Decreased susceptibility to tigecycline in Acinetobacter baumannii mediated by a mutation in trm encoding SAM-dependent methyltransferase.
OBJECTIVE
Acinetobacter baumannii is an important opportunistic pathogen and multidrug-resistant isolate. Although tigecycline is a potent antibiotic for treating infections with multidrug-resistant isolates, resistance is becoming a problem. This study aimed to explore the mechanism of tigecycline resistance in A. baumannii.


METHODS
A serial passage experiment was performed to collect isolates selected by tigecycline. The expression of efflux pumps was quantified in the final selected isolate, 19606-T8. The whole genome of 19606-T8 was sequenced and the putative mutations were confirmed using PCR and Sanger sequencing. A complementation experiment was performed to evaluate the contribution of the mutations to decreased susceptibility to tigecycline. The significance of a deletion mutation was further investigated in terms of growth rate and antibiotic susceptibilities.


RESULTS
We collected serial isolates by selective pressure of tigecycline, and designated them 19606-T1 to 19606-T8. The efflux pumps AdeABC, AdeFGH and AdeIJK were not overexpressed in 19606-T8, which had decreased susceptibility to tigecycline. Isolate 19606-T8 carried one deletion mutation in trm and three non-synonymous substitutions in msbA (A84V), lolA (P91L) and filC (N168K). The deletion mutation in trm (encoding S-adenosyl-L-methionine-dependent methyltransferase) resulted in decreased susceptibility to tigecycline as well as to minocycline and doxycycline. In complementation experiments, the MICs of tigecycline, minocycline and doxycycline in a tigecycline-resistant isolate were restored by complementation with wild-type trm.


CONCLUSIONS
Given that the deletion mutation in trm was associated with decreased susceptibility to tigecycline and that a wild-type trm plasmid could restore the susceptibility, trm is considered to play an important role in decreased susceptibility to tigecycline in A. baumannii.
Department of Infectious Diseases, Sir Run Run Shaw Hospital, College of Medicine, Zhejiang University, Hangzhou, Zhejiang 310016, China.
Qiong
Q
Chen
Xi
X
Li
Hua
H
Zhou
Yan
Y
Jiang
Yan
Y
Chen
Xiaoting
X
Hua
Yunsong
Y
Yu
D040981:Acinetobacter baumannii / Q000187:drug effects* / Q000201:enzymology / Q000235:genetics*; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D004252:DNA Mutational Analysis; D024881:Drug Resistance, Bacterial; D017353:Gene Deletion; D005816:Genetic Complementation Test; D008780:Methyltransferases / Q000235:genetics* / Q000378:metabolism; D008826:Microbial Sensitivity Tests; D008911:Minocycline / Q000031:analogs & derivatives* / Q000494:pharmacology; D010957:Plasmids; D012641:Selection, Genetic; D012692:Serial Passage; D000078304:Tigecycline
D000900:Anti-Bacterial Agents; D000078304:Tigecycline; D008780:Methyltransferases; D008911:Minocycline
efflux pumps; serial passage experiments; whole genome comparisons