TABLE 2.

Calculated detectability of representative mollicutes, if they were responsible for infectivity in TSE-infected brain^a

Organism (description)	No. of kbp/genome (reference)	No. of genomes/pg	Fold excess
Spiroplasma citri (largest known spiroplasma)	1,820 (56)	535	207
Spiroplasma mirum	1,300 (15)	750	148
Spiroplasma platyhelix (smallest known spiroplasma)	780 (53)	1249	89
Mycoplasma arginini	610 (51)	1598	69
Mycoplasma genitalium (smallest known bacterium)	580 (45)	1680	66

^aSince 1 pg of S. mirum or M. arginini was readily detected in these studies, the number of genomes present in 1 pg was calculated for each mollicute shown, assuming an average molecular mass of 618 Daltons per bp. This was compared with 111,000, the number of agent genomes that must be present in 40 ng of DNA from scrapie-infected hamster brain, given that 1 g of brain contains 10¹⁰ ID₅₀ of scrapie infectivity and yields 2.5 mg of total DNA and assuming that one genome is responsible for one infectious dose (which corresponds to 1.44 ID₅₀). The ratio between the number of genomes in 1 pg of each molicute DNA (which, when spiked, was readily detectable by 35 cycles of PCR) and the number of genomes present in the 40-ng hamster brain DNA sample is the “fold excess”over the detection limit. This is a worst-case calculation. If the specific infectivity is less than 1 infection per organism or if the 16S rRNA gene copy number is greater than 1 (see reference 20), then the “fold excess” ratio will increase proportionately.