Acid suppression and the gastric flora

Abstract

Until recent years the stomach and upper small intestine were believed to be relatively sterile (Hill, 1986; Drasar, 1989). The normal acid stomach (pH<4) has most commonly been found to have a sparse flora, consisting mainly of organisms from the mouth that have been swallowed with food or saliva and in the process of dying as a result of gastric acid. The major controlling factor for bacterial survival in the stomach thus appears to be pH. When pH is greater than 4 but less than 5 salivary organisms survive, particularly acid-tolerant lactobacilli and streptococci. Above pH 5 a resident gastric flora exists, including faecal streptococci and bacteroides (Hill, 1986; Drasar, 1989). This change is exemplified by studying the gastric bacterial flora of patients with pernicious anaemia and those who have had previous gastric surgery, which frequently contains faecal-type organisms such as Streptococcus faecalis and Bacteroides fragilis, which are also bile resistant. The jejunum and duodenum are also usually relatively free of bacteria in health. The sparse flora is due probably to the few live organisms entering from an acid stomach and the antibacterial effect of bile and pancreatic juice (Hill, 1986). In individuals with achlorhydria, small numbers of bacteria can be detected in jejunal juice but large numbers of organisms are only found in the presence of duodenal or jejunal diverticulae, blind loops or following previous gastric surgery (Drasar et al, 1.969). Our understanding of gastric and duodenal flora has been transformed by the discovery of Helicobacterpylori. This fascinating organism is now recognized to colonize the stomach in the majority of patients with active chronic gastritis and also those with peptic ulceration. It colonizes the duodenum as well in the presence of gastric metaplasia in most patients with chronic active duodenitis. H. pylori is found in the stomach in almost one-half to two-thirds of most populations, depending on the age and socioeconomic circumstances of those studied. The organism is of particular interest in relation to acid secretion since it is itself rapidly killed in an acid environment when the pH is less than 4 (Tompkins, 1989). The presence of this organism may also have effects on acid secretion by means of its association with elevated levels of circulating gastrin, although this is not completely resolved at present.