The pathogenic species Campylobacter fetus can be divided into the two subspecies C. fetus venerealis (Cfv) and C. fetus fetus (Cff), which are highly related on the genome level but show completely different infection route and host specificity. Cfv is mainly restricted to the bovine genital tract where it can lead to embryo death, abortion, and infertility. Cff in contrast enters the body via ingestion, withstands acid exposure in the stomach and colonizes the intestinal tract of animals as well as humans. Cff infection in humans causes systemic illness, including bacteremia and septicemia, in hosts either pregnant or having an underlying illness. In animals, Cff can lead to sporadic abortion but not infertility. Because host specificity and survival strategies of the pathogen are poorly understood this study investigates a Cff specific survival mechanism. A comparative analysis of the subspecies? genomes led to the discovery that the gmd gene is specific for Cff. In the closely related species Helicobacter pylori the gmd homolog is involved in lipopolysaccharide (LPS) biosynthesis and contributes to acid resistance in the host. To test whether Cff gmd contributes to acid survival in a similar fashion a wild type strain and a gmd knockout strain were challenged by exposure to decreasing pH, mimicking the passage through the host?s stomach. The mutant strain had a lower survival rate in comparison to the wild type strain when stressed with pH 3.4 for 30 minutes, whereas no significant difference in survival could be detected at a neutral pH. These results indicate that gmd contributes to acid resistance during stomach passage and might be important for virulence.