Helicobacter pylori (Hp), a gram-negative bacterium that causes gastritis and gastric duodenal ulcer, is the most well-known gastric Helicobacter species.1, 2, 3 Hp infection has long been associated with precancerous lesions, such as atrophic gastritis, gastric cancer, as well as gastric mucosa-associated lymphoid tissue (MALT) lymphoma.3,4 Even in developed countries, its prevalence in middle-aged adults is approximately 50%, though there is some evidence that its prevalence has decreased over the last century.5

In recent years, the characteristics of non-Helicobacter pylori Helicobacter (NHPHs; also referred to as H. heilmannii-like organisms or H. heilmannii sensu lato), a member of the Helicobacter genus other than Hp, have been reported.6,7 To date, five NHPH species have been known to colonize the human gastric mucosa: H suis, H felis, H bizzozeronii, H salomonis, and H heilmannii.8 Prevalence of NHPH infection in the human stomach has been reported to be 0.2–6%, which is highly diverse, depending on geographical location and the detection methods.9, 10, 11, 12 Prior to the development of polymerase chain reaction (PCR)-based diagnosis for NHPH infection, histological techniques were the most effective diagnostic approaches.13 The advent of modern techniques resulted in increased sensitivity of NHPH detection; thus, in recent reports, the NHPH infection rate has been higher than in previous reports.14,15 Recently, there have been reports from Japan that indicate the successful culture of H. suis species from human gastric mucosa, one of the most common causes of NHPH infection in humans.16 However, in a clinical setting, NHPH infections are still difficult to diagnose, and retrospective evaluation through culturing remains a challenge.

In cases of NHPH infection, neutrophil activity and infiltration are reported to be lower than that in a Hp infection. Adhesion to epithelial cells and surface epithelial damage are milder in cases of NHPH-associated gastritis than that caused by Hp infection.17,18 It has also been reported that the atrophic changes caused by NHPHs are mild when compared to Hp.13,17,18 NHPH infection has been reported to induce lymphoid hyperplasia, which does not occur on the surface mucosa, but in deep mucosal layers, such as the lamina propria.19,20 Hence, NHPH infection has been reported to be strongly associated with gastric MALT lymphoma that develops from the lamina propria mucosa of the stomach.21,22

In 2009, Haesebrouck et al. reported that NHPH infection is involved in the development of gastritis, gastric ulcer, and gastric MALT lymphoma.8 Our previous studies also reported NHPH infection rates of 10% in Hp-positive gastric MALT lymphomas and 55% in Hp-negative gastric MALT lymphomas.23 By contrast, there have been few reports on the association of NHPH with gastric cancer. This may be due to its poor affinity for the glandular epithelium, resulting in a lack of inducibility of atrophic gastritis and intestinal epithelialization, which are generally associated with precancerous lesions. Moreover, NHPH, unlike Hp, may not be involved in the pathogenesis of gastric cancer.24

In 2020, using PCR-based methods, Nakamura et al. screened Hp-negative cases to determine NHPH infection rates in various gastric diseases, such as gastritis and gastric MALT lymphoma, but no NHPHs-positive cases in gastric cancer cases were screened.25 Liu et al.14 investigated co-infections of NHPHs in Hp-positive cases and discovered an NHPH infection incidence of approximately 12% in chronic gastritis and gastric ulcer cases, but no NHPHs in gastric cancer cases. The relationship between NHPH infection and gastric cancer remains controversial.

In the past, frozen sections were required to extract DNA from the gastric mucosa. However, frozen sections are not routinely collected in daily medical practice. Even when they are collected for research purposes, the invasiveness remains problematic. Moreover, in retrospective studies, frozen sections are difficult to acquire, making it challenging to screen for NHPH infection in a large number of gastric cancer cases.

We have previously reported NHPH infection rate in gastric MALT lymphoma using PCR to diagnose NHPH infection using DNA extracted from formalin-fixed paraffin-embedded (FFPE) tissue.23,26, 27, 28 In the present study, we retrospectively investigated the rate of NHPH infection in patients with gastric cancer by extracting DNA from FFPE sections of gastric endoscopic submucosal dissection (ESD) specimens. We calculated NHPH infection rates in patients with gastric cancer according to Hp infection status and evaluated the association with gastric cancer. We also compared the gastritis-related markers, including serum markers and clinicopathological characteristics of gastric carcinoma, in the stomachs of NHPH-positive patients with those in the stomachs of NHPH-negative patients, with the aim of identifying not only the effects of NHPH on the gastric mucosa and gastric carcinogenesis but also the effects of NHPH on tumor characterization.