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New Clues on Salmonella

By Jim De Quattro
April 28, 1997

Several new studies cast fresh light on mysteries of Salmonella bacteria’s infection of chickens. They also show that, to protect consumers, egg producers can test spleens of mice from the poultry house to see if they harbor a Salmonella type that infects chickens more readily. In one study, two years of sampling more than 1,000 mice from commercial poultry houses turned up Salmonella enteritidis in the spleen of nearly one in five.

Scientists with USDA’s Agricultural Research Service performed the studies with colleagues in the U.S., England and Australia.

Salmonella contamination in eggs is rare, but S. enteritidis is the most common culprit. When consumed by a hen, the bacteria can multiply and invade organs including the reproductive tract.

S. enteritidis varies in its ability to infect. But researchers now have identified two of its most serious forms. One, named the lacy phenotype, is hardier. Compared with ordinary S. enteritidis forms, the lacy phenotype infects chickens more readily, but it’s not more likely to infect eggs or cause disease symptoms. But that’s not the case with the other serious form, called the SE-HCD phenotype (short for S. enteritidis-high cell density).

Injecting chicks with typical S. enteritidis or the lacy phenotype rarely kills them. But in tests, the SE-HCD phenotype killed 70 percent in three days. Current tests indicate it also lowers egg production in adult hens.

Until now, both the lacy phenotype and SE-HCD phenotype were difficult to study because they were indistinguishable from ordinary types. Also, the SE-HCD type loses its potent virulence in lab cultures. The researchers now have identified distinguishing features in carbohydrates called lipopolysaccharides on the surface of both phenotypes. They also developed the first lab-stable virulent strain.

The latest studies suggest a molecular approach to discovering environmental triggers that turn ordinary S. enteritidis into the more virulent type. This could lead to new tactics for lowering the Salmonella threat. Collaborators with ARS in the studies included USDA’s Food Safety and Inspection Service, the University of Pennsylvania, the University of Georgia, the University of New South Wales in Australia and, in Britain, the Public Health Laboratory Service and Centre for Applied Microbiology and Research.

Scientific contact: Jean Guard-Petter, ARS Southeast Poultry Research Laboratory, Athens, Ga., phone (706) 546-3446, fax 546-3161,