Citrobacter is a ubiquitous bacteria found in soil and water. It takes its name from the fact that it can survive solely on the carbon in citric acid. It can also be found in the intestines. It rarely causes disease in humans, but can cause a urinary tract infection, sepsis or meningitis in infants. In these case, it can be difficult to treat as it is naturally resistant to gram-positive antibiotics such as penicillin and can develop resistance to other antibiotics.

Citrobacter at Wikipedia