Monitoring phage infection and lysis of surface-immobilized bacteria by QCM-D

Abstract

While being a promising approach for the treatment of infections caused by drug-resistant, pathogenic bacteria, the clinical implementation of phage therapy still faces several challenges. One of these challenges lies in the high strain-specificity of most bacteriophages, which makes it necessary to screen large phage collections against the target pathogens in order to identify suitable candidates for the formulations of personalized therapeutic phage cocktails. In this work, we evaluate the potential of quartz crystal microbalance with dissipation monitoring (QCM-D) to identify and detect phage infection and subsequent lysis of bacteria immobilized on the surfaces of the QCM-D sensors. Using lytic Escherichia coli phage T7 as a model, we show that phage infection of E. coli cells results in various unique alterations in the behaviors of the frequency (Δf) and dissipation (ΔD) signals, which are not observed during exposure of the E. coli strain to non-infectious Bacillus subtilis phage phi29 at similar concentration. To aid future phage screening campaigns, we furthermore identify a single measurement parameter, i.e., the spread between the different overtones of ΔD, that can be used to detect phage-induced lysis. For T7 infection of E. coli, this is achieved within 4 h after inoculation, including immobilization and growth of the bacteria on the sensor surface, as well as the completed phage propagation cycle. Given the commercial availability of highly automated multichannel systems and the fact that this approach does not require any sensor modifications, QCM-D has the potential to become a valuable tool for screening medium-sized phage collections against target pathogens.

Keywords: Escherichia coli; Bacteriophages; Infection; Phage therapy; Quartz crystal microbalance with dissipation monitoring; T7.

Fig. 1

Change in frequency Δf (a) and dissipation ΔD (b) during E. coli adsorption and growth on the QCM-D sensor surface. Shaded regions indicate the injection of different media: yellow—PBS, gray—bacteria in medium, pink—medium. c SEM images of the sensor surface after the experiment at two different magnifications. Dividing bacteria are highlighted. Overtones in a and b are indicated by identical colors

Fig. 2

Change in frequency Δf (a) and dissipation ΔD (b) during E. coli adsorption, growth, and T7 infection on the QCM-D sensor surface. Shaded regions indicate the injection of different media: yellow—PBS, gray—bacteria in medium, pink—medium, green—T7 phages in medium (8 × 10⁷ PFU/mL). c SEM images of the sensor surface after the experiment at two different magnifications. Overtones in a and b are indicated by identical colors

Fig. 3

Change in frequency Δf (a) and dissipation ΔD (b) during E. coli adsorption, growth, and T7 infection on the QCM-D sensor surface. Shaded regions indicate the injection of different media: yellow—PBS, gray—bacteria in medium, pink—medium, green—T7 phages in medium (2 × 10⁶ PFU/mL). c SEM images of the sensor surface after the experiment at two different magnifications. Dividing bacteria are highlighted. Overtones in a and b are indicated by identical colors

Fig. 4

Change in frequency Δf (a) and dissipation ΔD (b) during E. coli adsorption, growth, and phi29 exposure on the QCM-D sensor surface. Shaded regions indicate the injection of different media: yellow—PBS, gray—bacteria in medium, pink—medium, green—phi29 phages in medium (2 × 10⁸ PFU/mL). c SEM images of the sensor surface after the experiment at two different magnifications. Dividing bacteria are highlighted. Overtones in a and b are indicated by identical colors

Fig. 5

a Normalized δD for the experiments shown in Figs. 1, 2, 3, 4, S5, S6, S9, and S11. Shaded regions indicate the injection of different media: yellow—PBS, gray—bacteria in medium, pink—medium, green—phages or medium (E. coli only). b Comparison of the normalized δD at 240 min (indicated by the dashed vertical line in a) averaged over all E. coli only experiments (− T7) and T7 experiments (all concentrations, + T7). Values are given as the mean ± standard deviation. Statistical significance was determined by two-tailed t-test and is indicated as ***p < 0.001