Inactivation of pathogenic bacteria on post-harvest produce by predatory bacteria and hurdle treatments

Authors: Olanya, Modesto; Niemira, Brendan; Ukuku, Dike; Mukhopadhyay, Sudarsan; Uknalis, Joseph

Submitted to: UJNR Food & Agricultural Panel Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 9/24/2019
Publication Date: 11/15/2019
Citation: Olanya, O.M., Niemira, B.A., Ukuku, D.O., Mukhopadhyay, S., Uknalis, J. 2019. Inactivation of pathogenic bacteria on post-harvest produce by predatory bacteria and hurdle treatments. UJNR Food & Agricultural Panel Proceedings. UJNR, Page 29-30.

Interpretive Summary:

Technical Abstract: Biological control may be applied as an alternative approach for inactivation of pathogenic bacteria at post-harvest, because of low host resistance, lack of chemical residues and cost-effectiveness. Bdellovibrio (predatory bacteria) are natural predators of Gram-negative bacteria. We evaluated the predation and survival of Escherichia coli O157:H7 and Salmonella enterica in co-cultures with Bdellovibrio bacterivorus 109J in HM buffer and on post-harvest produce. The sensitivity of Bb 109J to low dose gamma radiation was also determined as a prelude for potential applications in non-thermal hurdle treatments. SEM observations of cell suspensions at 24-hr showed Bb 109J curved-shaped predator cells as evidence of predation. This was confirmed by plaque assays on DNB, whereas ML35 host cells cultured in the absence of predator had no Bdellovibrio. The reductions of pathogenic E. coli and Salmonella by Bb 109J in co-cultures (HM buffer) by predation ranged from 1.00-3.85 CFU/mL after 24-h. On nalidixic acid-resistant strains (non-pathogens), Bb 109J predation of E. coli and Salmonella in co-cultures ranged from 0.91-3.11 and 0.98-2.04 Log CFU/mL of substrates, respectively. On inoculated post-harvest lettuce (E. coli O157:H7 (35150) and Bb 109J) stored at different temperatures, E. coli reductions varied and ranged from 0.32-1.84 Log CFU/g of produce. Although 26C may be suitable for Bb 109J growth, produce surfaces may impede motility of predator cells as opposed to aqueous substrates, limiting its ability to find host cells. In assessment of sensitivity of Bb 109J to irradiation, survival was significantly (P<0.05) decreased at subsequent radiation dosage from 0.25-1.0 kGy. The populations of ML35 (non-infectious host) also progressively decreased in buffer substrate and on lettuce as radiation dosage increased. There is limited research on sensitivity of Bb 109J to gamma radiation and quantification of Bb 109J exposed to gamma radiation is key to documenting predator survival. The influence of produce substrates and aqueous environment on predator survival may be crucial in assessing how this could be utilized to promote bacterial mortality and enhance produce safety. Predation of Gram-negative bacteria by Bb 109J may be a useful tool for post-harvest decontamination of produce, if delivery systems and predator titers are optimized.