ABSTRACT
The genus Citrobacter comprises a group of gram-negative bacilli belonging to the family Enterobacteriaceae, class Gammaproteobacteria, and phylum Proteobacteria. Description of this genera traces back to 1928, when an initial report described the isolation of a novel microorganism that was initially named Bacterium freundii1 and then renamed as Citrobacter freundii in 1932 for its abi lity to use citrate as the sole source of carbon.2 The genus Citrobacter was then enlarged with up to seven different species whose classiŠcation, however, underwent frequent modiŠcations, and over the years they were further relocated within the genera Colobactrum, Escherichia, Salmonella, Levinea, and Arizona, among others. For several years, the names Citrobacter diversus, Citrobacter koseri, and Levinea malonatica were used to designate the same microorganism, and it was not until 1990 that C. koseri prevailed over the other two, which were eliminated as valid species names. In 1993, Brenner et al. showed that Citrobacter strains could be classiŠed in 11 different species: C. koseri, C. amalonaticus, C. farmeri, C. youngae, C. braakii, C. werkmanii, C. sedlakii, C. freundii, and the unnamed genomospecies 9, 10, and 11, which were not named because only a very few strains had been isolated.2a Eventually, the isolation of additional strains led to their naming as C. rodentium, C. gillenii, and C. muraliniae, originating from the current classiŠcation for Citrobacter.3
