Coxiella burnetii Not a Rickettsia

Abstract

The recent article describing the outbreak of Q fever in the Netherlands (Microbe, April 2011, p. 159) was both well written and impactful in shedding light on what is very likely an underappreciated cause of febrile illness worldwide, especially where people live in close contact with ruminant livestock. In the article, the causative agent of Q fever, Coxiella burnetii, was erroneously described as a rickettsia. While previously Coxiella had been considered a monotypic genus within the family Rickettsiaceae, molecular approaches revealed that Coxiella and rickettsiae actually belong to separate classes within the phylum Proteobacteria: Coxiella within the Gammaproteobacteria and rickettsiae within the Alphaprotebacteria (W. G. Weisburg,, M. E. Dobson, et al., J. Bacteriol. 171:4202–4206, 1989). Coxiella is closely related to Legionella, which has been confirmed by genomic analyses (K. P. Williams, J. J. Gillespie, et al., J. Bacteriol. 192: 2305–2314, 2010). Importantly, this knowledge of the close relationship between Coxiella and Legionella has led to new molecular tools for understanding C. burnetii pathogenesis—a very recent example includes the work by Voth et al. (D. E. Voth, P. A. Beare, et al., J. Bacteriol. 193:1493–1503, 2011). Taxonomy aside, research on C. burnetii and rickettsial pathogens (which broadly can include all the Rickettsiales or more narrowly only the Family Rickettsiaceae or the genus Rickettsia) has been closely linked as experimental approaches to these intracellular pathogens are similar. This linkage continues to flourish today with similarities in intracellular invasion, survival, and growth interpreted in the context of convergent evolution—findings that are shared in forums such as those of the American Society for Rickettsiology, which highlights studies on Coxiella and Bartonella as well as those bacteria within the Order Rickettsiales.