Submitted to: LWT - Food Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/16/2020
Publication Date: N/A
Interpretive Summary: Microbial sensitivity to irradiation is important for quantifying survival potential under adverse environmental conditions and documenting its utility for inactivation or elimination of microbial populations from unwanted substrates. In this research, sensitivity of Bdellovibrio bacteriovorus 109J (predatory bacteria) and a non-infectious E. coli host to low-dose gamma radiation were evaluated in buffer substrates and on post-harvest lettuce. The populations of predatory bacteria and E. coli host decreased substantially on produce surfaces and in HM buffer as radiation dosage increased from 0.25 to 1.0 kGy. Predatory bacteria in buffer and on lettuce were not detected beyond 0.75 kGy level, indicating that the treatment resulted in microbial inactivation or mortality. The non-infectious E. coli were severely reduced by radiation treatment as low levels of recovery were recorded at 1.0 kGy. Predator survival on produce substrates and aqueous environment may enhance bacterial mortality. Potential applications of predatory bacteria against food-borne bacteria may improve food safety.
Technical Abstract: Bdellovibrio bacteriovorus 109J is a natural predator of Gram-negative bacteria and its utility with irradiation in hurdle treatments would be beneficial for post-harvest produce. However, its sensitivity to gamma irradiation is unknown. The objective of this research was to evaluate the survival of B. bacteriovorus 109J and E. coli ML35 (ATCC 43827, a non-infectious host) following gamma radiation in HM buffer and lettuce substrates. B. bacteriovorus 109J (Bb109J) was cultured on ML35, filtered from host cells and suspended in HM buffer prior to radiation. Similarly, filtered Bb109J (attack-phase cells) were inoculated on lettuce, then irradiated. In replicated experiments, radiation dose rate of 4.42 kGy/h, corresponding to 0.25, 0.50, 0.75 and 1.0 kGy treatments and 0 (control) were applied. Scanning electron micrographs revealed curved-shaped predator cells (Bb109J) almost devoid of host cells. Bb109J populations in HM buffer (0 kGy) were 2.6×107± 8.6×106 plaque forming units (PFU)/mL, while at 0.5 kGy of irradiation, they were 5×102±0.4×102 PFU/mL. The survival of B. bacteriovorus (PFU/mL) in buffer substrate was reduced to below detection levels at = 0.75 kGy. On lettuce, predator cells were reduced from 1.8 ×108 ± 0.4×108 (control) to 0.5×102±0.4×102 PFU/mL at 0.5 kGy. No attack-phase cells were recovered on lettuce at = 0.5 kGy, indicating treatment inactivation. Similarly, linear decreases of E. coli ML35 in HM buffer from 8.1±0.12 Log CFU/mL (0 kGy) to 2.9±0.60 Log CFU/mL (1.0 kGy) were obtained; indicating its survival. On lettuce, ML35 were reduced from 7.3±0.11 Logs (0 kGy) to 4.1±0.35 Log CFU/g of produce at 1.0 kGy. The mean D10 values for Bb109J was 0.71 kGy as opposed to 0.41 kGy for ML35. These results imply that utility of predator cells in hurdle interventions of post-harvest produce may be possible. Inactivation parameters could provide data on Bdellovibrio sensitivity in radiation environments.