Antimicrobial susceptibility

BACLIB detects antimicrobial susceptibility and resistance two ways:

■ Some forms of antimicrobial resistance are detected as part of the initial, one-hour ID test.

■ MICs (minimum inhibitory concentrations) of antibiotics are provided by a second testing step within six hours of sample collection.


Ultra-rapid antimicrobial resistance testing

Some forms of antimicrobial resistance (AMR) are expressed in microbial membranes. For example, bacteria make their membranes less electrically negative to become resistant to antibiotics including colistin and polymyxins. These membrane changes are expressed even in the absence of antibiotics, and are readily detected by BACLIB, as part of ID testing.

Below, BACLIB spectra from four Gram-negative strains transformed with the mcr-1 plasmid (“Resistant”) are compared to un-transformed strains (“Susceptible”). The mcr-1 plasmid adds a phosphoethanolamine to lipid A molecules, which reduces colistin’s ability to cross the bacterial membrane. This also increases the mass of modified lipid A by 123 Da, which is easily observed by mass spectrometry. Other membrane modifications conferring antimicrobial resistance, such as addition of aminoarabinose, can also be observed in this way.

Modified membranes from mcr-1 transformation are easily observed by BACLIB

Antimicrobial resistance of this type is detected automatically by BACLIB at the same time as ID, for Ultra-rapid AMR testing in one hour, direct from specimen.


Rapid antimicrobial susceptibility testing

Membrane-expressed AMR is detected as part of the BACLIB ID test; other forms of AMR are detected in a second step called “BACLIB Rapid AST.” In six hours, direct from specimen, BACLIB Rapid AST determines minimum inhibitory concentrations (MICs).

BACLIB Rapid AST detects microbial growth by adding a small percentage of inexpensive, non-toxic deuterium oxide (D2O) to growth media. The initial inoculum is not labeled with D2O. Any growth in labeled media will increase the mass of lipids, which is easily observed by mass spectrometry.

BACLIB Rapid AST works just like traditional AST: samples of a specimen are grown with a range of antibiotic concentrations; the lowest concentration that inhibits growth indicates MIC. The difference is that BALIB Rapid AST determines MICs in six hours, direct from a clinical sample such as a urine specimen –days faster than with traditional AST.

Below, BACLIB Rapid AST spectra are shown for Pseudomonas fluorescens with Kanamycin. The MIC at 50.0 µg/mL is easily observed.

Together, BACLIB ID and BACLIB Rapid AST allow patients to get appropriate therapy in hours, instead of days.


Further reading:

Colistin heteroresistance is largely undetected among carbapenem-resistant Enterobacterales in the United States
Band V, Satola S, Smith R, et al.
mBio 2021

Rapid microbial identification and colistin resistance detection via MALDI-TOF MS using a novel on-target extraction of membrane lipids
Sorensen M, Chandler CE, Gardner F, et al.
Sci Rep. 2020

A Prospective Study of Acinetobacter baumannii Complex Isolates and Colistin Susceptibility Monitoring by Mass Spectrometry of Microbial Membrane Glycolipids
Leung LM, McElheny CL, Gardner FM, et al.
J Clin Microbiol. 2019

Rapid Microbial Identification and Antibiotic Resistance Detection by Mass Spectrometric Analysis of Membrane Lipids
Liang T, Leung LM, Opene B, et al.
Analytical Chemistry 2019

Structural modification of LPS in colistin-resistant, KPC-producing Klebsiella pnemoniae
Leung LM, Cooper VS, Rasko DA, et al.
J Antimicrob Chemother. 2017

Structural Modification of Lipopolysaccharide Conferred by mcr-1 in Gram-Negative ESKAPE Pathogens
Liu YY, Chandler CE, Leung LM, et al.
Antimicrob Agents Chemother. 2017

Unique structural modifications are present in the lipopolysaccharide from colistin-resistant strains of Acinetobacter baumannii
Pelletier MR, Casella LG, Jones JW, et al.
Antimicrob Agents Chemother. 2013

Unique lipid A modifications in Pseudomonas aeruginosa isolated from the airways of patients with cystic fibrosis
Ernst RK, Moskowitz SM, Emerson JC, et al.
J Infect Dis. 2007

pmrA(Con) confers pmrHFIJKL-dependent EGTA and polymyxin resistance on msbB Salmonella by decorating lipid A with phosphoethanolamine
Murray SR, Ernst RK, Bermudes D, et al.
J Bacteriol 2007