Abstract: Rapid determination of antimicrobial susceptibility/resistance is an important factor in selecting an appropriate antimicrobial treatment and eradicating infections promptly. Conventional antimicrobial susceptibility tests (ASTs) are very time consuming. Thus, we developed a liquid chromatography-mass spectrometry (LC-MS/MS) method for rapidly determining the resistance of Staphylococcus aureus  to penicillin-G in an animal-infection model. This technique will be able to detect those resistant strains whose resistance mechanism specifically controlled by penicillinase. The resistance status of S. aureus against penicillin-G was determined by conventional AST. Cultured S. aureus  cells were inoculated to chicken for developing bacteraemia. The solution of penicillin-G was intravenously administered (10 mg/kg b.w.) to chickens just after infection detection. Blood samples were collected at different intervals after drug administration. The concentration of active penicillin-G and its metabolites were determined from the bacteria-free blood supernatant by utilizing the LC-MS/MS method. Evidence of infection in chicken was observed within 5 h of bacterial inoculation. The penicillinase enzyme generated by S. aureus  transforms the active penicillin-G to an inactive metabolite by hydrolysis, which is evident by the mass shift from 335.10600 to 353.11579 Da as quantified using liquid chromatography quadrupole time-of-flight mass spectrometry (LC/Q-TOF/MS). The signal intensity of inactive/hydrolysed penicillin-G is several-fold greater than that of the active penicillin-G in the blood sample of chicken infected with resistant strain and treated with penicillin-G. The antimicrobial resistance index (ARI) value of resistant S. aureus  strain was more than 1, demonstrating the penicillin-G-resistance pattern of that strain. This method is able to determine the extent of β-lactam antimicrobial resistance within 1.5 h from the patient’s blood and is complementary with those existing AST methods which are usually practicing in the evaluation of β-lactam antibiotic resistance.
Keywords: β-lactamase, spectrometry, antibacterial susceptibility test, chicken infection model, antibacterial resistance