Correlation of Antibiotic Resistance with Vmax/Km Ratio of Enzymatic Modification of Aminoglycosides by Kanamycin Acetyltransferase

Kinetic data for the antibiotic-modifying enzyme kanamycin acetyltransferase AAC(6')-IV have been determined for five aminoglycoside antibiotics (amikacin, gentamicin C1a, kanamycin A, sisomicin, and tobramycin) and compared with close-interval determinations of the minimal inhibitory concentrations of the same antibiotics against Escherichia coli W677 harboring the resistance plasmid pMH67. These minimal inhibitory concentrations for the resistant bacteria varied from 80 to 800 micrograms/ml. Of the kinetic parameters Vmax, Km, and Vmax/Km ratio only Vmax/Km ratio had a linear correlation with minimal inhibitory concentrations (r = +0.818) at pH 7.8, where all antibiotics produced substrate inhibition, but not at pH 6.0, where they did not. The correlation with only Vmax/Km ratio has implications regarding the expression of resistance within the dynamics of the bacterial cell (i.e., antibiotic uptake versus modification), whereas substrate inhibition presents an opportunity to search for new chemotherapeutic agents which will combat resistance directly.