Propose a life cycle for Coxiella

Coxiella burnetti

The life cycle of Coxiella burnetii wasn’t described until 1981, although the bacterium had been recognized more than 40 years earlier. Observations made by many researchers were finally assembled to show that this bacterium has a more complex life cycle than most. See if you can propose a life cycle for this bacterium from the information provided.

Coccoid and bacillary forms of Coxiella burnetii were first described in 1938. Subsequently, other researchers described round particles that passed through bacteriological filters (0.45 µm) and were capable of infecting guinea pig cells.

In 1981, electron microscopy studies of Coxiella revealed a large cell variant (LCV) and a small cell variant (SCV). The LCV has inner and outer membranes separated by a periplasm containing little peptidoglycan. The SCV lacks a periplasm and has a large peptidoglycan layer. LCVs develop a dense area in the periplasm at one end of the cell when nutrients are depleted or the pH increases. This area contains DNA and ribosomes.

In one study, suspensions of C. burnetii were put in distilled water, exposed to sonication (high-frequency vibration used to disrupt cells), and incubated at 45° C for 3 hr. Only SCVs were present after this treatment. Coxiella undergo binary fission in a host cell phagolysozyme. LCVs metabolize and divide more rapidly than SCVs.

* Please be sure to submit your answers to the following questions as an attachment to the dropbox within this week’s assignment section.

1.  Propose a life cycle for Coxiella.

2.  Why do Coxiella show variable Gram stain results-that is, they may stain gram-positive or gram-negative? Should they be classified as gram-positive or gram-negative?

3.  What disease does C. burnetii cause? Why can this disease be transmitted by airborne routes while other (closely related) rickettsia require insects and ticks for transmission to humans?