Monitoring the effect of pH on the growth of pathogenic bacteria using electrical impedance spectroscopy

pH value is a significant environmental factor controlling the bacterial growth, activity and affecting their metabolic properties. In the present study, the effect of the different pH values (5, 6, 7, 8, and 9) of the cultivation medium on the growth rate of Escherichia coli ATCC 8739, Bacillus cereus ATCC 10876 and Pseudomonas aeruginosa 134,909/2 were studied. Plate count method and spectrophotometric measurement of optical density of the cultivation broth were used to assess growth of the pathogenic bacteria. Moreover, impedance spectroscopy measurement was applied to monitor the impedance change caused by the bacterial growth and activity at different pH values. Although the results showed that the initial pH did not completely inhibit the growth of the bacteria, the bacterial growth varied with the pH change showing the interference of pH with cell metabolism. Based on the results, the optical density of the cultivation broth measured spectrophotometrically for E. coli and B. cereus has shown good correlation with cell number determined by the plate count method in each phase of bacterial cultivation. Moreover, the results indicated that for E. coli and B. cereus the impedance value of the bacteria and the medium decreased from the beginning of the experiment till its end after 96 h from the start The impedance value during the first 24 h increased for P. aeruginosa, and then was decreasing until the end of the experiment. Nevertheless, the change in the resistance of the bacteria and the medium was proportionate to the change in cell number. For E. coli and B. cereus the resistance was decreasing with the growth of the bacteria. The linear dependency between the impedance and cell number was observed at low frequencies around 10-100 Hz. For P. aeruginosa, the resistance was increasing during the first 24 h and was decreasing afterwards. Furthermore, the resistance for P. aeruginosa was decreasing with the increase in cell number. Nonetheless, the impedance of the medium with P. aeruginosa was in linear dependency on the logarithm of cell number with the best R2 at high frequencies (0.1-1 MHz).