Diagnosing Rickettsia: culture, molecular testing and paired serology
Rickettsia replicate only inside living host cells, so diagnosis combines early detection of the organism's genetic material, when microbial material is present, with antibody tests (serology) on paired samples.
Remember
Because the organism lives inside cells and antibodies appear late, look for the organism's genetic material early, when microbial material is present, and for a rising antibody level in paired samples later.
Memory aid
Rickettsia needs a living house; agar is only an empty plate of food.
Agar supplies food but not a living host cell, which is what Rickettsia needs.
Rickettsia are small obligate intracellular bacteria, which means they can complete their replication only inside living host cells. Many replicate within vascular endothelial cells, the cells lining blood vessels. Routine agar (a cell-free plate that supplies nutrients and nothing else) cannot provide a living cell. Culture of Rickettsia needs living cells, and it is described as difficult and not clinically useful for routine diagnosis.
The practical approach to diagnosis follows the biology. Direct molecular detection, such as PCR (a molecular test for genetic material) on blood or a rash biopsy, can pick up the organism's genetic material early, when microbial material is present. A molecular test can only find genetic material that actually reached the tube, so the specimen has to come from a site, and a phase of illness, where the organism is present.
Serology tests serum (the liquid part of blood) for antibodies, so it reflects the host's response and not the organism itself. Antibodies take time to appear, and titres (antibody levels) are frequently still negative in the first week of illness, so a single early test can falsely reassure. The usual approach is therefore to use paired samples: an acute (early) sample is compared with a convalescent sample (taken during recovery) weeks later, and a rise in titre, often at least fourfold, shows that the body has responded.
One late sample cannot show a rise, because with no early serum stored there is nothing to compare. Serology tracks the host over time and does not replace early direct detection.
Key facts
- Culture
- needs living cells and specialist containment, and is described as difficult and not clinically useful
- Early detection
- direct molecular detection, such as PCR on blood or a rash biopsy, can find the organism's genetic material when microbial material is present
- Specimen
- a molecular test can only find genetic material that reached the tube, so the sample must come from the right site and phase of illness
- Antibody timing
- antibodies take time to appear, and titres are frequently negative in the first week of illness
- Paired samples
- an acute sample is compared with a convalescent sample, and a rise in titre, often at least fourfold, shows that the body has responded
- No early sample
- with nothing to compare, a rise cannot be demonstrated from one late result
Exam traps
- Adding blood or serum to agar supplies extra nutrients, but a richer plate would still fail for Rickettsia, because nutrients are not the barrier and the organism needs a living host cell.
- However long a cell-free agar plate is incubated, Rickettsia forms no colonies, because the organism needs the environment of a living cell.
- A single acute titre does not on its own distinguish protective immunity from active infection, because titres are interpreted by their change over time.
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