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Multidrug-Resistant Acinetobacter baumannii Chloramphenicol Resistance Requires an Inner Membrane Permease

Andrew P-A Karalewitz¹, Samuel I Miller^2,3,4

¹Department of Microbiology, University of Washington, Seattle, Washington, USA.

Antimicrobial Agents and Chemotherapy

|June 13, 2018

Abstract:

Acinetobacter baumannii is a Gram-negative organism that is a cause of hospital-acquired multidrug-resistant (MDR) infections. A. baumannii has a unique cell surface compared to those of many other Gram-negative pathogens in that it can live without lipopolysaccharide (LPS) and it has a high content of cardiolipin in the outer membrane. Therefore, to better understand the cell envelope and mechanisms of MDR A. baumannii, we screened a transposon library for mutants with defective permeability barrier function, defined as a deficiency in the ability to exclude the phosphatase chromogenic substrate 5-bromo-4-chloro-3-indolylphosphate (XP). We identified multiple mutants with mutations in the ABUW_0982 gene, predicted to encode a permease broadly present in A. baumannii isolates with increased susceptibility to the ribosome-targeting antibiotic chloramphenicol (CHL). Moreover, compared to other known CHL resistance genes, such as chloramphenicol acyltransferase genes, we found that ABUW_0982 is the primary determinant of intrinsic CHL resistance in A. baumannii strain 5075 (Ab5075), an important isolate responsible for severe MDR infections in humans. Finally, studies measuring the efflux of chloramphenicol and expression of ABUW_0982 in CHL-susceptible Escherichia coli support the conclusion that ABUW_0982 encodes a single-component efflux protein with specificity for small, hydrophobic molecules, including CHL.

Insights

A newly discovered permease, ABUW_0982, is the main cause of intrinsic chloramphenicol resistance in multidrug-resistant Acinetobacter baumannii. This finding helps understand hospital-acquired infections and develop new treatments.

Area of Science:

Microbiology
Molecular Biology
Drug Resistance

Background:

Acinetobacter baumannii is a Gram-negative bacterium causing hospital-acquired multidrug-resistant (MDR) infections.
It possesses a unique cell envelope, lacking lipopolysaccharide (LPS) and having high cardiolipin content, contributing to its resistance.
Understanding A. baumannii's cell envelope and MDR mechanisms is crucial for combating infections.

Purpose of the Study:

To identify genes responsible for permeability barrier function in A. baumannii.
To investigate the role of specific genes in chloramphenicol (CHL) resistance.
To elucidate the mechanism of intrinsic CHL resistance in A. baumannii.

Main Methods:

Screening a transposon library for mutants with defective permeability barrier function using a chromogenic substrate (XP).
Identifying mutations in the ABUW_0982 gene in susceptible mutants.
Measuring chloramphenicol efflux and ABUW_0982 expression in susceptible Escherichia coli.

Main Results:

Mutants with defective permeability barrier function were identified, many with mutations in ABUW_0982.
ABUW_0982 mutations led to increased susceptibility to chloramphenicol (CHL).
ABUW_0982 was identified as the primary determinant of intrinsic CHL resistance in A. baumannii strain 5075.
ABUW_0982 encodes a single-component efflux protein specific for small, hydrophobic molecules, including CHL.

Conclusions:

ABUW_0982 is a key factor in intrinsic chloramphenicol resistance in Acinetobacter baumannii.
The ABUW_0982 efflux pump contributes to the multidrug resistance phenotype of A. baumannii.
Targeting ABUW_0982 may offer a strategy to overcome chloramphenicol resistance in A. baumannii infections.

Read full analysis

Keywords:

antibiotic resistance drug efflux

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