Cellular Injuries in Cronobacter sakazakii CIP 103183T and Salmonella enterica Exposed to Drying and Subsequent Heat Treatment in Milk Powder

Abstract

Because of the ability of foodborne pathogens to survive in low-moisture foods, their decontamination is an important issue in food protection. This study aimed to clarify some of the cellular mechanisms involved in inactivation of foodborne pathogens after drying and subsequent heating. Individual strains of Salmonella Typhimurium, Salmonella Senftenberg, and Cronobacter sakazakii were mixed into whole milk powder and dried to different water activity levels (0.25 and 0.58); the number of surviving cells was determined after drying and subsequent thermal treatments in closed vessels at 90 and 100°C, for 30 and 120 s. For each condition, the percentage of unculturable cells was estimated and, in parallel, membrane permeability and respiratory activity were estimated by flow cytometry using fluorescent probes. After drying, it was clearly observable that the percentage of unculturable cells was correlated with the percentage of permeabilized cells (responsible for 20–40% of the total inactivated bacteria after drying), and to a lesser degree with the percentage of cells presenting with loss of respiratory activity. In contrast, the percentages of unculturable cells observed after heat treatment were strongly correlated with the loss of respiratory activity and weakly with membrane permeability (for 70–80% of the total inactivated bacteria after heat treatment). We conclude that cell inactivation during drying is closely linked to membrane permeabilization and that heat treatment of dried cells affects principally their respiratory activity. These results legitimize the use of time–temperature scales and allow better understanding of the cellular mechanisms of bacterial death during drying and subsequent heat treatment. These results may also allow better optimization of the decontamination process to ensure food safety by targeting the most deleterious conditions for bacterial cells without denaturing the food product.

Management of Listeria monocytogenes on Surfaces via Relative Air Humidity: Key Role of Cell Envelope

“…Thus, plasma membrane permeabilization played a significant role in cell death at 68% and 43% RH because >70% of bacteria were permeabilized. This is also consistent with previous studies showing that permeabilization of the plasma membrane occurred during a dehydration–rehydration cycle in bacteria and yeast [ 29 , 31 ].…”

“…L. monocytogenes strains EGDe and SLCC2755 exhibit similar resistance to desiccation [ 22 ], whereas SLCC2755 displays a nonsense mutation in the lmo2768 gene (encoding a potential permease ABC transporter with unknown function [ 26 ]), which could play a role in responses to desiccation and osmotic stresses [ 27 ] and in cell wall homeostasis [ 28 ]. A complementary study also showed that the plasma membrane plays a main role in pathogen inactivation during drying, as cell death was closely related to membrane permeabilization in C. sakazakii and S. enterica dried in milk powder [ 29 ].…”

Natural Antimicrobials Suitable for Combating Desiccation-Resistant Salmonella enterica in Milk Powder

“…Based on these findings, the inclusion of the membrane-active food additive, carvacrol, during drying is advantageous since the compound manipulates Salmonella ’s physiology in a manner that disrupts the desiccation resistance in pathogen cells. The additive pre-treatment made it easy to inactivate Salmonella cells by mild processing such as drying, a process which was also recognized to cause cell death of S. enterica by increasing cytoplasmic membrane permeabilization [ 33 ]. An additional advantage is that the application of the antimicrobial food additive carvacrol at 5000 ppm with freeze-drying caused more than 4.5 log CFU/g reduction in S. enterica .…”

High-Throughput 16S rRNA Gene Sequencing of Butter Microbiota Reveals a Variety of Opportunistic Pathogens

“…Neonatal infections caused by C. sakazakii are often epidemiologically linked to the consumption of contaminated powdered infant formula [35]. It is known that C. sakazakii can live in low-moisture foods [36] and has high tolerance to elevated temperatures [37], which explains the presence of Cronobacter bacteria in dried food products. Cronobacter spp.…”