Thermal-Stability and Reconstitution Ability of Listeria Phages P100 and A511

Abstract

The study evaluated the thermal-stability of Listeria phages P100 and A511 at temperatures simulating the preparation of ready-to-eat meats. The phage infectivity after heating to 71°C and holding for a minimum of 30 s, before eventually cooling to 4°C were examined. Higher temperatures of 75, 80, and 85°C were also tested to evaluate their effect on phages thermal-stability. This study found that despite minor differences in the amino acid sequences of their structural proteins, the two phages responded differently to high temperatures. P100 activity declined at least 10 log (PFU mL-1) with exposure to 71°C (30 s) and falling below the limit of detection (1 log PFU mL-1) while, A511 dropped from 108 to 105 PFU mL-1. Cooling resulted in partial reconstitution of P100 phage particles to 103 PFU mL-1. Exposure to 75°C (30 s) abolished A511 activity (8 log PFU mL-1) and both phages showed reconstitution during cooling phase after exposure to 75°C. P100 exhibited reconstitution after treatment at 80°C (30 s), conversely A511 showed no reconstitution activity. Heating P100 to 85°C abolished the reconstitution potential. Substantial differences were found in thermal-stability and reconstitution of the examined phages showing A511 to be more thermo-stable than P100, while P100 exhibited reconstitution during cooling after treatment at 80°C which was absent in A511. The differences in predicted melting temperatures of structural proteins of P100 and A511 were consistent with the observed differences in thermal stability and morphological changes observed with transmission electron microscopy.

Examination of the Use of Bacteriophage as an Additive and Determining Its Best Application Method to Control Listeria monocytogenes in a Cooked-Meat Model System

“…This observation is markedly different from that reported in a previous study involving the heating of A511 in a buffer system at 71°C for 30 s, which resulted in >3.0 log reduction in A511 titers ( Ahmadi et al, 2017 ), as compared to 1.5 log reduction in titers observed in the present study. The differences in thermal stability could be attributed to the composition of the meat slurry that provided thermal protection to A511 or reduced ramp-up time required to reach 71°C in the meat slurry [>7 min in buffer system ( Ahmadi et al, 2017 ) compared 56 s in meat slurry samples]. It is important to point out that the differences in ramp-up time in buffer ( Ahmadi et al, 2017 ) versus the meat slurry is not surprising and could be attributed to the differences in the experimental system.…”

“…Holding for 30 s at 71 • C resulted in ∼1.5 log PFU/g reduction in A511 titers and increasing the holding time to 240 s resulted in 3.5 log PFU/g reduction of A511 titers. This observation is markedly different from that reported in a previous study involving the heating of A511 in a buffer system at 71 • C for 30 s, which resulted in >3.0 log reduction in A511 titers (Ahmadi et al, 2017), as compared to 1.5 log reduction in titers observed in the present study. The differences in thermal stability could be attributed to the composition of the meat slurry that provided thermal protection to A511 or reduced ramp-up time required to reach 71 • C in the meat slurry [>7 min in buffer system (Ahmadi et al, 2017) compared 56 s in meat slurry samples].…”

“…In contrast, the meat slurry system used in the present study utilizes very thin (2 mm) meat slices containing A511 which explains the very short temperature ramp-up time. Therefore, one has to be very careful drawing any conclusions regarding thermal stability of A511 comparing results from the present study to that of Ahmadi et al (2017). Nevertheless, the findings reported here are helpful in preparing a model meat system that could be used to determine efficacy of bacteriophages as a biocontrol agent when used as an additive.…”

“…The differences in thermal stability could be attributed to the composition of the meat slurry that provided thermal protection to A511 or reduced ramp-up time required to reach 71°C in the meat slurry [>7 min in buffer system ( Ahmadi et al, 2017 ) compared 56 s in meat slurry samples]. It is important to point out that the differences in ramp-up time in buffer ( Ahmadi et al, 2017 ) versus the meat slurry is not surprising and could be attributed to the differences in the experimental system. The buffer system utilized by Ahmadi et al (2017) consisted of 160 mL glass dilution bottles with 100 mL A511 suspension.…”

“…It is important to point out that the differences in ramp-up time in buffer ( Ahmadi et al, 2017 ) versus the meat slurry is not surprising and could be attributed to the differences in the experimental system. The buffer system utilized by Ahmadi et al (2017) consisted of 160 mL glass dilution bottles with 100 mL A511 suspension. In contrast, the meat slurry system used in the present study utilizes very thin (2 mm) meat slices containing A511 which explains the very short temperature ramp-up time.…”