Sunday, March 27, 2011

Bacteria on the move

Have you ever seen a time-lapse video of people on a city block?  New York's Times Square, maybe, or the Embarcadero in San Francisco.  It's amazing to watch people weave in and out, following their individual paths.  This might surprise you, but the microscopic world often resembles that busy city block!  Many bacteria are motile, which means that they can propel themselves in a particular direction and even change directions.  Motility is very important to bacteria...and sometimes bacterial motility has serious consequences for us.

In the video below (courtesy of the wonderful MicrobiologyBytes Video Library), you can see an incredible range of motile bacteria.  

Notice the little elliptical cells, such as those shown at the beginning of the video. Although you can't see it in the video, these cells have one or more whip-like structures called flagella protruding from their cell surface.  Here's an image of a bacterial cell and its flagella.

Sunday, March 6, 2011

Matters of size

We're used to thinking about bacteria as tiny invisible creatures, and for the most part that's true. However, the diversity of the bacterial world holds a bit of a surprise for us. There are giants among the some cases, they are the size of the period at the end of this sentence. One such giant microbe was discovered off the coast of Namibia in southwestern Africa. It was named Thiomargarita namibiensis...this Latin name means "Sulfur pearl of Namibia". How big is this bacterium, and how did it get its beautiful name?

Let's talk about matters of size first. E. coli is a common bacterium used in laboratory studies, and it is a member of the microbial community in the human gut (yes, you are carrying E. coli around with you every day). A typical E. coli cell is about 1 micrometer (or micron) wide and 2 micrometers long. There are 1000 micrometers in one millimeter. Now, this is where I admit that I was never very good at visualizing the metric system without a reference of some sort. So, for comparison, a strand of human hair is about 100 micrometers thick. Below, I've drawn a comparison of the size of an E. coli cell and the size of a human cell. Human cells actually vary a reasonable amount in their size, but this is a fairly typical representative.

Not to tangent, but this size difference explains why your body can contain more bacterial cells than human cells (a point made in my introductory post). Now, let's bring out the giant! Below, I've changed the scale (compare the two bars in the top and the bottom drawings) to compare the same human cell to a Thiomargarita namibiensis cell. Look at this whopper!

It's huge! To make this point even more clearly, here is an incredible picture from the original article published about this bacterium (Schulz et al. Science 1999). The white glob at the top left of the picture is a Thiomargarita namibiensis cell. A single cell! For comparison, it is laid out next to a fruit fly.

That is one big cell. Cells of Thiomargarita range from as "small" as 100 micrometers in diameter to upwards of 750 micrometers in diameter. How can this be possible? It turns out that most of the interior of a Thiomargarita cell is occupied by a large sac called a vacuole where the bacteria store nitrate. Thiomargarita uses the nitrate along with sulfur from its environment to produce energy. Nitrate can sometimes be rare in their environment, so the bacteria have evolved a structure to hoard enough nitrate to ensure their survival.

So...our giant microbe gets its name from "Thio" = containing sulfur, "margarita" = pearl in Spanish, "namibiensis" = from Namibia....Thiomargarita namibiensis = "Sulfur pearl of Namibia". Looking at the picture with the fruit fly, I can see how this giant microbe looks like a pearl!

Primary literature: