Determination of the mass of Mycobacterium leprae by electron microscopy

The mass of Mycobacterium leprae, obtained as a pure suspension from tissues of infected armadillos, was measured electron microscopically using a technique that avoids the need for standards of mass. The mean mass of an individual bacterium was 3-9+/-1-0 (S.D.) X 10(-14) g. Comparative measurements were also made on a small sample of M. lepraemurium (whose mass is known). Calculation of the mass of an individual bacterium allows numbers of bacteria in samples to be estimated by direct weighing rather than by counting.