Survival of acid-adapted and non-adapted Shiga toxin-producing Escherichia coli using an in vitro model

Determination of the changes in the gastric fluid endurance of O157 and non-O157 Shiga toxin-producing Escherichia coli during storage of experimentally produced beef frankfurter

Resistance of Shiga toxin-producing Escherichia coli (STEC) O157:H7 and serogroups O103, O26 and O145 to synthetic gastric fluid (SGF, pH 1.5) were investigated during frankfurter storage. Pathogens were inoculated (5 ± 1 log₁₀ cfu g^-1) on frankfurters and frankfurters were stored at 4 °C for 75 days in vacuum packages. Population changes of the competitive flora and STEC, changes in the pH of the frankfurters and resistance of STEC to SGF were monitored on days 0, 15, 30, 45, 60 and 75 of frankfurter storage. Direct synthetic gastric fluid (DSGF) challenges were also conducted to assess pathogen resistance without being effected by frankfurters, by inoculating pathogen cultures directly into SGF. Results showed that acid resistance of O145 and O26 was stronger than that of O103 and O157 during frankfurter storage. Resistance of O103 to SGF was better than that of O157 during frankfurter storage but, was similar to that of O157 during DSGF challenges. Results indicate that acid resistance of some strains of STEC pathogens might differentiate during storage of frankfurters. Different resistance capabilities to SGF were observed in the STEC strains when inoculated and stored on frankfurters than directly inoculated in the SGF.

New insights into thermo-acidophilic properties of Alicyclobacillus acidoterrestris after acid adaptation

Alicyclobacillus acidoterrestris has unique thermo-acidophilic properties and is the main cause of fruit juice deterioration. Given the acidic environment and thermal treatment during juice processing, the effects of acid adaptation (pH 3.5, 3.2, and 3.0) on the resistance of A. acidoterrestris to heat (65 °C, 5 min) and acid (pH = 2.2, 1 h) stresses were investigated for the first time. The results showed that acid adaptation induced cross-protection against heat stress of A. acidoterrestris and acid tolerance response, and the extent of induced tolerance was increased with the decrease of adaptive pH values. Acid adaptation treatments did not disrupt the membrane potential stability and intracellular pH homeostasis, but reduced intracellular ATP concentration, increased cyclic fatty acids content, and changed the acquired Fourier transform infrared spectra. Transcription levels of stress-inducible (dnaK, grpE, clpP, ctsR) genes and genes related to spore formation (spo0A, ctoX) were up-regulated after acid adaptation, and spore formation was observed by scanning electron microscopy. This study revealed that the intracellular microenvironment homeostasis, expression of chaperones and proteases, and spore formation played a coordinated role in acid stress adaptive responses, with implications for applications in fruit juice processing.