae and Escherichia coli causing neonatal septicaemia (2007–10) and role of an efflux pump in tigecycline non-susceptibility

Methods: MICs of tigecycline for the isolates were determined. The isolates were evaluated for b-lactamases and carbapenemases. Molecular typing of the tigecycline-resistant isolates was performed. Expression of efflux pump genes (acrA, acrB and tolC) and regulators (soxS and ramA) was examined by real-time RT–PCR and western blotting. Sequencing of the ramA and ramR genes was carried out to identify mutations within these genes. Results: Tigecycline susceptibility was evaluated in all K. pneumoniae (n¼57) and E. coli (n¼ 19) blood isolates. The prevalence of extended-spectrum b-lactamase (ESBL)-producing organisms was high, but tigecycline nonsusceptibility remained low in these isolates. Though MIC values of tigecycline remained in the susceptible range, there was a 2-fold increase in the value of MIC90 from 2007 to 2010. Over the 4 year period K. pneumoniae showed higher MIC values of tigecycline in comparison withE.coli. Tigecycline non-susceptibility was not observed among carbapenem-resistant isolates. Only two ESBL-producing clonally distinct K. pneumoniae isolates showed tigecycline resistance with overexpression of ramA and the AcrAB–TolC pump. No mutations were present within the ramA and ramR genes that might enhance the expression of the pump. Conclusions: The study showed for the first time the trend of tigecycline susceptibility inE.coli andK.pneumoniae causing neonatal septicaemia. Tigecycline still has potent antimicrobial effects against most ESBL- or carbapenemase-producing K. pneumoniae and E. coli, but the increasing MIC values make it essential to be vigilant.