Academic journal article The American Biology Teacher

Beguiling Bacteria

Academic journal article The American Biology Teacher

Beguiling Bacteria

Article excerpt

I will admit that this title might be considered on the cute side, but it's too accurate to reject. To me, bacteria are enthralling. As a case in point, they weren't supposed to be the subject of this month's column. I was going through my folder of recent items of interest with the aim of focusing on some molecular rather than microbiological topic. But those bacteria kept coming to the fore, and there were just too many good items to ignore. By "good" I mean recent findings revealing new aspects to the microbial world, surprises indicating that the terrain is less well-charted than we might have thought. It's probably because I like good surprises that I've always been interested in bacteria, and my recent foray into the literature just deepened my conviction that there's nothing better to gladden the heart of biologist than a peak into this small world.

The first article to catch my eye was "Bacteria's New Bones" (Callaway, 2008). No, bacteria aren't that amazing--they don't have real bones, but rather a molecular skeleton that gives them shape. In other words, bacteria are more than just sacks of interesting molecules and can now be considered worthy of study in cell biology. A number of cytoskeletal proteins have been identified. FtsZ, a distant relative of the eukaryotic protein tubulin, creates a belt around the middle of most bacteria and cinches the dividing cell closed. Without FtsZ, rod-shaped bacilli grow longer and longer, never splitting. These bacteria also need MreB, which forms a helical pattern inside the cell wall, and probably directs the activity of wall-building enzymes. Without MreB, bacilli become spherical. For more complicated bacterial shapes, other proteins are required. The crescent-shaped Caulobacter needs crescentin or it will straighten out. Work is still being done on how spirochetes retain their spiral form; some have an internal tail or filament that gives them their twist.

These skeletal structures in bacteria are now coming to light for two reasons. One is the identification of a number of misshapen mutants. As with many biological functions, it's easier to figure them out when things go wrong. Also, the relatively new field of cryo-electron microscopy (cryo-EM) provides enhanced pictures of microbial structures. It gives three-dimensional images without the use of harsh chemicals needed in traditional EM. Among the amazing pictures generated by this technique is one of an actin-like filament, MamK, decorated with a chain of iron-containing magnetosomes in Magnetospirillum, a bacterium that can orient itself in relation to the earth's magnetic field. Cryo-EM is likely to reveal much more about bacterial "bones," features that were invisible with traditional EM. This is an important point to make to students: Biologists study what they can find to study. Often, new fields open up because of new techniques that detect the previously undetectable. It's difficult to keep this very basic limitation in mind, that is why we so frequently make the mistake of thinking that we understand the whole picture when we only comprehend what our senses and our technologies make available to us.

Bacterial Diversity

Another example of this involves bacterial diversity: Just how many different kinds of bacteria are there? If you go by the number of bacteria that have been cultured and studied in some depth, then the number is a few thousand. However, some estimate that 99.9% of microbes will not grow on nutrient rich agar, the microbiologist's medium of choice. Difficulties in growing bacteria are hardly a new problem. Even today, the bacterium that causes leprosy can only be grown in the footpads of animals like rabbits or in the armadillo but what is becoming more and more apparent is just how many species are included in that 99.9%. One reason for this greater awareness is the increasing use of genomic analysis on microbial samples (Glausiusz, 2007). Last year, ocean water samples yielded 6 million new genes and thousands of new protein families. …

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