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

OBJECTIVES: To investigate the trend of tigecycline susceptibility and mechanisms behind tigecycline non-susceptibility in Klebsiella pneumoniae and Escherichia coli isolates causing neonatal septicaemia (2007-10). METHODS: MICs of tigecycline for the isolates were determined. The isolates were evaluated for ß-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 (n = 57) and E. coli (n = 19) blood isolates. The prevalence of extended-spectrum ß-lactamase (ESBL)-producing organisms was high, but tigecycline non-susceptibility 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 with E. 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 in E. coli and K. 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.