A "bad egg" will stand upright when immersed in water, because of methanogenic bacteria.
Non immunized chickens are only an issue if theyve GOT salmonella . The symptoms are easily noticeable and involve some pretty gross symptoms. A farmer can see if a chicken comes down with salmonella and its unusual because big farm FOOD is medicated for the chicks and young pullets. They dont immunize or treat them after about 19 weeks because its a rare condition that, like bird flu, is a mess in the flock.
All my hens are "TOTAL FREE RANGE". They live among the sheep and will , like tick birds, remove bot-fly and larvae on sheep that are less thrifty.
Chickens that are immunized (And this is mostly marketing secrets) can still give a dose of salmonella because the Brits an French dont , for some damned reason, WASH their eggs.
American farmers have egg washers that use cold water and fine soft brushs, and the eggs are usually harder shell .
We feed our chickens a major meal of shell building calcium/magnesium carbonate crushed oyster shell, and they seek it out like salt.
Salmonella can occur iin or outside the eggs shell and what everyone worries about is the "inside the shell" pathogen. (This can be avoided by the feed meds in early months of life and keeping the chickens on qatch so they can be assessed daily for their health. Its usually done by color scan cameras nowadays. (The chickens with salmonella develop blue wattles and feet and the scanning cameras use color comparitors tuned to the specific light frequency of salmonella wattles, and then there are very frequent bact samples taken)
The Commercial houses will hold over 200K chickens so its poor business to dismiss the potential for salmonella . If a bunch of chickens in a house maybe developing salmonella, their initial egg production goes down(normal optimum is 5 eggs perweek per bird) and the egg counters on each cage will signal to a puter that somthing's wrong. Then the sampling and isolation of potentially sick birds goes into high gear.
We only have like 25 +/- chickens now . So we must hunt up the eggs (chickens , not in tight cages, dont lay their eggs in the same spot no matter what anybody sez). Therefore we keep an eye on the girls health and , like I said, we dont worry about the salmonells issue because we
1dont keep any cracked eggs
2we wash the eggs in cold water
3we let em cool on the sinkboard
4we will let eggs in the fridge for weeks and even then they are too fresh to hardboil.
Lots of Americns like eggs and, as far as I know, we dont have a pndemic or even a statitically significant breakout of egg salmonella any more than any other developed country. Where most of our salmonella omes from is infected vegetbles sprouts , live poultry killed for restaurants and pet reptiles