Rapid detection of antibiotic resistance based on mass spectrometry and stable isotopes.

With the emergence and growing complexity of bacterial drug resistance, rapid and reliable susceptibility testing has become a topical issue. Therefore, new technologies that assist in predicting the effectiveness of empiric antibiotic therapy are of great interest. Although the use of matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF MS) for the rapid detection of antibiotic resistance is an attractive option, the current methods for MALDI-TOF MS susceptibility testing are restricted to very limited conditions. Here, we describe a technique that may allow for rapid susceptibility testing to an extent that is comparable to phenotypic methods. The test was based on a stable isotope labelling by amino acids in cell culture (SILAC)-like approach. This technique was used to visualise the growth of bacteria in the presence of an antibiotic. Pseudomonas aeruginosa was chosen as the model organism, and strains were incubated in normal medium, medium supplemented with (13)C6-(15) N2-labelled lysine and medium supplemented with labelled lysine and antibiotic. Peak shifts occurring due to the incorporation of the labelled amino acids were detected by MALDI-TOF MS. Three antibiotics with different mechanisms of action, meropenem, tobramycin and ciprofloxacin, were tested. A semi-automated algorithm was created to enable rapid and unbiased data evaluation. With the proposed test, a clear distinction between resistant and susceptible isolates was possible for all three antibiotics. The application of SILAC technology for the detection of antibiotic resistance may contribute to accelerated and reliable susceptibility testing.

Electrophoresis-related protein modification: alkylation of carboxy residues revealed by mass spectrometry.

In recent years, the combination of gel electrophoresis and mass spectrometry has developed into one of the most powerful approaches for the analysis of proteins. However, a number of gel electrophoresis-induced protein modifications have been described. Cysteine is the most endangered amino acid readily reacting with mercaptoethanol or free acrylamide. In the course of studies on glucan phosphorylases (E.C.2.4.1.1) from white potato (Solanum tuberosum L.) and the T cell receptor, we noticed that proteolytic peptides from these proteins can undergo an unexpected modification, giving rise to a mass increment of 14 Da. By post-source decay (PSD) analysis the modification was identified as methylation of the glutamic acid side chain carboxyl group. The methylation takes place during Coomassie blue staining of proteins if both trichloroacetic acid and methanol are present in the staining solution. Replacement of methanol by ethanol under otherwise unchanged conditions results in ethylation of the peptides. The in vitro alkylation was further studied by using synthetic peptides which contain, at different positions: glutamic acid, aspartic acid or the corresponding amides. The kinetic analysis of the observed reactions revealed that glutamic acid is preferentially methylated. The three other amino acid residues can be methylated but with a velocity at least one order of magnitude lower. Although these modifications complicate the interpretation of the spectra, they provide valuable structural information.