Different bacteria give different coloured rots when they spoil an egg, because each species produces its own characteristic pigment as it grows. Serratia species are well known for producing a red pigment called prodigiosin, giving the whole egg a reddish tint as it spoils. Pseudomonas fluorescens instead causes a greenish rot, and Aeromonas and Acinetobacter are usually linked with black rot. So the organism behind red rot in eggs is Serratia.