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Chapter 1 Humans and the Microbial World

PERSPECTIVE 1.1 The Long and the Short of It

We might assume that because prokaryotes have been so intensively studied over the past hundred years, no major surprises are left to be discovered.This, however, is far from the truth. In the mid-1990s, a large, peculiar-looking organism was seen when the intestinal tracts of sturgeon fish from both the Red Sea in the Middle East and the Great Barrier Reef in Australia were examined.This organism, named Epulopiscium, cannot be cultured in the laboratory (figure 1). Its large size, 600 mm long and 80 mm wide, which makes it clearly visible without any magnification, suggested that this organism was a eukaryote. It did not, however, have a nuclear membrane. A chemical analysis of the cell confirmed that it was a prokaryote and a member of the Domain Bacteria. This very long, slender organism is an exception to the rule that prokaryotes are always smaller than eukaryotes.

In 1999, an even larger prokaryote in volume was isolated from the sulfurous muck of the ocean floor off the coast of Namibia in Africa. It is a spherical organism 70 times as big in volume as Epulopiscium. Since it grows on sulfur compounds and contains glistening globules of sulfur, it was named Thiomargarita namibiensis, which means "sulfur pearl of Namibia" (figure 2). Although scientists were initially skeptical that prokaryotes could be so large, there is no question in their minds now. In contrast to these large bacteria, a cell was isolated in the Mediterranean Sea that is 1 mm in width. It is a eukaryote because it contains a nucleus and a mitochondrion, even though it is about the size of a typical bacterium. ■ Thiomargarita, p. 270

On the other side of the coin, investigators are asking how small can a living organism be. Finnish scientists claim that they have discovered a new form of life, the nanobacteria, which are roughly the size of viruses, about 50 to 500 nanometers (nm) in diameter (see figure 1.13). These nanobacteria are coated with thick shells of minerals, which render them very resistant to heat and chemicals. There is no good evidence that the nanobacteria are living, however, and their apparent multiplication could result from crystal formation.

More recently, Australian investigators have observed fuzzy tangles of filaments, 20 to 150 nm in length, projecting from sandstone retrieved from an oil-drilling site in the ocean off western Australia (figure 3). They appear to be another form of nanobacteria.The Australian researchers reported that the filaments appear to reproduce quickly and form colonies, suggesting that they are alive.

Most scientists are very skeptical that "organisms" of such a small size can live independent lives.The molecules required for life such as nucleic acids and the protein-synthesizing machinery cannot be packed into "organisms" of this small size. If they are not free-living, might they be part of a larger biological entity? Might they have a biological function? Although there is great skepticism about the biological significance of these extraordinarily small "organisms," most microbiologists are convinced that many unusual microbes exist in nature that have not yet been discovered. Are nanobacteria one example?

Figure 1 Longest known bacterium, Epulopiscium, mixed with paramecia Note how large this prokaryote is compared with the four eukaryotic paramecia.

Figure 3 Filaments projecting from sandstone

The filaments, 20 to 150 nanometers in size, are smaller than cells.

Figure 2 Thiomargarita namibiensis The average Thiomargarita namibiensis is two-tenths of a millimeter, but some reach three times that size.

Figure 1 Longest known bacterium, Epulopiscium, mixed with paramecia Note how large this prokaryote is compared with the four eukaryotic paramecia.

Figure 2 Thiomargarita namibiensis The average Thiomargarita namibiensis is two-tenths of a millimeter, but some reach three times that size.

Figure 3 Filaments projecting from sandstone

The filaments, 20 to 150 nanometers in size, are smaller than cells.

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