Corynebacterium diphtheriae causes diphtheria. The toxin is the product of a latent bacteriophage in the bacterium. It catalyzes the inactivation of a factor necessary for amino acids to be added to the polypeptide chain being synthesized on the ribosome. Treatment of the toxin with formaldehyde converts it into a harmless toxoid . Both these organisms have had their complete genomes sequenced.
Some mycoplasmas have the distinction of being the smallest living organisms.
Mycoplasma pneumoniae has 816,394 base pairs of DNA encoding 679 genes. Rickettsias Rickettsias are also too small to be clearly seen under the light microscope. Almost all are obligate intracellular parasites. Rickettsia prowazekii causes typhus fever when it is transmitted to humans by lice. Rocky Mountain spotted fever is a rickettsial disease transmitted by ticks.