Pus often results from an infection and is usually made up of dead white blood cells, debris from other damaged cells and tissue, and bacteria. Cells called neutrophils can kill the invaders, but are often killed in the process; bacteria in pus are therefore sometimes called pyogenic. These are often cocci, which are spherical organisms with thick cell walls, and include various Staphylococcus and Streptococcus varieties. Staphylococcus epidermidis bacteria typically live on the skin and sometimes prevent fungal infections. These rarely cause disease, but the aureus variety is often found in wounds, boils, and pimples.
The bacteria in pus can be organisms that cause strep throat and tonsillitis. Streptococci can be seen in many skin infections. They are generally capable of causing suppurative infections, which are those that lead to the formation of pus. Some infections that trigger this reaction can lead to diseases like rheumatic fever, or bacteria can release toxins that cause shock or scarlet fever. Whether a bacterium is disease-causing or harmless depends on its genetics, chemistry, and structural composition.
Streptococcus pneumoniae is the organism that usually triggers bacterial pneumonia, and can also be present in middle ear infections. These bacteria in pus sometimes have an outer shell that prevents the immune system cells from engulfing and killing them. Encapsulated forms can make someone sick, but when a bacterium is non-encapsulated, it can usually be removed by blood cells. This type can be called non-virulent because it doesn't usually cause disease.
Other kinds of bacteria in pus include Neisseriae, which can trigger meningitis and gonorrhea. Many of these types live normally in mucus membranes of the body, but a couple of varieties can be dangerous and even lethal. The color of pus can vary from yellow to whitish-yellow, to yellow-brown depending on the type of bacteria and other cellular material present. Some bacteria have pigments that cause pus to be blue-green, and some white blood cells can release proteins that kill organisms while adding a distinct green color.
The bacteria in pus are typically pathogens. Pus can form on the skin as well as inside the body. By scoping the inside of the abdomen, surgeons can view infections of organs such as the liver or pancreas. Other bacteria can infect the urinary tract or even the lungs. In general, bacteria can infect a person through any cut in the skin, and pus often interferes with the healing process; shock, gangrene, or lymph node infections are sometimes the result.