Shigella is closely related to Escherichia and is considered by some, to be another strain of E. coli. However, because Shigella is anaerogenic(does not produce gas from carbohydrates) and lactose(-), it is easily discernable from E. coli. In many cases, a Shigella infection will lead to diarrhea accompanied by fever. Shigella is also an invasive pathogen which can be recovered from the bloody stool of an infected host. Invasive pathogens colonize the host's tissues as opposed to growing on tissue surfaces. The four species in this genus are sometimes referred to by a letter designation based on their serologocal antigen: