Changes in Thermal Resistance of Three Salmonella Serovars in Response to Osmotic Shock and Adaptation at Water Activities Reduced by Different Humectants

Abstract

The purpose of this study was to investigate the effect of osmotic shock and adaptation at low water activity (aw) and the type of humectant used to lower the aw, on heat resistance of three Salmonella enterica serovars (Saintpaul 02-109, Tennessee 2053H, and Elmsbuettel 1236H). The serovars were grown (adapted) or transferred (osmotic shocked) in low-aw broths and subjected to heat treatment at 55°C for up to 45 min; samples were removed at 5-min intervals and immediately placed in an ice-water bath until plating. The aw of tryptic soy broth (TSB) was lowered by the addition of 20% (wt/wt) glycerol (aw 0.94), 4% (wt/wt) sodium chloride (NaCl; aw 0.97), or 35% sucrose (wt/wt) (aw 0.95). The type of humectant and cell adaptation significantly affected the D55°C-value. Cells merely suspended in 20% glycerol broth (i.e., nonadapted) prior to heat treatment showed a larger D55°C-value (3.0 to 3.9 min), when compared with that of cells adapted in the same medium (D55°C-values of 0.86 to 0.98 min). Interestingly, cells adapted to TSB plus glycerol were not more resistant to heat than were the controls. NaCl and sucrose showed a net protective effect for all serovars under both the adapted and nonadapted conditions, with sucrose providing the most protection. Highest D55°C-values were obtained for cultures adapted to TSB plus sucrose. Based on these results, the effect of reduced aw on thermal resistance of Salmonella serovars varies greatly, depending on medium constituents and adaptation of the pathogen in these media.

Assembly and Characterization of a Pathogen Strain Collection for Produce Safety Applications: Pre-growth Conditions Have a Larger Effect on Peroxyacetic Acid Tolerance Than Strain Diversity

“…While it has been well established that different strains and genetic lineages of a pathogen may differ in their ability to survive and grow under different stress conditions, the physiological state of bacterial cells and the conditions under which bacteria are pre-grown also have a considerable impact on the ability of foodborne pathogens to survive subsequent stress conditions, including produce-relevant interventions (such as chlorine washes) as specifically documented for L. monocytogenes (Bergholz et al, 2012, 2013; Poimenidou et al, 2016), S. enterica (Yang et al, 2001; Asakura et al, 2002; Gruzdev et al, 2011; Stackhouse et al, 2012; He et al, 2013) and STEC (Gawande and Griffiths, 2005; Shuai et al, 2017). One of the most well documented examples of the effect of environmental conditions on stress tolerance is the observation that S. enterica present in low water activity (a w ) environments are considerably more tolerant to heat treatment than S. enterica present in high water activity environments (He et al, 2013; Peña-Meléndez et al, 2014). For example, S. enterica serotype Tennessee (“ Salmonella Tennessee”) present in peanut butter with an a w of 0.2 showed less than a 3 log reduction after treatment at 90°C for 20 min, while reduction in peanut butter with an a w of 0.8 was around 5 log, a difference of at least 2 log reduction (He et al, 2013).…”

Determination of D and z values for Salmonella Typhimurium inoculated in an egg-based pastry

“…According to the range of aw, 0.94-0.99 (International Commission on Microbiological Specifications for Foods, 1996), for Salmonella growth, 0.944 obtained for "Cavacas de Resende" is at the minimum growth limit. Furthermore, the ability of some Salmonella serotypes to survive in environments with aw ≤ 0.85 and dry environments has already been demonstrated (Kapperud et al, 1990;Peña-Meléndez et al, 2014). The pH value obtained, 7.26, is in the range of optimum pH for Salmonella grow (6.5-7.5) (D' Aoust & Maurer, 2007;International Commission on Microbiological Specifications for Foods, 1996;Jay et al, 2005).…”

“…Decrease of aw, exposure to sublethal temperatures before heat treatment, low pH, absence of oxygen and food composition play an important role, increasing the microorganism's thermal resistance. An increase in Salmonella thermal resistance has been mentioned as related with the addition of humectants agents like sugar, as well as the presence of lipids in food composition that may cause a protective effect on the microorganism (Bell & Kyriakides, 2002;Doyle & Mazzotta, 2000;Peña-Meléndez et al, 2014).…”

“…Thermal resistance tends to increase with the addition of humectants such as sucrose and salt with the consequent decrease of aw. In addition, other factors influence the increase of thermal resistance, such as lack of oxygen, use of sub lethal incubation temperatures before heat treatment and food composition, where the presence of lipids has been mentioned as having a protective effect on microbial cells (Bell & Kyriakides, 2002;Doyle & Mazzotta, 2000;Peña-Meléndez et al, 2014).…”

Survival of Salmonella on Red Meat in Response to Dry Heat

“…Some strains may have had a better adaptation towards heat by upregulating these genes or expression of heat shock proteins. The alternative sigma factor RpoS is involved in regulating the expression of several genes when Salmonella is exposed to low water activity and high temperature (40). Transcriptomic studies may be useful to determine the expression of these iron responsive and stress responsive genes enhancing the survival of Salmonella on different red meat.…”