Effect of the growth rate of Pseudomonas aeruginosa biofilms on the susceptibility to antimicrobial agents: beta-lactams and fluoroquinolones.

The growth rate of biofilm bacteria of a leucine-requiring mutant Pseudomonas aeruginosa HU1 was regulated by the leucine concentration in a chemically-defined medium. The semiquantitative measurement of glycocalyx and scanning electron microscopy revealed that the kinetics of HU1-biofilm formation were dependent on the incubation time and the leucine concentration in the medium. The effect of the growth rate of biofilm cells on their susceptibility to antimicrobial agents, three beta-lactams and four fluoroquinolones, was evaluated. beta-Lactams showed weak bactericidal activity to biofilm cells; the activity was greater in younger biofilm cells growing in high concentrations of leucine. Fluoroquinolones revealed strong bactericidal activity to biofilm bacteria regardless of the growth rate. The following is suggested: the bactericidal action of beta-lactams against biofilm cells is affected by the cell growth rate, while that of fluoroquinolones is considerably greater and independent on the growth rate.