Which microbe is smallest?

The smallest known microorganisms are typically bacteria or archaea. Among them, the group of bacteria known as "Mycoplasma" species are some of the smallest known cells. Mycoplasmas lack a cell wall, which contributes to their small size. One example is Mycoplasma genitalium, which has one of the smallest genomes of any free-living organism and is considered one of the smallest known bacteria.

Viruses, while often referred to as microorganisms, are not considered cells and are even smaller than bacteria. Viruses are composed of genetic material (DNA or RNA) surrounded by a protein coat and sometimes an envelope. They can be incredibly tiny, with sizes ranging from about 20 to 300 nanometers (nm), making them smaller than most bacteria.

It's important to note that sizes can vary within these categories, and new discoveries might change our understanding of the smallest microbes.