Regarding their high bacterial strain specificity, rapid and accurate selection of therapeutic bacteriophages is crucial in phage therapy clinical protocols. Here, we report the use of photonic crystal cavities as on-chip nanotweezers for ultrafast phage susceptibility testing (PST) at the single bacterium level. We used an integrated optical trap based on silicon hollow photonic crystal (HPhC) nanocavities to trap a single Escherichia coli B cell. The optical cavity has a characteristic resonance wavelength, which is modified when an object is trapped. This translates in a transmission jump DT. The collected transmitted signal depends on the characteristics of the trapped entity. E. coli cells were put in contact with T4 Myoviridae phages before being injected in the trapping device. We report direct observation of a bacterium-phage lytic event in the optical cavity. Bacterial burst likely allows the liquid medium to enter the cytoplasm, which implies reduction of the refractive index contrast between the bacterium and the medium. The resonant mode of the cavity being less perturbed, the transmission signal decreases. Accordingly, only 40min ± 5min after the mixing of phages and bacteria (t = 0), we observe an abrupt decrease of the transmission signal correlating with the bursts of the trapped bacterium. The detection of the lysis event is much faster than current culture-based phagograms requiring 16h-24h incubation times. This innovative phagogram approach paves the way to ultrafast PST at the single bacterium level.