The number of cases of salmonellosis has steadily increased during the last decade. Until the end of the 70s, the serotype Salmonella typhimurium was most commonly isolated, but since 1986 the serotype S. enteritidis has become more common. Recent research has shown that the colon is more frequently infected than was earlier recognized, but most common is still an acute enterocolitis during an infection with salmonella. In severe cases, salmonella can spread from the gut, the site of infection, to cause a systemic disease. As in vitro studies have shown, gastroenteritic salmonellae seem to possess a number of so-called virulence factors that help them to evade the host's defenses against bacterial infection. Colonisation in the gut seems to be dependent on a 66 kD heat shock protein (HSP), which binds to a 15 kD glycoprotein present in gut mucus. For penetration into the mucosa, by invasion of the epithelial cells, bacterial invasions are newly synthesized. These proteins distort the epithelial membrane and induce a port of entrance. Macrophages will attack and engulf the bacteria, but salmonella can withstand these professional killers, and even grow intracellularly in them. In order to do so, not only LPS is required, but also specific outer membrane proteins (OMP), porins. Moreover, during their intracellular survival the bacteria need to adapt to the new microenvironment, and in reaction to stress, new HSPs are expressed. Finally, the O-specific sugar chains prevent the otherwise lytic action of complement in blood serum.(ABSTRACT TRUNCATED AT 250 WORDS)