The Microbe Blog (at http://www.smallthingsconsidered.us)

Elio Schaechter and Merry Youle, fellow writer and editor

Coxiella Escapes from Cell!
http://schaechter.asmblog.org/schaechter/2009/04/coxiella-escapesfrom-cell.html

Surprise! Coxiella burnetii, the agent of Q fever, has been grown in cell-free media. This is news because, along with rickettsiae and a few others, C. burnetii is the stereotypical obligate intracellular bacterial parasite. Until now, coxiellae could only be grown within a host or in host cells in culture. So how did researchers at the NIH's Rocky Mountain Laboratories come up with a complex-but otherwise fairly ordinary-laboratory medium that supports their growth? It wasn't just trial and error, as you might suppose. The researchers arrived at the right conditions by evaluating the metabolic skills of the organism (but usually without overtly relying on its genome annotation). It's interesting that this accomplishment came in stages. Before finding the medium that supports growth of C. burnetii, they had developed one called CCM (Complex Coxiella Medium) that supports metabolic activities of the organism for 24 hours. Their next step was to improve on it.

Several observations suggested that C. burnetii likes a low-oxygen environment. Thus, it has genes for cytochrome bd, which has a high affinity for oxygen and is found in other intracellular bacteria, such as the tubercle bacillus, chlamydiae, and rickettsiae. The cellular environment where such organisms grow-a lysosome-like vesicle-is low in oxygen. The upshot? Give them 2.5% oxygen. Those intracellular vacuoles are also acidic, so give them a pH of 4.5.

Earlier, the researchers had noticed that ribosomal gene expression was reduced in C. burnetii in CCM, but apparently not for lack of energy. Perhaps lack of amino acids? It turned out that including casamino acids and a high concentration of cysteine in the medium did the trick. Presto, the bacteria now grow at a rate nearly that of intracellular growth (doubling every 9 hours). Perhaps the most telling finding is that these cells are fully infectious for Vero cells in culture. Moreover, in the cell-free medium, coxiellae undergo a characteristic developmental transition from nonreplicative small forms to large replicating ones.

The genome of one strain of C. burnetii has been sequenced. With nearly 2 Mb, its genome is large compared to other "obligate" intracellular parasites, which generally underwent greater genome reduction. Although the C. burnetii genome encodes a lot of metabolic and transport genes, more than in most obligate intracellular bacteria, one cannot immediately deduce from this that it could be free living. Still, it did suggest to the authors of the genome paper that its "obligate intracellular lifestyle" may be of recent origin and that genome reduction may be in progress. If so, the authors of the growth paper may have been lucky in choosing an intracellular bug that happens to still remember how to grow by itself.

This bacterium is the agent of a somewhat rare disease called Q fever. (The Q is for "query," even though Queensland is where it was first described.). Q fever is a debilitating disease, something like a bad flu, which affects both humans and domestic animals. It's highly contagious and highly infective-perhaps a single cell can cause disease. It is classified as a class B bioterrorism agent that "has been weaponized and mass-produced under various biological warfare programs." That may be easier to do now that you can grow it with relative ease.

Omsland, A., D. C. Cockrell, D. Howe, E. R. Fischer, K. Virtaneva, D. E. Sturdevant, S. F. Porcella, and R. A. Heinzen. 2009. Host cell-free growth of the Q fever bacterium Coxiella burnetii. Proc. Natl. Acad. Sci. USA 106:4430-4434.


Talmudic Question of the Month*

Some believe that the extent of anaerobic respiration on Earth is usually underestimated. What is your guess for the proportion of biogenic CO₂ that is made by this mechanism?

Answers? Add a comment online to Talmudic Question #54, October 15, 2009.

http://schaechter.asmblog.org/schaechter/2009/10/talmudic-question-54.html

*We use this term to denote questions whose answers cannot be found by a Google search.