Helicobacter hepaticus, a recently recognized bacterial pathogen, associated with chronic hepatitis and hepatocellular neoplasia in laboratory mice.

Gastric carcinoma, one of the most prevalent human cancers worldwide, is among the neoplasms for which epidemiologic evidence of environmental causes is strongest. The exact nature of these environmental causes was obscure until mounting evidence recently linked chronic infection of the gastric antrum mucosa by Helicobacter pylori (a microaerobic, gram-negative, spiral bacterium) with elevated cancer risk (1). It is now recognized that gastric B-cell lymphoma of mucosa-associated lymphoid tissue is also closely linked to gastric H. pylori infection, and eradication of the infection with antibiotics can result in regression of the lymphoma (2,3). This startling finding has stimulated intense interest in the genus Helicobacter and related organisms; as a result, additional species of Helicobacter are now frequently isolated and characterized from many non-human hosts. Until 1994, however, only H. pylori was known to be associated with tumor development, in humans or in any other animal species. In 1992, at the National Cancer Institute’s Frederick Cancer Research and Development Center (FCRDC) in Frederick, Maryland, a high prevalence of liver disease was observed among certain strains of mice; these mice were untreated controls in long-term chemical carcinogenesis experiments. Affected strains, notably A/JCr, had been bred at FCRDC under pathogen-free conditions and were free of known serologically detectable murine viruses and parasites; moreover, they had no histologically demonstrable hepatic abnormalities, except for a very low incidence (1% to 2%) of hepatocellular tumors in mice 15 months of age or older. Over a very short period, the prevalence of a histologically distinctive form of hepatitis increased to virtually 100% in male mice at 1 year of age (Table 1). The earliest demonstrable lesions were small, undistinctive foci of hepatic necrosis seen in young mice aged 2 to 6 months. In older mice, aged 6 to 10 months, there was a highly distinctive pericholangitis, consisting of abundant mononuclear cell infiltrates around bile ducts within portal triads. The biliary epithelium within affected ducts was focally swollen, and the luminal surfaces of damaged epithelial cells were poorly defined in hematoxylin and eosin-stained sections (4,5). In livers with extensive lesions, bile ductular (oval cell) hyperplasia was also prominent. Moreover, mice with hepatitis usually had hepatocellular tumors, often multiple, that included both adenomas and carcinomas (4). Hepatocellular tumors in mice are one of the most common endpoints in bioassays for chemical carcinogens. They were not, at that time, known to be associated with infectious agents. Accordingly, initial efforts to identify the cause of the hepatitis/hepatocellular tumor syndrome were directed toward possible sources of chemical exposure. The possibility of accidental exposure to experimental substances within the research animal facilities was ruled out when liver disease was identified in mice that had never left the breeding areas which are located in separate buildings. Extensive chemical analyses of food, bedding, water, and other possible sources of toxic substances had negative results. Detailed pathologic examination by light microscopy of tissue sections from diseased livers was continued, and many special stains were used. One such stain, Steiner’s silver impregnation procedure for spirochetes (6), revealed in hepatic tissue uniform bodies that were consistent in size and shape with bacteria. Homogenates of fresh liver tissue from diseased mice proved effective in transmitting hepatitis to A/J mice purchased from commercial sources outside FCRDC, when given by intraperitoneal injection (5). In addition, from these homogenates, a motile, spiral bacterium could be cultivated on blood agar plates incubated at 37o C under anaerobic or microaerobic conditions. This organism was subsequently characterized by ultrastructural morphologic examination, biochemical characteristics, and 16S rRNA gene sequence. Determined to be a new species related to H. pylori, it was given the name H. hepaticus (7). The bacterium is motile and gram negative, 0.2 to 0.3 μm in diameter, 1.5 to 5.0 μm long, and curved to spiral in shape, with one to several spirals; it has bipolar sheathed flagella (one at each end) but Dispatches