INTERNATIONAL JOURNAL of SYSTEMATIC BACTERIOLOGY Vol. 21, No. 4 October 1971, pp. 332-334 Printed in U.S.A. Copyright 0 1971 International Association of Microbiological Societies Taxonomy of the Chlamydiae: Reasons for Classifying Organisms of the Genus Chlamydia, Family Chlamydiaceae, in a Separate Order, Chlamydiales ord. nov. JOHANNES STORZ and LESLIE A. PAGE Department of Microbiology, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, Colorado 80521, and National Animal Disease Laboratory, USDA-ARS, Veterinary Sciences Research Division, Ames, Iowa 5001 0 It is proposed that the family Chlamydiaceae, which contains only one recognized genus, Chlamydia, be placed in a new order, Chlamydiales, coordinate with Rickettsiales Buchanan and Buchanan 1938. The principal argument for this change is that chlamydiae multiply by a developmental cycle that is unique among all other forms of bacteria. Classification of the family Chlamydiuceae Subcommittee on Chlamydiaceae of the Tax- alongside the families Rickettsiaceae and Bar- onomy Committee of the American Society for tonellaceae in the order Rickettsiales was first Microbiology on 4 May 197 1. made by R. S. Breed, E. G. 9. Murray, and A. Placement of the family Chlamydiaceae in a P. Hitchens acting as the Editorial Board for separate new order is based on characteristics of Bergey’s Manual, 6th ed. (1948). In this edi- organisms of the genus Chlamydia because at tion, G. W. Rake described the family (then present Chlamydia is the only recognized genus called Chlamydozoaceae Moshkovsky 1945) in the family (6). Three other genera, Colesiota, and included three genera (Chlamydozoon, Ricolesiu, and Colettsia, along with their respec- Miyagawanella, and Colesiota) and 12 species. tive species, were also placed in this family by In the 7th edition (1957) of the manual, the Rake (Bergey’s Manual, 7th ed.), but these family Anaplasmataceae was added to the organisms are now considered to be genera et order, and Rickettsiales was assigned to a new species incertae sedis because none of the class, Microtatobiotes Philip 1956. The reasons organisms has been isolated or cultivated in for placing Chlarnydiaceae in the order Rickett- pure culture, and their description is not siales were not explained in either edition, but adequate to include or exclude them from the by 1957 it appeared that this order had family. accumulated several groups of very diverse The principal argument for establishing the organisms, most of which shared only one new order is that the intracellular develop- characteristic: they could not be cultivated mental cycle of chlamydiae as originally de- outside of living animal cells. Obligate intra- scribed by Bedson and Bland (1) and reviewed cellular parasitism was the principal reason used by Moulder (5) is unique not only among in 1938 by Buchanan and Buchanan (2) to organisms of the other families in the order establish the order initially, and it has uninten- Rickettsiales but is without parallel among tionally served to lump “viral and rickettsia1 organisms in all the other orders of bacteria. agents” together in the minds of many authors Briefly, the cycle consists of the reorganization of medical texts. and growth of the small, rigid-walled, infectious The concept that fundamental biological form of the organism within a cytoplasmic differences between organisms of the family vacuole of the host cell resulting in develop- Chlamydiaceae and those of the families ment of a large vegetative form that has a Ric kettsiaceae, Bartonellaceae, and Anaplas- fragile, nonrigid wall. The large form multiplies mataceae warranted placing Chlamydiaceae in a by fission, but it either is noninfectious or does separate taxon at the ordinal level was initially not survive to infect other host cells if it is suggested by Storz (7). This concept was separated from the original host cell. Daughter unanimously endorsed by members of the organisms again reorganize and gradually con- 332 VOL. 21, 1971 TAXONOMY OF THE CHLAMYDIAE 333 dense to become the small infectious forms dependent upon the host cell for many essential which are released after rupture of the host-cell metabolites (5, 8, 10). Although chlamydiae membranes to repeat the cycle in other host have some independent catabolic abilities, they cells. The ratio of chlamydial deoxyribo- are not able to generate adenosine triphosphate nucleic acid to ribonucleic acid changes signifi- for high-energy storage and utilization (10). cantly during reorganization changes. The They have never been cultivated extracellularly. various stages are illustrated in Fig. 1. Rickettsiae, on the other hand, have been Chlamydiae differ significantly from shown to have the capability of generating rickettsiae in ways other than mode of multipli- adenosine triphosphate and to contain cyte cation. Metabolically, the strains of ChZamydiu chromes (4, 9). One species has been cultivated that have been studied thus far are anaerobic, on a lifeless medium. Furthermore, arthropods cytochrome-less “energy parasites” that are play a significant role in the perpetuation and FIG. 1. Electron micrograph of ultrathin section through a microcolony of Chlamydia psittaci organisms in the cytoplasm of a McCoy cell at 48 hr after infection. The various developmental forms of chlamydiae are labeled. X41,OOO. Relabeled photograph originally published by Cutlip (3)and used by permission. 3 34 STORZ AND PAGE INT. J. SYST. BACTERIOL. transmission of many rickettsial species in LITERATURE CITED nature, whereas arthropods apparently play a 1. Bedson, S. P., and J. 0. W. Bland. 1932. Morpho- very reduced role, if any at all, in the logical study of psittacosis virus with a descrip- epizootiology of chlamydiae. tion of a developmental cycle. Brit. J. Exp. Chlamydiae also differ significantly from Pathol. 13:46 1-466. organisms of the families Bartonellaceae and 2. Buchanan, E. D., and R. E. Buchanan. 1938. Anaplasmataceae in mode of multiplication, Bacteriology. MacMillan Co., New York, N.Y. morphology, and other biological features de- p. 64. scribed for the latter organisms in the BergeyS 3. Cutlip, R. C. 1970. Electron microscopy of cell cultures infected with a chlamydia1 agent Manual, 7th ed. causing polyarthritis of lambs. Infec. Immun. On the basis of the above reasoning, we 1~499-502. propose that, the monogeneric family Chlam- 4. Hayes, J. E., F. E. Hahn, Z. A. Cohn, E. B. ydiaceae be placed in a new and separate taxon, Jackson, and J. E. Smadel. 1957. Metabolic Chlamydiales ord. nov. Chlamydiales is to be studies of Rickettsiae. IV. Terminal respiration considered coordinate with Rickettsiales in a enzymes in Rickettsia mooseri. Biochim. class whose name is yet to be determined. The Biophys. Acta 26570-576. type family is Chlamydiaceae Rake 1957, and 5. Moulder, J. W. 1966. The relation of the psittacosis group (Chlamydiae) to bacteria and the type genus is Chlamydia Jones, Rake, and viruses. Ann. Rev. Microbiol. 20:107-130. Stearns 1945. A description of the order 6. Page, L. A. 1966. Revision of the family Chlamy- Chlamydiales is as fpllows. diaceae Rake (Rickettsiales): unification of the (Ch1a.my.di.a les. M. L. fem. noun p sitt a c o si s -1y m p h o gr anu lo ma v ene reum- Chlamydia type genus of the order; -ales ending trachoma group of organisms in the genus to denote an order; M. L. pl. fem. noun Chlamydia Jones, Rake, and Stearns, 1945. Int. Chlamydiales the Chlamydia order.) J. Syst. Bacteriol. 16:223-252. 7. Storz. J. 1971. Chlamydia and Chlamydia-induced Gram-n e gative, coccoid microorganisms diseases, p. 12. Charles C. Thomas, Publisher, whose obligately in tracellular mode of multipli- Springfield, Ill. cation is characterized by change of the small 8. Weiss, E. 1968. Comparative metabolism of (0.2 to 0.5 pm), rigid-walled, infectious form of rickettsias and other host dependent bacteria. the organism (elementary body) into a larger Zentralbl. Bakteriol. Parasitenk. Infektionskr. (0.6 to 1.5 pm), thin-walled, noninfectious form Hyg. Abt. Orig. 206:292-298. (initial body) that divides by fission. The 9. Weiss, E., E. M. Neptune, Jr., and R. W. Gaugler. developmental cycle is complete when daughter 1968. Influence of gas environment on catabolic cells reorganize and condense to become ele- activities and on reoxidation of reduced mentary bodies which survive extracellularly to nicotinamide adenine dinucleotide phosphate in Chlamydia. J . Bacteriol. 96 :1 5 6 7 - 15 73. reinfect other host cells. Metabolically limited, 10. Weiss, E., and N. N. Wilson. 1969. Role of they are parasites of vertebrates, in which they exogenous adenosine triphosphate in catabolic may cause various diseases, and are occasionally and synthetic activities of Chlamydia psittaci. J. found in arthropods. Bacteriol. 97 :7 19-7 24. .