Bacterial contamination of food. Coloured scanning electron micrograph (SEM) of the surface of a piece of cooked roast beef showing colonies of rod-like bacteria (green) reposing on the ends of the muscle fibres. The result of a visit from an itinerant housefly, the bacteria are probably Salmonella sp. , a genus responsible for food poisoning. The housefly's visit may further disturb the surface, since any eggs laid hatch quickly and the maggots released rapidly begin breaking down the muscle fibres by secreting an enzyme-containing liquid. The resulting soup" is then re-ingested. Magnification: X1920 at 6x4.5cm size."