Yesterday, it was a first look although there's a more deep research on the subject. After reading different sources, we found an entire book from Spanish scientists about the egg. It dedicates a chapter to microbiological contamination.
Trying to reduce the information toward few key aspects from our viewpoint, the first thing to mention is how the egg has a barrier composed by the shell and internal structures made from proteins or other chemical compounds that protect the yolk. The yolk is the main interest for microorganism as it's full of nutrients for achieving a successful multiplication.
This barrier lasts in ambient temperature about 10, 20 days. Why does the egg have that barrier? Well, at the exterior part of the shell it's full of microorganisms that came from different sources and they want to invade the egg for the aforementioned reasons. If the eggs come with dirt or even fecal rests, it has more microorganisms and available nutrients for reproduction, so the danger of invasion is higher.
The more common microorganism are bacteria from Pseudomonas, Flavobacterium, and Alcaligenes branch. These are behind the putrefaction and strange coloration. But the risk should be higher when other pathogens like Staphylococcusor even a variety of Salmonella called enteritidis try to live in the shell and colonize the yolk. The advancing method is still being studied in the case of Salmonella. Salmonella Enteritidis are nowadays very rare although there's still chance of having someday one in your kitchen. Fortunately, with easy methods, recalled below, they will never reach the interior of a human organism.
Hygiene is fundamental, so any egg coming with visible dirt should be cleaned to give less chance to the growth of microorganisms. The natural barriers last more with refrigeration, and there's a partially safe (it's impossible to say entirely safe) period of three, four weeks. That's why eggs have a reference date and nobody should purchase eggs if they are not going to be consumed in that period. Finally, although with exceptions, no common microorganism will survive high temperatures of more than 55-75ºC, 131-167ºF.
A fried egg with a partially cooked yolk seems a problematic bridge to the human organism in some circumstances, so please remind that this has a potential relationship with severe health problems. Cooking at high temperatures is really helpful to vanish possible risks.
The egg industry and the government establish quality controls and they are constantly promoting new methods to handle those more dangerous microorganisms. Perhaps is then better to know and trust as much as possible the source and controls!