The pyogenic cocci are spherical bacteria which cause various suppurative (pus-producing) infections in animals. Included are the Gram-positive cocci Staphylococcus aureus, Streptococcus pyogenes and Streptococcus pneumoniae, and the Gram-negative cocci, Neisseria gonorrhoeae and N. meningitidis. These bacteria are leading pathogens of humans. It is estimated that they produce at least a third of all the bacterial infections of humans, including strep throat, pneumonia, food poisoning, various skin diseases and severe types of septic shock, gonorrhea and meningitis. Staphylococcus aureus is arguably the most successful of all bacterial pathogens because it has a very wide range of virulence determinants (so it can produce a wide range of infections) and it often occurs as normal flora of humans (on skin, nasal membranes and the GI tract), which ensures that it is readily transmitted from one individual to another. In terms of their phylogeny, physiology and genetics, these genera of bacteria are quite unrelated to one another. They share a common ecology, however, as parasites of humans.

Figure 15. Gallery of pyogenic cocci, Gram stains of clinical specimens (pus), L to R: Staphylococcus aureus, Streptococcus pyogenes, Streptococcus pneumoniae, Neisseria gonorrhoeae, Neisseria meningitidis. The large cells with lobed nuclei are neutrophils. Pus is the outcome of the battle between phagocytes (neutrophils) and the invading cocci. As the bacteria are ingested and killed by the neutrophils, the neutrophils eventually lyse (rupture) and release their own components, plus the digested products of bacterial cells, which are the make-up of pus. As a defense against phagocytes the staphylococci and streptococci produce toxins that kill the neutrophils before they are able to ingest the bacteria. This contributes to the pus, and therefore these bacteria are "pyogenic" during their pathogenic invasions.

Two species of Staphylococcus live in association with humans: Staphylococcus epidermidis which lives normally on the skin and mucous membranes, and Staphylococcus aureus which may occur normally at various locales, but in particular on the nasal membranes (nares). S. epidermidis is rarely a pathogen and probably benefits its host by producing acids on the skin that retard the growth of dermatophytic fungi.  Staphylococcus aureus  always has the potential to cause disease and so is considered a pathogen. Different strains of S. aureus differ in the range of diseases they can cause, including boils and pimples, wound infections, pneumonia, osteomyelitis, septicemia, food intoxication, and toxic shock syndrome. S. aureus is the leading cause of nosocomial (hospital-acquired) infections by Gram-positive bacteria. Also, it is notoriously resistant to penicillin and many other antibiotics. Recently, a strain of S. aureus has been reported that is resistant to EVERY known antibiotic in clinical usage, which is a grim reminder that the clock is ticking on the lifetime of the usefulness of current antibiotics in treatment of infectious disease.

 Streptococcus pypgenes, more specifically the Beta-hemolytic Group A Streptococci, like S. aureus, causes an array of suppurative diseases and toxinoses (diseases due to the production of a bacterial toxin), in addition to some autoimmune or allergic diseases. S. pyogenes is rarely found as normal flora (<1%), but it is the main streptococcal pathogen for man, most often causing tonsillitis or strep throat. Streptococci also invade the skin to cause localized infections and lesions, and produce toxins that cause scarlet fever and toxic shock. Sometimes, as a result of an acute streptococcal infection, anomalous immune responses are started that lead to diseases like rheumatic fever and glomerulonephritis, which are called post-streptococcal sequelae. Unlike the staphylococci, the streptococci have not developed widespread resistance to penicillin and the other beta lactam antibiotics, so that the beta lactams remain drugs of choice for the treatment of acute streptococcal infections.

Streptococcus pneumoniae is the most frequent cause of bacterial lobar pneumonia in humans. It is also a frequent cause of otitis media (infection of the middle ear) and meningitis. The bacterium colonizes the nasopharynx and from there gains access to the lung or to the eustachian tube. If the bacteria descend into the lung they can impede engulfment by alveolar macrophages if they possess a capsule which somehow prevents the engulfment process. Thus, encapsulated strains are able to invade the lung and are virulent (cause disease) and noncapsulated strains, which are readily removed by phagocytes, are nonvirulent.

The Neisseriaceae  comprise a family of Gram-negative Beta Proteobacteria with metabolic characteristics similar to pseudomonads. The neisseriae are small, Gram-negative cocci usually seen in pairs with flattened adjacent sides. Most neisseriae are normal flora or harmless commensals of mammals living on mucous membranes. In humans they are common residents of the throat and upper respiratory tract. Two species are primary pathogens of humans, Neisseria gonorrhoeae and Neisseria meningitidis, the bacterial causes of gonorrhea and meningococcal meningitis.

Neisseria gonorrhoeae is the second leading cause of sexually-transmitted disease in the U.S., causing over three million cases of gonorrhea annually. Sometimes, in females, the disease may be unrecognized or asymptomatic such that an infected mother can give birth and unknowingly transmit the bacterium to the infant during its passage through the birth canal. The bacterium is able to colonize and infect the newborn eye resulting neonatal ophthalmia, which may produce blindness. For this reason (as well as to control Chlamydia which may also be present), an antimicrobial agent is usually added to the neonate eye at the time of birth.

Neisseria meningitidis is one bacterial cause of meningitis, an inflammation of the meninges of the brain and spinal cord. Other bacteria that cause meningitis include Haemophilus influenzae, Staphylococcus aureus and Escherichia coli. Meningococcal meningitis differs from other causes in that it is often responsible for epidemics of meningitis. It occurs most often in children aged 6 to 11 months, but it also occurs in older children and in adults. Meningococcal meningitis can be a rapidly fatal disease, and untreated meningitis has a mortality rate near 50 percent. However, early intervention with antibiotics is highly effective, and with treatment most individuals recover without permanent damage to the nervous system.

Lactic acid bacteria are Gram-positive, nonsporeforming rods and cocci which produce lactic acid as a sole or major end product of fermentation. They are important in the food industry as fermentation organisms in the production of cheese, yogurt, buttermilk, sour cream, pickles, sauerkraut, sausage and other foods. Important genera are Streptococcus, Enterococcus, Lactococcus and Lactobacillus. Some species are normal flora of the human body (found in the oral cavity, GI tract and vagina); some streptococci are pathogens of humans (see pyogenic cocci above). Certain oral lactic acid bacteria are responsible for the formation of dental plaque and the initiation of dental caries (cavities). Enterococcus faecalis is a consistent member of the GI tract of humans and an important purveyor of drug resistance via horizontal gene transmission in the intestine.

Lactococcus lactis has been nominated as the "state microbe" of Wisconsin. The bacterium is one of the most important microbes involved in the dairy industry. It is non-pathogenic and critical for manufacturing dairy products like buttermilk, yogurt and cheese. When  L. lactis ssp. lactis is added to milk, the lactic acid produced by the bacterium curdles the milk that then separates to form curds, which are used to produce cheese and whey.

Lactococcus lactis is also used to prepare pickled vegetables, beer, wine, some breads and sausages and other fermented foods. Researchers anticipate that understanding the physiology and genetic make-up of this bacterium will prove invaluable for food manufacturers as well as the pharmaceutical industry, which is exploring the capacity of  L. lactis to serve as a vehicle for delivering drugs.