Mycoplasma is a tiny genus of bacteria which is especially interesting, as they lack a cell wall around their cell membrane. They are considered pathogens in humans and animals. Their evolution to a parasitic lifestyle has led to a dramatic reduction of their genome. Mycoplasma have the smallest genome of all bacteria, some having as few as 580.000 base pairs (in comparison, a human genome has about 3 billion). The small size of their genome have made mycoplasma one of the most interesting objects of study in order to understand how life works. Mycoplasma was also the first bacteria, the genome of which has been entirely synthesized (Gibson et al. 2010). The fact that mycoplasma is both pathogenic and genetically of such small size makes it a prime candidate for engineering its genome to create a new type of vaccine.
Source: Gibson, D. G.; Glass, J. I.; Lartigue, C.; Noskov, V. N.; Chuang, R.-Y.; Algire, M. A.; Benders, G. A.; Montague, M. G.; Ma, L.; Moodie, M. M.; Merryman, C.; Vashee, S.; Krishnakumar, R.; Assad-Garcia, N.; Andrews-Pfannkoch, C.; Denisova, E. A.; Young, L.; Qi, Z.-Q.; Segall-Shapiro, T. H.; Calvey, C. H.; Parmar, P. P.; Hutchison, C. A.; Smith, H. O.; Venter, J. C. (20 May 2010). "Creation of a Bacterial Cell Controlled by a Chemically Synthesized Genome". Science. 329 (5987): 52–56. doi:10.1126/science.1190719. PMID 20488990.