In 1992, a team of scientists led by T.J. Rowbotham identified a microorganism growing in amoebae during a pneumonia outbreak investigation in Bradford , England . Attempting to isolate Legionella-like Amoebal Pathogens (LLAP) involved in the outbreak, the investigators cultured amoebae growing in the water of a hospital cooling tower and found what they believed to be a gram-positive bacterium which they first called " Bradford coccus." Nevertheless, attempts at extracting the amoebae-associated bacteria using universal 16S ribosomal RNA (rRNA) PCR amplification and sequencing failed, challenging the microbe's hypothesized status as a bacterium.
Observation of the microorganism within infected Acanthamoebae polyphaga under electron microscope revealed mature intra-cytoplasmic particles with a characteristic icosahedral viral morphology and a diameter of 400 nm. When the observations were reproduced using the cell-free supernatant of an infected A. polyphaga culture, the microbe was a declared the largest known human virus. Because of its resemblance to bacteria upon Gram staining, the virus was called mimivirus (for Mimicking Microbe) and was later given its own viral family due to its distinction from all other viruses.
Pictures: Figure 1 (A and B) in Suzan-Monti et al (2005)