Mechanistic basis of how LD-transpeptidases protect against outer membranedefects

Project Summary/Abstract
The emergence of multidrug and extensively drug-resistant Gram-negative bacteria is a growing problem that
threatens established antimicrobial treatment protocols. Acinetobacter baumannii is a critical threat pathogen
notorious for its ability to rapidly develop multidrug resistance. A. baumannii causes hospital-acquired infections,
which manifest as bacteremia, urinary tract and wound infections. In the US, an estimated 60% of hospital-acquired
A. baumannii infections were multidrug-resistant, often including carbapenem resistance, which leaves colistin as
the “last-resort” treatment option. However, colistin resistance has also emerged. There is an urgent need to
understand intrinsic mechanisms that promote antibiotic resistance in A. baumannii to guide alternative antimicrobial
strategies. Our preliminary work has identified two LD-transpeptidases that promote viability of colistin resistant
lipooligosaccharide-deficient A. baumannii. Specifically, LD-transpeptidase-dependent cell envelope modifications
are key for the resistance phenotype, where alternative crosslinks compensate for outer membrane defects. In this
proposal, we will address three important questions to understand the function and regulation of LD-transpeptidases
in A. baumannii, including (I) how does LD-transpeptidase activity counter otherwise lytic mechanical forces
produced by outer membrane defects?; (II) how are LdtK lipoprotein substrates regulated in A. baumannii?; and (III)
how do class A penicillin-binding proteins impact LD-transpeptidase activity in A. baumannii? Collectively, these
studies will address key questions in bacterial physiology and cell envelope assembly, which will enable us to
build a model of intrinsic factors in A. baumannii that contribute to multidrug resistance. Furthermore, these analyses
will aid in the design of combinatorial drug regimens that target both essential outer membrane and peptidoglycan
layers of the cell envelope, thus precluding resistance; consequently, our findings support the National
Institute of Health mission, which aims to foster fundamental discoveries to reduce human disease.

Grant Number: 5R01AI168159-05
NIH Institute/Center: NIH
Principal Investigator: Joseph Boll