Rickettsia

Key takeaways

Rickettsia belongs to biology; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Rickettsia to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Rickettsia from memory before moving on to harder problems.

Reference excerpt

Rickettsia is a genus of nonmotile, gram-negative, nonspore-forming, highly pleomorphic bacteria that may occur in the forms of cocci (0.1 μm in diameter), bacilli (1–4 μm long), or threads (up to about 10 μm long). The genus was named after Howard Taylor Ricketts in honor of his pioneering work on tick-borne spotted fever. Properly, Rickettsia is the name of a single genus, but the informal term "rickettsia", plural "rickettsias", usually not capitalised, commonly applies to any members of the order Rickettsiales. Being obligate intracellular bacteria, rickettsias depend on entry, growth, and replication within the cytoplasm of living eukaryotic host cells (typically endothelial cells). Accordingly, Rickettsia species cannot grow in artificial nutrient culture; they must be grown either in tissue or embryo cultures. Mostly chicken embryos are used, following a method developed by Ernest William Goodpasture and his colleagues at Vanderbilt University in the early 1930s. Many new strains or species of Rickettsia are described each year. Some Rickettsia species are pathogens of medical and veterinary interest, but many Rickettsia are non-pathogenic to vertebrates, including humans, and infect only arthropods, often non-hematophagous, such as aphids or whiteflies. Many Rickettsia species are thus arthropod-specific symbionts, but are often confused with pathogenic Rickettsia (especially in medical literature), showing that the current view in rickettsiology has a strong anthropocentric bias. Pathogenic Rickettsia species are transmitted by numerous types of arthropods, including chiggers, ticks, fleas, and lice, and are associated with both human and plant diseases. Most notably, Rickettsia species are the pathogens responsible for typhus, rickettsialpox, boutonneuse fever, African tick-bite fever, Rocky Mountain spotted fever, Flinders Island spotted fever, and Queensland tick typhus (Australian tick typhus). The majority of pathogenic Rickettsia bacteria are susceptible to antibiotics of the tetracycline group.

Classification The classification of Rickettsia into three groups (spotted fever, typhus, and scrub typhus) was initially based on serology. This grouping has since been confirmed by DNA sequencing. All three of these groups include human pathogens. The scrub typhus group has been reclassified as a related new genus, Orientia, but they still are in the order Rickettsiales and accordingly still are grouped with the rest of the rickettsial diseases. Rickettsias are more widespread than previously believed and are known to be associated with arthropods, leeches, and protists. Divisions have also been identified in the spotted fever group and this group should probably be divided into two clades. Arthropod-inhabiting rickettsiae are generally associated with reproductive manipulation (such as parthenogenesis) to persist in host lineage. In March 2010, Swedish researchers reported a case of bacterial meningitis in a woman caused by Rickettsia helvetica previously thought to be harmless.

Spotted fever group Rickettsia rickettsii (Western Hemisphere) Rocky Mountain spotted fever Rickettsia akari (USA, former Soviet Union) Rickettsialpox Rickettsia conorii (Mediterranean countries, Africa, Southwest Asia, India) Boutonneuse fever Rickettsia sibirica (Siberia, Mongolia, northern China) Siberian tick typhus or North Asian tick typhus Rickettsia australis (Australia) Australian tick typhus Rickettsia felis (North and South America, Southern Europe, Australia) Flea-borne spotted fever Rickettsia japonica (Japan) Oriental spotted fever Rickettsia africae (South Africa) African tick bite fever Rickettsia hoogstraalii (Croatia, Spain and Georgia USA) Unknown pathogenicity Rickettsia lanei (Western USA) Severe Rocky Mountain spotted fever-like illness

Typhus group Rickettsia prowazekii (worldwide) Epidemic typhus, recrudescent typhus, and sporadic typhus Rickettsia typhi (worldwide) Murine typhus (endemic typhus)

Scrub typhus group The causative agent of scrub typhus formerly known as R. tsutsugamushi has been reclassified into the genus Orientia.

Flora and fauna pathogenesis Plant diseases have been associated with these Rickettsia-like organisms (RLOs):

Beet latent rosette RLO Citrus greening bacterium possibly this citrus greening disease Clover leaf RLO Grapevine infectious necrosis RLO Grapevine Pierce's RLO Grapevine yellows RLO Witch's broom disease on Larix spp. Peach phony RLO Papaya Bunchy Top Disease Infection occurs in nonhuman mammals; for example, species of Rickettsia have been found to afflict the South American guanaco, Lama guanacoe potentially marsupials and reptiles.

Pathophysiology

Rickettsial organisms are obligate intracellular parasites and invade vascular endothelial cells in target organs, damaging them and producing increased vascular permeability with consequent oedema, hypotension, and hypoalbuminaemia.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Rickettsia

Start with the simplest possible case. Write down what Rickettsia claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Rickettsia before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Rickettsia ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Rickettsia

In research: Rickettsia appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Rickettsia in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Rickettsia is common in secondary-school and first-year university syllabi. It links to neighbouring topics Bacteria genera, Pathogenic bacteria, Rickettsiaceae, so understanding it makes those chapters shorter.
In everyday life: Look for Rickettsia outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Rickettsia in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Rickettsia means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Rickettsia out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Rickettsia in simple terms?

Rickettsia is a genus of nonmotile, gram-negative, nonspore-forming, highly pleomorphic bacteria that may occur in the forms of cocci (0.1 μm in diameter), bacilli (1–4 μm long), or threads (up to about 10 μm long). The genus was named after Howard Taylor Ricketts in honor of his pioneering work on…

Why does Rickettsia matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Rickettsia?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Rickettsia.