Investigation of Escherichia coli O157:H7 Survival and Interaction with Meal Components during Gastrointestinal Digestion.

Escherichia coli O157:H7 is responsible for foodborne poisoning, incriminating contaminated animal food and especially beef meat. This species can survive in the digestive tract, but, up to now, very few studies have considered its survival during the gastrointestinal digestion of meat. The present study aimed to investigate the survival of the pathogenic strain E. coli O157:H7 CM454 during the gastrointestinal digestion of ground beef meat and its interactions with meal components using a semidynamic digestive model. The CM454 strain in meat survived throughout digestion despite acidic pH (pH 2) and the presence of bile salts. The addition of nitrite and ascorbate in the digestion medium led to a decrease in strain survival. During digestion, a release of free iron was observed, which was accentuated in the presence of the CM454 strain. In addition, the strain modified the Fe2+/Fe3+ ratio, in favor of Fe2+ compared to the noninoculated meat sample. In the presence of nitrite, nitroso compounds such as nitrosamines, nitrosothiols, and nitrosylheme were formed. E. coli O157:H7 CM454 had no impact on N-nitrosation but seemed to decrease S-nitrosation and nitrosylation.

Using a dynamic artificial digestive system to investigate heme iron nitrosylation during gastro-intestinal transit.

The International Agency for Research on Cancer recently classified cured meats as carcinogenic for humans and red meats as probably carcinogenic. Mutagens can be formed during meat process or digestion. In a previous study, we used a dynamic artificial digestive system (DIDGI®) to investigate protein oxidation and N-nitrosation during bovine meat digestion. This new paper completes the previous one by focusing on the endogenous heme iron nitrosylation. Low nitrosylation due to nitrate initially present in meat and to ammonia oxidation in the stomach was observed in the digestive tract even in conditions in which no nitrite was added to the model. The endogenous addition of nitrite (1 mM) considerably increased heme iron nitrosylation while a significant decrease was observed with prior meat cooking (30 min at 60 and 90 °C).

To what extent does the nitrosation of meat proteins influence their digestibility?

Nitrosation can occur during meat digestion due to the physicochemical conditions of the stomach (low pH and reducing conditions). The aim of the present study was to elucidate the link between the nitrosation of proteins from beef meat and their digestibility by comparing cooked meat digested with and without the addition of nitrite. To do this, a dynamic in vitro artificial digestive computer-controlled system (DIDGI®) was used to reflect human gastro-intestinal conditions. Peptides and proteins from gastrointestinal digestion were identified by high-resolution LC-MS/MS mass spectrometry. The results showed a dynamic digestion pattern of meat proteins according to their cellular localization. A combined effect of the digestive compartment and the addition of nitrite was established for the first time on peptides profile, linking the nitrosation of proteins and their digestibility.

Iron-catalysed chemistry in the gastrointestinal tract: Mechanisms, kinetics and consequences. A review.

Chemical changes that occur during the storage and processing of food can affect its nutritional content. During digestion, the exposure of food to considerable variations of pH and high oxygen and peroxide concentrations also participates in the deterioration of nutrients, with a negative impact on the nutritional value of the diet and harmful consequences for human health. Iron plays a key role in gastrointestinal chemistry. Haem iron, which exists only in meat, and non-haem iron, present in most foods, are catalysts of most of the reactions implicated in the deterioration of nutrients. Disintegration of food matrix due to mechanical forces and enzymatic hydrolysis favour this endogenous process. This paper provides a review of what is known in the literature concerning the mechanisms and kinetics of endogenous reactions catalysed by iron. The main consequences on nutrient bioavailability are reported and protective strategies against the deleterious effect of iron are discussed.

Mechanisms and kinetics of heme iron nitrosylation in an in vitro gastro-intestinal model.

Fresh red meat and cured meat consumption increases the risk of gastro-intestinal cancers and it is strongly suspected that nitrosylheme is implicated by stimulating the endogenous production of mutagenic aldehydes and N-nitroso compounds. To investigate the extent of endogenous heme iron nitrosylation an experimental in vitro model that mimics the physicochemical conditions of the gastro-intestinal tract was used in association with a mathematical model of chemical reaction kinetics. The combined effect of pH (from 7.2 to 3.2) and myoglobin oxidation state was evaluated in the reaction of nitrite with heme iron, and the observed rate constants of the reactions were determined. Nitrosylation was optimal under mildly acidic conditions (pH 6.5-4.7). Up to 20% of myoglobin can be nitrosylated under gastro-intestinal conditions in this pH range. The effect of various antioxidants (from meat or vegetables) on the endogenous nitrosylation process was also tested.

Oxidation and nitrosation of meat proteins under gastro-intestinal conditions: Consequences in terms of nutritional and health values of meat.

The chemical changes (oxidation/nitrosation) of meat proteins during digestion lead to a decrease in their nutritional value. Moreover, oxidized and nitrosated amino acids are suspected to promote various human pathologies. To investigate the mechanisms and the kinetics of these endogenous protein modifications, we used a dynamic artificial digestive system (DIDGI®) that mimics the physicochemical conditions of digestion. The combined effect of meat cooking and endogenous addition of ascorbate and nitrite was evaluated on protein oxidation (by measuring carbonyl groups), protein nitrosation (by measuring nitrosamines), and proteolysis. Considerable carbonylation was observed in the digestive tract, especially under the acidic conditions of the stomach. Nitrosamines, caused by ammonia oxidation, were formed in conditions in which no nitrite was added, although the addition of nitrite in the model significantly increased their levels. Meat cooking and nitrite addition significantly decreased protein digestion. The interactions between all the changes affecting the proteins are discussed.

Mechanisms and kinetics of tryptophan N-nitrosation in a gastro-intestinal model.

The reaction of nitrite with different amino acids containing secondary amino groups was tested under simulated in-vitro conditions of the digestive tract. After treatment, tryptophan was the only amino acid that exhibited specific UV absorbance of nitrosamines at 335nm, supporting the assumption that it is the main source of endogenous nitrosamines. The combined effect of pH (from 2 to 6.5) and nitrite (from 0.1 to 20mM) was analyzed and the mechanisms and kinetic laws of tryptophan N-nitrosation were determined. The model was then completed by the addition of iron and various antioxidants in concentrations reflecting different diets. The results clearly demonstrated that, in the presence of iron, large amounts of N-nitroso-tryptophan can be formed even at neutral pH, as in the intestine. Antioxidants (ascorbic acid, trolox C, ß carotene, chlorogenic acid, phytic acid and butylated-hydroxytoluene) had various impacts on the extent of N-nitrosation, depending on the iron level.

Impact of the Fenton process in meat digestion as assessed using an in vitro gastro-intestinal model.

The production of oxygen free radicals catalysed by non-haem iron was investigated in an in vitro mimetic model of the digestive tract using specific chemical traps. Superoxide radicals (O2(∗-)) and their protonated form (hydroperoxyl radicals, HO2(∗)) were detected by the reduction of nitroblue tetrazolium into formazan, and hydroxyl radicals (OH(∗)) were detected by the hydroxylation of terephthalate. Under gastric conditions, O2(∗-)/HO2(∗) were detected in higher quantity than OH(∗). Increasing the pH from 3.5 to 6.5 poorly affected the kinetics of free radical production. The oxidations generated by these free radicals were estimated on myofibrils prepared from pork rectus femoris muscle. Myofibrillar lipid and protein oxidation increased with time and oxidant concentration, with a negative impact on the digestibility of myofibrillar proteins. Plant food antioxidants considerably decreased free radical production and lipid oxidation but not protein oxidation.