The smallest of the gram positive Eubacteria are the mycoplasma. The smallest mycoplasma are as small as 50-70 nanometers in diameter. Mycoplasma are responsible for a form of pneumonia (mycoplasmal pneumonia, distinct in origin from viral or other bacterial pneumonia), some sexually transmitted diseases, and "lethal yellowing", a disease of coconut palms. Since they are often parasitic organisms, they can afford to jettison most of thier metabolism, and simply feed off the host. This allows them to be much smaller, since they do not need to synthesise amino acids, but simply pump them across the cell wall.

Using the scale from the largest known bacterial cell, this is an ant to the normal bacteria's mouse - 100 times smaller (10 micrometers = 10,000 nm; 10,000 nm / 100 nm = 100 fold). That is assuming a mouse is 10 cm and an ant 1mm (damn, that's a small ant!). Sooooo, we're talking about a scale range of Blue Whale (for T.namibiensis) to tiny ant (for Mycoplasma). Strangely similar to macroscopic life, therefore..

Mycoplasmas are very tiny microorganisms (usually spherical with a diameter of .3 to .8 micrometers, the smallest cells known) which are able to slip through most filters and are therefore often found as contaminating organisms in cultures, vaccines and other supposedly sterile preparations.

Some types of mycoplasmas cause pneumonia. Mycoplasmal contamination is an important problem in biotechnology, but fortunately the microbes can usually be killed with antibiotics.

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From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.