Submitted to: LWT - Food Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/12/2014
Publication Date: 2/20/2015
Citation: Olanya, O.M., Niemira, B.A., Phillips, J.G. 2015. Effects of gamma irradiation on the survival of Pseudomonas fluorescens inoculated on romaine lettuce. LWT - Food Science and Technology. 62:55-61.
Interpretive Summary: We determined the sensitivity of Pseudomonas fluorescens (biocontrol agent, non pathogenic) to gamma irradiation in broths, on spinach and romaine lettuce in order to assess the irradiation levels that can best be combined with biocontrol measure for effective control of food-borne bacteria. Our research showed that the irradiation levels required to reduce Pseudomonas fluorescens by 90% in broths were in the range of 0.09 to 0.12 kGy. When bacteria were surface inoculated on spinach and romaine lettuce and irradiated, the doses for microbial inactivation were lower than in broths and ranged from 0.04 to 0.06 kGy. These results indicate that P. fluorescens has high sensitivity to irradiation. Therefore, irradiation should be used on leafy greens prior to the application of P. fluorescens as a combined measure for control of food-borne bacteria.
Technical Abstract: Irradiation of fresh fruits and vegetables has been utilized as a post-harvest intervention measure for inactivation of microbial pathogens. Pseudomonas fluorescens 2-79, Q8R1, Q287 are non-plant pathogenic, non-pectolytic strains of a potential biocontrol agent. We tested the sensitivity of these strains to gamma irradiation to determine irradiation levels that can be applied when biocontrol and irradiation are utilized simultaneously to optimize pathogen control. The effects of gamma irradiation at doses of 0 (control), 0.25, 0.5, 0.75 and 1.0 kGy on P. fluorescens surface inoculated on romaine lettuce and baby spinach and stored for 24 hrs at 5 deg C were assessed. The sensitivity of P. fluorescens to gamma irradiation was also determined on bacteria cells suspended in buffered peptone water (BPW) in tubes. In the non-irradiated control, cell populations ranged from 9.08 to 9.69 log CFU/ml of BPW. The bacteria counts on the non-irradiated spinach and romaine lettuce ranged from 8.14 to 8.95 and from 8.32 to 8.63 log CFU/g, respectively. The radiation D10 values of P. fluorescens suspended in BPW and irradiated ranged from 0.09 to 0.12 kGy when surviving populations were enumerated on Pseudomonas Agar F and Typticase Soy Agar media. On spinach and romaine lettuce, the D10 values of P. fluorescens ranged from 0.04 to 0.05 and from 0.05 to 0.06 kGy, respectively. These results indicate that P. fluorescens has a relatively high sensitivity to irradiation. Populations of P. fluorescens varied on spinach and on romaine lettuce, perhaps due to produce type. These results will guide future research on the combination of irradiation with biological control that can best be utilized to optimize pathogen control.