While researching Salmonella enteritidis in poultry, ARS
veterinary medical officer Jean Guard Bouldin (nee Petter) found an
interesting phenomenon. Not only is Salmonella present inside
seemingly uncracked chicken eggs, but other bacteria are there too.
These other bacteria are usually found in eggs that have cracks in
the shell. How could they get into unbroken eggs?
Bouldin and her colleagues at the Southeast Poultry Research Laboratory
in Athens, Georgia, noticed a decrease in shell quality among infected
birds in some experiments. They guessed that the reason other bacteria
besides Salmonella gained entry into the egg was that shell integrity
was being compromised.
To test her theory, Bouldin consulted with Jeff Buhr, of the ARS Poultry
Processing and Meat Quality Research Unit in Athens, Georgia, on mechanical
methods that could be used to measure the strength of the shell. They
inoculated chickens with S. enteritidis and evaluated eggs with
an Instron, a standard laboratory instrument used to test food firmness
through compression. The eggs were compressed until a hairline crack
formed. Eggs from Salmonella-infected hens cracked more easily
than those from non-infected hens. "Salmonella enteritidis seems
to target the hen's reproductive tract, which sometimes results in an
egg with a less resilient shell," says Bouldin.
Other experiments that used a high dose of bacteria in hens confirmed
that S. enteritidis targeted the avian reproductive tract, because
the reproductive tract organs involuted, or shrank, after exposure,
even though the hen continued to appear healthy.
At a low-dose infection, Bouldin found that S. enteritidis actually
stimulated egg production, particularly in older hens. This increased
production may have stretched the limited eggshell materialcalciuma
bit too thin, literally. Other diseases of chickens can also decrease
shell quality, but they usually decrease egg production and cause symptoms
Though eggshell quality normally decreases over the lifespan of a laying
hen, Bouldin wonders whether the decline also occurs from pathogen infection.
Either way, her discovery could inadvertently lead to a way to detect
Salmonella-infected birdsby testing eggshell integrity.
"Our next step is to develop methods to use as a sensitive assay
of bird health in general, leading to a possible control of S. enteritidis
infection and a way to detect other pathogens that result in poor shell
quality," says Bouldin.By Sharon
Durham, Agricultural Research Service Information Staff.
Jean Guard Bouldin
is with the USDA-ARS Southeast
Poultry Research Laboratory, 934 College Station Rd., Athens, GA
30605; phone (706) 546-3446, fax (706) 546-3161.
"Effect of Salmonella on Eggshell Quality"
was published in the October
2004 issue of Agricultural Research magazine.