Coxiella burnetii is an obligate intracellular bacterium and pathogen closely related to Rickettsia bacteria, but distinguished from them by multiple extraordinary characteristics. Those characteristics include the presence of small and large cell variants with differing structures, strong resistance to adverse environmental conditions, variable reactions to Gram staining, and airborne transmission capability. The peculiarities of Coxiella burnetii are probably connected and show through in the bacterium’s life cycle and behavior as a pathogen.
The existence of two cell variants presents the key to understanding this bacterium’s unusual nature. Whereas the small cell variant (SCV) has no periplasm but possesses a dense peptidoglycan layer, the large cell variant (LCV) has low peptidoglycan and a periplasm with an area containing DNA and ribosomes. SCVs are more resistant to adverse conditions and can invade hosts. Once they enter a more favorable environment inside host cells, SCVs transform into LCVs, which are less resilient but more reproductively active. After dividing, the new cells can spread throughout the organism and infiltrate other hosts. The differences in cell structures likely account for the discrepancy in Gram stain results. Nevertheless, it seems appropriate to classify Coxiella burnetii as a Gram-negative bacterium because its reproducing form possesses the characteristics of such organisms, such as a periplasm and a thin peptidoglycan layer. Coxiella burnetii is known to cause Q fever in humans (Kudva et al., 2020). Unlike similar diseases, Q fever can spread through the air thanks to the resilience of SCVs, which do not require a host carrier. This versatility is what makes Coxiella burnetii such a dangerous pathogen.
The life cycle of Coxiella burnetii is based on the collaboration of two cell variants. While the highly resilient SCV is well-suited for survival in adverse environments, it has a low reproductive capability. However, by infiltrating a host body and entering a suitable environment, it could transition into a reproductively active LCV. The variance in forms causes discrepant Gram stain results, though LCV exhibits a Gram-negative bacterium’s characteristics. The heightened environmental resistance of SCV enables airborne transmission.
References
Kudva, I. T., Cornick, N. A., Plummer, P. J., Zhang, Q., Nicholson, T. L., Bannantine, J. P., & Bellaire, B. H. (Eds.). (2020). Virulence mechanisms of bacterial pathogens. (5th ed.) John Wiley & Sons.