Inorganic salts and intracellular polyphosphate inclusions play a role in the thermotolerance of the immunobiotic Lactobacillus rhamnosus CRL 1505.

María A Correa Deza,Mariana Grillo-Puertas,Viviana A Rapisarda,Graciela Font De Valdez,Carla L Gerez,Susana Salva,Shihui Yang

doi:10.1371/journal.pone.0179242

Abstract

In this work, the thermotolerance of Lactobacillus rhamnosus CRL1505, an immunobiotic strain, was studied as a way to improve the tolerance of the strain to industrial processes involving heat stress. The strain displayed a high intrinsic thermotolerance (55°C, 20 min); however, after 5 min at 60°C in phosphate buffer a two log units decrease in cell viability was observed. Different heat shock media were tested to improve the cell survival. Best results were obtained in the mediumcontaining inorganic salts (KH2PO4, Na2HPO4, MnSO4, and MgSO4) likely as using 10% skim milk. Flow cytometry analysis evinced 25.0% live cells and a large number of injured cells (59.7%) in the inorganic salts medium after heat stress. The morphological changes caused by temperature were visualized by transmission electronic microscopy (TEM). In addition, TEM observations revealed the presence of polyphosphate (polyP) granules in the cells under no-stress conditions. A DAPI-based fluorescence technique, adjusted to Gram-positive bacteria for the first time, was used to determine intracellular polyP levels. Results obtained suggest that the high initial polyP content in L. rhamnosus CRL 1505 together with the presence of inorganic salts in the heat shock medium improve the tolerance of the cells to heat shock. To our knowledge, this is the first report giving evidence of the relationship between polyP and inorganic salts in thermotolerance of lactic acid bacteria.

Highlights

In the last decades, the food industry evinced the necessity of transforming traditional foods into functional ones by adding bioactive ingredients and probiotic lactic acid bacteria (LAB) [1, 2]
The subsequent exposure to heat shock of these cells with low polyP content resulted in greater lost in cell viability than the results previously described in Table 1 (CRL-1505 with high polyP content), regardless the heat shock medium used
In this work the thermotolerance of the immunobiotic strain L. rhamnosus CRL 1505 was studied since cellular damage by heat stress is one of the major disadvantages of industrial dehydration processes such as spray drying [27,28,29]

Summary

Introduction

The food industry evinced the necessity of transforming traditional foods into functional ones by adding bioactive ingredients and probiotic lactic acid bacteria (LAB) [1, 2]. The demand for probiotics has been on the upswing.

Objectives

Methods

Results

Conclusion