Postprocessing in vitro digestion challenge to evaluate survival of Escherichia coli O157:H7 in fermented dry sausages

Abstract

Fermented dry sausages, inoculated with Escherichia coli O157:H7 during batter preparation, were submitted to an in vitro digestion challenge to evaluate the extent to which passage through the human gastrointestinal tract could inactivate the pathogenic cells, previously stressed by the manufacturing process. The numbers of surviving E. coli O157:H7 cells remained constant after a 1-min exposure of the finely chopped sausage to synthetic saliva or during the following 120-min exposure to synthetic gastric juice at an initial pH of 2.0. However, significant (P < or = 0.05) growth of the pathogen (1.03 to 2.16 log10 CFU/g) was observed in a subsequent 250-min exposure to a synthetic pancreatic juice at pH 8.0. In a different set of experiments, fractions from the gastric suspension were transferred into the synthetic pancreatic juice at 30-min intervals to mimic the dynamics of gastric emptying. Concurrently, the pH of the remaining gastric fluid was reduced to 3.0, 2.5, and 2.0 to simulate the gradual reacidification of the stomach contents after the initial buffering effect resulting from meal ingestion. Under these new conditions, pathogen growth during pancreatic challenge was observed for the first few fractions released from the stomach (90 min of exposure [pH 2.5]), but growth was no longer possible in the fractions submitted to the most severe gastric challenge (120 min of exposure [pH < 2.2]).

FIG. 1.

Survival and/or growth of E. coli O157:H7 cells experimentally inoculated in dry sausage during an in vitro digestion challenge under static conditions. Values reported represent the average results of at least two independent experiments carried out in duplicate. Bars represent standard deviations. Open squares and filled circles represent strains 5-1 and ATCC 43894, respectively. Data represent total counts for injured and uninjured cells measured on MSA after resuscitation on TSA plates. For each strain, means bearing identical letters are not significantly different (P > 0.05).

FIG. 2.

Survival of E. coli O157:H7 cells experimentally inoculated in dry sausage during a gastric challenge under dynamic conditions. Values reported represent the average results of at least two independent experiments carried out in duplicate. Bars represent standard deviations. Open circles, squares, and triangles represent strains F-90, 5-1, and ATCC 43894, respectively. Data represent total counts for injured and uninjured cells measured on MSA after resuscitation on TSA plates. For each strain, means bearing identical letters are not significantly different (P > 0.05).

FIG. 3.

Survival or growth of E. coli O157 in pancreatic juice following 30, 60, 90, and 120 min of prior exposure to gastric challenge under dynamic conditions. Values reported represent the average results of at least two independent experiments carried out in duplicate. Filled circles, open squares, open circles, and filled squares represent cells previously challenged in the gastric juice for 30, 60, 90, and 120 min. Bars represent standard deviations. Data represent total counts for injured and uninjured cells measured on MSA after resuscitation on TSA plates. For each gastric treatment, means bearing identical lowercase letters are not significantly different (P > 0.05). For each intestinal challenge time, means bearing identical uppercase letters are not significantly different (P > 0.05).