<prompt> The first pathogenic bacterium to be observed under microscope was-Vibrio choleraBacillus anthracisStaphylococcus aureusStreptococcus pyogenes  <response>: The first pathogenic bacterium to be observed under microscope was Bacillus anthracis. Bacillus anthracis: First pathogenic bacterium to be observed under microscope. First bacterium isolated in pure culture. First communicable disease shown to transmit by inoculation of infected blood. First used for preparation of attenuated vaccine- Pasteurs vaccine-nontoxigenic strain. Recently used for biological warfare as spores enclosed in envelopes. 