Of the bacterial group of gram-positive cocci comprising the genera Streptococcus, Enterococcus, and Peptostreptococcus, streptococci constitute the most significant pathogens of swine. Streptococci are also associated with infectious conditions of people, cattle, sheep, goats, and horses. Relative to pigs, S suis (an alpha-hemolytic Streptococcus) is by far the most important agent of infectious diseases in this group, affecting mainly nursing and recently weaned pigs. Septicemia, meningitis, polyserositis, polyarthritis, and bronchopneumonia are associated with S suis infections.
Streptococcus dysgalactiae equisimilis is considered the most important beta-hemolytic Streptococcus involved in lesions in pigs, and it has been judged to be of etiologic significance in autopsy reports. S porcinus, another beta-hemolytic Streptococcus, has been associated particularly in the USA with a contagious clinical entity in growing pigs known as streptococcal lymphadenitis, jowl abscesses, or cervical abscesses. Enterococci reside in the intestinal tract and may cause disease in multiple species. In pigs, the E faecium species group, mainly E durans and E hirae, are especially associated with enteritis and diarrhea.
Other streptococci have occasionally been isolated from diseased pigs, such as S pluranimalium, S parcorum, S hyovaginalis, S gallolyticus gallolyticus, S plurextorum, and S porci. So far, there are no clear data about the habitat and/or virulence properties of these streptococcal species. In addition, S parasuis (which includes previous S suis serotypes 20, 22,and 26) and S orisratti (previous S suis serotypes 32 and 24) are sometimes isolated from diseased pigs.
Enterococci are known as part of the intestinal flora, but some strains can extensively colonize the mucosal surface of the small intestine. Some enterococcal species that show typical adhesion to the apical surface of the enterocytes of the small intestine of young animals have been described as associated with diarrhea in different species, including piglets 2–20 days old. Taxonomic studies have shown that most of these enterococci are members of the E faecium species group, mainly E durans and E hirae. Enterococci are usually considered a reservoir for antimicrobial resistance genes.