L-Glutamate is involved in the perception of umami taste, intermediary metabolism, and excitatory neurotransmission. In addition, recent studies have uncovered a variety of physiological roles for dietary glutamate, as evidenced by the fact that intragastric glutamate infusions induce flavor preference learning in rats. Moreover, glutamate increases digestive juice secretion and gastric emptying of protein-rich meals. Glutamate levels in blood and brain remain stable all day long even after the food intake since most of glutamate absorbed is oxidized in the mucosa of the small intestine as a primary energy source. Chronic ad libitum ingestion of glutamate solution contributes to reducing weight gain, fat deposition, and plasma leptin levels in comparison to ingestion of water. Glutamate receptors and their cellular transduction molecules have recently been identified in gut epithelial cells. Stimulation of gut glutamate receptors enhances the apical expression of glutamate transporters and also triggers the release of nitric oxide. Nitric oxide in its turn induces gut serotonin release, which increases vagal afferent inputs to different brain regions. Notably, three brain areas, i.e., the medial preoptic area, the dorsomedial nucleus of the hypothalamus, and the habenular nucleus are activated by intragastric glutamate infusions. Total subdiaphragmatic vagotomy abolishes this response. Consistent with the above, vagotomy specifically reduces the overall intake of glutamate. Taken together, these findings contribute to the growing body of evidence indicating that glutamate signaling via dedicated taste and gut receptors influences multiple physiological functions including gut secretion, motility, digestion, absorption, metabolism and energy homeostasis.
While a rationale for the use of probiotics can be developed for a number of gastrointestinal symptoms and syndromes and an experimental basis for their use continues to emerge, irritable bowel syndrome (IBS) has become the focus of much interest in this regard. IBS has also attracted attention because of recent revelations with regard to the potential roles of the enteric flora and immune activation in the pathogenesis of IBS, thereby, leading to a re-awakening of interest in bacteriotherapy in this common and challenging disorder. Some recent, randomized, controlled studies attest to the efficacy of some probiotics in alleviating individual IBS symptoms while selected strains have a more global impact. Evidence for long-term efficacy is also beginning to emerge though more studies are needed in this regard. Several other issues complicate the interpretation of much of the literature in this area: lack of quality control, use of many different species and strains and, above all, significant deficiencies in trial methodology.
The stress response affects virtually every organ in the body and constitutes a coordinated behavioral and physiological response to potentially threatening stimuli that may be physiological or psychological in nature. There is a substantial amount of research focusing on how the stress response affects health, but relatively few studies have focused on the ability of the stress response to affect indigenous populations of bacteria in the intestines, referred to as the intestinal microbiota. Research from our lab, and from others, have demonstrated that psychological stressors early in the life span significantly changes the levels of different types of microbiota that are shed from the intestines in the stool. In our studies, stress in young rhesus monkeys, and even in the prenatal period, led to a significant reduction in the levels of lactobacilli and bifidobacteria shed in the stool. In rodents, prolonged restraint stress resulted in a significant overgrowth of aerobic microbiota, particularly Gram-negative aerobes, in the intestines. Interestingly, there is increasing evidence that alterations in the microbiota are associated with a variety of diseases that are known to be exacerbated during periods of psychological stress, including irritable bowel syndrome and the inflammatory bowel diseases. Thus, our data provide a compelling rationale to test the hypothesis that stress-induced exacerbations of intestinal diseases are in part due to stress-induced alterations of the microbiota.
Salmonella is the most common food-borne pathogen worldwide. Rapid dissemination of multidrug-resistant strains, in particular from animal origins, is a growing concern to human and animal health, and it is mostly attributed to conjugative DNA exchange in the intestinal tract of food animals. To understand the potential role of pigs as a reservoir for antimicrobial-resistant Salmonella, we isolated Salmonella from slaughtered pigs and examined their drug resistance and gene transfer ability. We collected fecal and carcass swabs from 104 healthy pigs at an abattoir in Obihiro, Hokkaido, Japan. A total of 15 Salmonella strains were isolated, the most common serotypes being S. Typhimurium (7/15), S. Derby (2/15), S. Southampton (2/15) and S. O4:d:- (4/15). All of S. Typhimurium isolates were resistant to at least one of five antibiotics (ampicillin, kanamycin, sulfisoxazole, tetracycline and streptomycin). Pulsed-field gel electrophoresis (PFGE) profiles after XbaI and BlnI digestion were analyzed. S. Typhimurium isolates from 3 farms located in different regions clustered together and showed genetic relatedness. In conjugation experiments, one multidrug-resistant S. Typhimurium isolate showed the ability to transfer not only antibiotic resistance genes but also virulence genes such as spvABC to recipient bacteria. These results suggest that the spread of S. Typhimurium had occured in pig farms and that asymptomatic Salmonella-infected pigs should be considered as a significant source of antibiotic-resistant bacteria.