Coxiella burnetti

Question

Background
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

Answer 1

1. Proposed Life Cycle for Coxiella burnetii

Coxiella burnetii has a complex life cycle characterized by two distinct forms: the Large Cell Variant (LCV) and the Small Cell Variant (SCV). Here's a proposed life cycle based on the provided information:

1. Infective Form (SCV): The life cycle starts with the SCV, which is the infectious form of the bacterium. It is resistant to environmental stresses and can persist outside of the host. These SCVs can be aerosolized and are susceptible to infection by the host.

2. Entry into Host Cell: The SCVs enter a host cell (e.g., macrophages) via phagocytosis. The bacterium resides within a phagolysozyme, where it is initially exposed to the harsh environment of the lysosome.

3. Transformation to Large Cell Variant (LCV): Once inside the phagolysozyme, SCVs can undergo transformation into LCVs. LCVs metabolize and replicate more efficiently. They develop a specialized internal structure filled with DNA and ribosomes, which allows for robust growth and division.

4. Replication: The LCVs utilize nutrients from the host cell to rapidly divide through binary fission. This stage allows the bacteria to proliferate, leading to an increased bacterial load within the host cell.

5. Release and Propagation: Eventually, the host cell may undergo lysis, releasing SCVs back into the extracellular environment. These SCVs can then infect other host cells or be aerosolized, continuing the cycle of infection.

6. Environmental Persistence: Outside the host, SCVs can survive in various environmental conditions, contributing to their transmission via airborne routes.

2. Variable Gram Stain Results

Coxiella burnetii exhibits variable Gram stain results due to its unique cell wall structure. It possesses characteristics of both gram-positive (thick peptidoglycan layer) and gram-negative (outer membrane, periplasmic space) bacteria. The presence of both an outer membrane and a substantial peptidoglycan layer leads to inconsistency in Gram staining.

• Classification: While Coxiella has a thick peptidoglycan layer similar to gram-positive bacteria, the presence of an outer membrane aligns more closely with gram-negative bacteria. Therefore, it should be classified as a gram-negative bacterium, although its atypical characteristics may not align perfectly with standard definitions.

3. Disease Caused by Coxiella burnetii and Transmission Routes

Coxiella burnetii causes Q fever, which can present as acute or chronic illness. Acute Q fever often resembles a viral illness, while chronic Q fever can lead to more severe complications, such as endocarditis.

Transmission Mechanisms:

• Airborne Routes: Q fever can be transmitted through inhalation of contaminated aerosols, primarily from infected animals (especially livestock) or their products (e.g., birth fluids, feces, urine). The SCV form of C. burnetii is stable in the environment and can be aerosolized, aiding in this transmission route.
• Rickettsia Comparison: In contrast, many closely related Rickettsia species require insect vectors, such as ticks or lice, for transmission. These organisms typically rely on vector-borne transmission because their survival outside of a host depends significantly on a blood meal from the host, which is not required for Coxiella. Coxiella’s environmental resilience and ability to persist as infectious particles in the air facilitate its airborne transmission, making it distinct from many of its rickettsial relatives.