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Types of Round-Shaped Bacteria

Bacteria are the simplest life forms. They can come in many shapes, from spheres to spirals and rods. They reproduce by fission, or splitting in two and can pass through many generations in a short amount of time. Spherical bacteria sometimes come in pairs or in clumps or strings. Spherical bacteria usually have the suffix "cocci" at the end of their name.

Streptococcus
- Streptococcus pyogenes is 0.6 to 1 micrometer in diameter. Like most streptococci it's Gram positive, which means it stains violet or blue when a Gram stain is applied to it. Gram stains are useful in determining the kind of bacteria. S. pyogenes occurs in pairs or in chains. It's anaerobic, which means it functions without oxygen. It doesn't form spores. S. pyogenes is the bacteria that, when invasive, causes scarlet fever, pharyngitis, rheumatic fever and impetigo. Streptococcus pneumoniae is also a Gram positive bacteria that causes pneumonia and meningitis, but it's a normal part of the flora in the respiratory tract until it becomes invasive.
Staphylococcus
- Staphylococcus aureus occurs in clusters that resemble grapes. It's normal on the skin and in the noses of humans. There are over 20 species of staphylococcus but only aureus and epidermidis are significant. They are about 1 micrometer in diameter and can cause toxic shock syndrome, food poisoning and suppurating diseases.
Lactococcus
- Lactococcus lactis is Wisconsin's state microbe. It's 1.2 to 1.5 micrometers in diameter. It's also found in chains or pairs. It's Gram positive, doesn't move much, and doesn't form spores. It's formed on plant material, usually grasses eaten by cows and easily gets into the cow's milk. Lactococcus is most likely the bacteria that causes milk to sour. But two lactococcus species lactis and cremoris are used to make cheese and other fermented milk products.
Others
- Sarcina is another Gram positive bacteria in the Clostridiaceae family. It's found in the gut, and on the skin in such great numbers that it can't be completely removed, even after repeated washings. Thiomargarita namibiensis is the largest bacteria ever found. It has a diameter of 100 to 750 micrometers, which means it can be seen with the naked eye -- it's about the size of a 12 pt. period. The cell contains sulfur and glows when a light is shone on it. It was found off the coast of Namibia, and so it's name means sulfur pearl of Namibia. The large size is due to a fluid-filled sack that takes up 98 percent of the bacteria's interior and allows it to store nitrates that are used to oxidize sulfur for energy.

Nature