Is chlamydia caused by bacteria or a virus?

Chlamydia. Science Picture Co./Collection Mix:Subjects/Getty Images

Question: Is chlamydia caused by bacteria or a virus?

Answer: Chlamydia trachomatis, the organism that causes both chlamydia and lymphogranuloma venereum infections, is a bacterium, but in some ways, it's one that acts more like a virus.

Most bacteria are capable of reproducing on their own as long as they're in a hospitable environment. Not chlamydia. Chlamydia is virus-like, because it is dependent on molecules from its host organism to reproduce.

It can't make copies of itself without help from the human cells it infects.

Chlamydia can not survive without humans because it needs ATP, an energy molecule, to reproduce -- as well as other nutrients and supplies that it can't make it on its own. Basically, chlamydia treats the insides of human cells as great big grocery stores, and, since the bacteria can't live without groceries, C. trachomatis is as an obligate (can't survive without) intracellular (living inside cells) parasite (where it takes but does not give back).

Chlamydia travels between cells, and between people, in the form of an elementary body, which is a small, dense, spore-like, metabolically-inactive structure, almost like a virus. This elementary body doesn't do much of anything. It's infectious but inactive.

When the elementary body enters a host cell, it changes into a reticulate body and uses supplies from the host cell to make copies of itself inside the cell.

These reticulate bodies can grow, divide, and metabolize, but they're not infectious. Infections can persist in this manner for a while, or, once there are enough copies, the reticulate bodies can turn back into elementary bodies, burst the cell open, and escape to infect new cells or new people.

In other words, Chlamydia basically has a two stage life cycle.

The elementary body is the infectious form. It's how the bacteria travel between cells and between people to create new infections. These bodies don't replicate or change. They just are carried around in bodily fluids so that, when the opportunity presents itself, they can infect things.

Then, once the elementary body infects a new cell, Chlamydia enters the second stage of its life cycle -- the reticulate body. This form of chlamydia grows and reproduces inside cells. It makes new chlamydia. Then, once there are two many bacteria to survive inside the cell, the reticulate bodies turn into elementary bodies, escape the cell, and start the process again.

This is a pretty weird lifecycle that doesn't really follow the paradigm for either bacterial infection or viral infection, which is one of the reasons that chlamydia is so interesting (and important!) to study. It's a clear example of how infectious processes don't always conform to expectations, one that encourages people to think outside the box when looking for forms of treatment, prevention, or cures.

Fun Fact: A lot of chlamydia vaccine research takes place in Koalas, which have significant problems with chlamydial infections in the wild. They are infected by a different type of chlamydia than humans, one which causes far more severe health consequences, but the research has cross-species implications

Craig AP, Hanger J, Loader J, Ellis WA, Callaghan J, Dexter C, Jones D, Beagley KW, Timms P, Wilson DP. A 5-year Chlamydia vaccination programme could reverse disease-related koala population decline: predictions from a mathematical model using field data. Vaccine. 2014 Jul 16;32(33):4163-70. doi: 10.1016/j.vaccine.2014.05.049.
A . Zomorodipour. "Obligate intracellular parasites: Rickettsia prowazekii and Chlamydia trachomatis. " FEBS Letters , Volume 452 , Issue 2 , Pages 11 - 15
Subtil A, Dautry-Varsat A."Chlamydia: five years A.G. (after genome)." Curr Opin Microbiol. 2004 Feb;7(1):85-92. Review.
L.A. Page "The Chlamydiae" in Wm. M. O'leary(Ed.), Practical Handbook of Microbiology (pp.187-194). Boca Raton, FL: CRC Press.

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