Because sugar doesn't dissolve in automotive fuel, it doesn't carmelize, and so it does not turn into the debilitating gunk this well-known entry in the revenge canon calls for. Instead, sugar poured into a car's gas tank stays intact. While sugar could still cause harm if it reached the engine (but in the same way sand would, by virtue of its being a granular contaminant, not because the sugar would turn into a syrup), even that potential harm is generally prevented by filtration.
The stool culture is a test that detects and identifies bacteria that cause infections of the lower digestive tract. The test distinguishes between the types of bacteria that cause disease (pathogenic) and the types that are normally found in the digestive tract (normal flora). The test helps to determine if pathogenic bacteria are the cause of gastrointestinal symptoms (gastroenteritis). The bacteria found in stool are representative of the bacteria that are present in the gastrointestinal tract. Certain bacteria and fungi called normal flora inhabit everyone's gastrointestinal tract.
In brief: No. The ct contrast is dilute, a small amount of either barium or iodine in a large amount of water. It shouldn't be visible in your stool. Black tarry stools are a sign of internal bleeding. It could be a sign of colon cancer or other important problem. Ask your doctor.