The Genus Enterobacter

The genus Enterobacter was first proposed by Hormaeche and Edwards (1960a). However, the history of some species now placed in the genus Enterobacter can be traced, albeit with some confusion, to the end of the 19th century. “Bacillus lactis aerogenes” was isolated by Escherich (1885) from milk and renamed “Bacillus aerogenes” by Kruse (1896) and “Aerobacter aerogenes” by Beijerinck (1900). Until 1955, differentiation of this organism from Friedländer’s bacillus (now called Klebsiella pneumoniae) was not clear, and most authors considered “B. lactis aerogenes” or “Aerobacter aerogenes” to be either nonmotile or to contain both motile and nonmotile strains (Grimbert and Legros, 1900; Edwards and Fife, 1955). This led Edwards and Fife (1955) to state that “A. aerogenes” strains were in fact Klebsiella strains. “Bacterium cloacae” was described by Jordan (1890) and transferred to a new genus “Cloaca” as “Cloaca cloacae” by Castellani and Chalmers (1920). In the first edition of Bergey’s Manual (Bergey et al., 1923), this species was transferred to the genus “Aerobacter” as “A. cloacae.” Since “Aerobacter aerogenes” was at that time indistinguishable from Klebsiella pneumoniae, Edwards and Fife (1955) proposed that the species “A. aerogenes” not be used, although disappearance of the type species (“A. aerogenes”) implied disappearance of the genus (“Aerobacter”). Because of this, two solutions were suggested for “A. cloacae”: 1) the reintroduction of the genus “Cloaca” with the species “Cloaca cloacae” (reference is not an exact match Kauffman, 1954); and 2) the redefinition of the genus “Aerobacter” with “A. cloacae” as type species (Edwards and Fife, 1955). However, this latter proposal did not conform to the rules of nomenclature. A significant step forward occurred when Møller (1955) devised some simple methods for testing amino acid decarboxylases. Since the “Cloaca” group was arginine-positive it could now easily be distinguished from the Klebsiella group, which was arginine-negative. This led to the finding of motile strains of the “Cloaca” group, which were arginine-negative and produced gas from inositol and glycerol (Hormaeche and Munilla, 1957). These strains were called “Cloaca B” (arginine-positive strains forming the “Cloaca A group”). Then, after reexamination of many cultures using the decarboxylase test, Hormaeche and Edwards (1958) redefined the genus “Aerobacter” to include two species, “A. aerogenes” (“Cloaca B”) and “A. cloacae” (“Cloaca A”), with the type species reaffirmed as “A. aerogenes.” Grimes and Hennerty (1931) described a group of strongly proteolytic strains they named “Aerobacter liquefaciens.” However, this name was illegitimate since the same name had been used by Beijerinck (1900) for bacteria which could have been aeromonads, so that Grimes (1961) renamed his group as “Aerobacter lipolyticus.” Eventually, however, Beijerinck’s “A. liquefaciens” was placed on the list of rejected names (Judicial Commission, 1973), and “A. liquefaciens” Grimes and Hennerty became accepted. In an attempt to avoid confusion resulting from the reclassification in the genus Klebsiella of many nonmotile strains previously labeled “A. aerogenes,” Hormaeche and Edwards (1960a 1960b) proposed a new genus Enterobacter as a substitute for “Aerobacter.” This genus was then composed of E. cloacae (the type species) and E. aerogenes. In 1963, the Judicial Commission of the International Committee on Nomenclature of Bacteria placed the name Enterobacter on the list of conserved names (Judicial Commission, 1963). “Aerobacter liquefaciens,” included in the genus Enterobacter as E. liquefaciens (reference is not an exact matchEdwards and Ewing, 1972), was then transferred to the genus Serratia as S. liquefaciens (Bascomb et al., 1971). DNArelatedness studies have clearly demonstrated it to be a member of the genus Serratia (Steigerwalt et al., 1976). A group of strains named Hafnia by Møller (1954) was included in the genus Enterobacter E. hafniae (Ewing and Fife, 1969) although it was Prokaryotes (2006) 6:197–214 DOI: 10.1007/0-387-30746-x_9