Antimicrobial resistance is a growing problem worldwide. Clinical laboratories often determine the susceptibility of the bacterial isolates to a number of different antibiotics in order to establish the most effective antimicrobial for treatment. Early targeted antimicrobial therapy helps decrease of cost and prevents spreading of antimicrobial resistance. Unfortunately, conventional antimicrobial susceptibility tests (ASTs) are very time consuming, and insufficiently precise to promptly select a proper antimicrobial treatment. This difficulty disrupts the management of infections and exacerbates the development of antimicrobial resistance. Hence, rapid and reliable susceptibility testing has become a topical issue.
Generally, antimicrobial resistance involves the chemical modification of an antimicrobial compound to an inactive form by an enzyme released by bacteria. This modification causes a structural change and is followed by a characteristic mass shift of the antimicrobials. Using this mechanism we developed a new liquid chromatography-mass spectrometry method to rapidly determine the degree of resistance of Salmonella enterica subspecies enterica serovar Typhimurium (Salmonella Typhimurium), Escherichia coli, and Staphylococcus aureus to amoxicillin, ampicillin, and penicillin G, respectively. This method was successfully applied to bacterial isolates from Korean slaughterhouses and farms. There were 18-Da mass shifts in resistant strains compared with susceptible strains of Salmonella Typhimurium, E. coli, and S. aureus, and the intensities of the hydrolyzed penicillin mass spectra were much higher in resistant strains than those in susceptible strains, which together indicate the reliability of this method. A comparison of the mass spectrometry-derived results with that from conventional ASTs revealed an identical classification of the tested bacteria according to sensitivity and resistance. Notably, this assay method requires only 2 hours for determining the susceptibility status of a strain. This newly developed method is able to determine the extent of antimicrobial resistance qualitatively and quantitatively within a very short time and could be used to replace conventional AST methods.