A Bacterium in Disguise
In 1992, microbiologists were investigating a pneumonia outbreak in Bradford, England. They traced the source to a water cooling tower, a known reservoir for bacteria. Inside samples of an amoeba, Acanthamoeba polyphaga, they observed what appeared to be a large, Gram-positive bacterium. They named it Bradfordcoccus and, after initial studies, stored it away in a freezer.
For over a decade, the sample remained misidentified. It wasn't until 2003 that researchers in Marseille, France, re-examined the organism with an electron microscope. They discovered its true identity: not a bacterium, but a virus of unprecedented scale. Its icosahedral (20-sided) structure confirmed it was a virus, and they renamed it Acanthamoeba polyphaga mimivirus (APMV), or "mimivirus," for its ability to resemble a microbe.
A Giant Revealed
The Mimivirus is physically enormous by viral standards. Its central protein shell, or capsid, is about 500 nanometers in diameter. This is surrounded by a dense layer of 125-nanometer-long protein fibers, bringing the virus's total diameter to around 750 nanometers. This makes it larger than some of the smallest bacteria, like Mycoplasma genitalium, which measures only about 200-300 nanometers across.
The virus's genetic makeup is equally massive. Its double-stranded DNA genome contains approximately 1.2 million base pairs and codes for an estimated 979 to 1,262 proteins. For comparison, the influenza virus has only 8 genes. Uniquely for a virus, the mimivirus genome includes genes for producing aminoacyl-tRNA synthetases, enzymes that are essential for translating genetic code into proteins. Viruses are typically defined by their reliance on the host cell's machinery for such tasks. Mimivirus's partial autonomy blurs the line between viruses and cellular life. Its replication occurs entirely within the host's cytoplasm, inside a complex structure called a "virus factory".