Staphylococci

Staphylococci are Gram-positive cocci that form clusters ( Fig. 3.2 ). They are a component of the skin microbiome and are opportunistic pathogens, in that they cause disease if given the right environment or opportunity. Staphylococcus aureus is the most frequent cause of staphylococcal sepsis. Infection results in severe disease in humans due to its ability to produce numerous virulence factors ranging from those causing tissue damage and disease spread to toxic shock from superantigens. Given its niche, S. aureus is one of the leading causes of skin and soft tissue infections. However, because of its ability to spread, S. aureus is also a common cause of bacteremia, endocarditis, and abscesses throughout the body ( Table 3.2 ).

The Gram stain shows the characteristic “bunch of grapes” appearance that gave staphylococci their name.

From Gillespie SH. Gram-positive cocci. In SH Gillespie, ed. Medical Microbiology Illustrated . Waltham, MA: Butterworth-Heinemann; 1994: 12–29, Fig. 2.1a .

Streptococci

Streptococci are Gram-positive cocci that tend to form chains rather than clusters in culture ( Fig. 3.3 ). Once a streptococcus is identified, further speciation is warranted, given the large number of streptococci and various virulence capabilities. Traditionally, laboratories have separated streptococci via two different but useful ways: hemolysis patterns and Lancefield antigen testing. When growing on a blood agar plate, streptococci can cause three different hemolysis patterns: beta (full hemolysis), alpha (partial hemolysis), and gamma (no hemolysis) ( Fig. 3.4 ). Beta-hemolytic species tend to cause fairly destructive clinical presentations, given their ability to destroy tissue, as evidenced by the hemolysis. However, alpha- and gamma-hemolytic species can cause human disease as well, and depending on the host factors, quite dramatically at times.

Streptococci form long chains of cocci, as in this preparation of S. pyogenes .

From Gillespie SH. Gram-positive cocci. In SH Gillespie, ed. Medical Microbiology Illustrated . Waltham, MA: Butterworth-Heinemann; 1994: 12–29, Fig. 2.1a .

Examples of various types of hemolysis on blood agar. (A) Streptococcus pneumoniae showing alpha-hemolysis (i.e., greening around colony). (B) Staphylococcus aureus showing beta-hemolysis (i.e., clearing around colony). (C) Enterococcus faecalis showing gamma-hemolysis (i.e., no hemolysis around colony).

From Bailey & Scott’s Diagnostic Microbiology . 14th ed. St. Louis, MO: Elsevier; 2017; Fig. 5-2 .

Along with hemolysis patterns, Lancefield antigens are used to name specific streptococci. Streptococcus pyogenes contains the “A” Lancefield antigen, and thus is also called group A streptococci (GAS). GAS are beta-hemolytic and well known as the agent that causes streptococcal pharyngitis. It can also cause severe skin and soft tissue infections (cellulitis and necrotizing fasciitis, respectively). The human immune response to GAS antigens can also be associated with disease, most commonly rheumatic fever and post-streptococcal glomerulonephritis. Group B streptococci (GBS) correspond to the Streptococcus agalactiae species, whose niche is the gastrointestinal (GI) and genitourinary tracts. Given this location, it is responsible for neonatal sepsis and infections. Mothers who test positive for GBS are treated with penicillin before delivery to prevent disease in the newborns.

Alpha-hemolytic streptococci known to cause disease include Streptococcus pneumoniae, which is typically found in the nasopharyngeal niche, and viridans group streptococci, typically found in the oropharyngeal niche. S. pneumoniae is a well-known cause of otitis media, lower respiratory tract infections, and disseminated infections such as meningitis and bacteremia. Patients without functioning spleens are at increased risk, given their inability to effectively clear this encapsulated organism ( Table 3.3 ).

Enterococci are also Gram-positive cocci that form chains. It is worth noting that though they are in a separate genus from streptococci, they do have a Lancefield carbohydrate antigen (group D). This was the reason why previously, enterococci used to be included in the Streptococcus genus as group D streptococci. However, further identification methods led to the creation of a separate genus. The Enterococci niche is the GI tract, with a special predilection for the biliary tract. Thus these organisms can cause cholangitis, but are also known for bacteremia and subacute endocarditis.

Though most Gram-positive disease is caused by cocci, Gram-positive rods are components of the human microbiome and pathogens. Important elements of the human microbiome include Cutibacterium (formerly Propionibacterium ) acnes on the skin, Actinomyces species in the mouth, and Lactobacillus species in the vagina. Gram-positive rods are more often associated with disease. Listeria monocytogenes can cause food-borne illnesses ranging from mild, in immunocompetent hosts, to severe disseminated disease, such as bacteremia and meningitis, in pregnant and immunocompromised hosts. Some toxin-producing Gram-positive organisms include Clostridium botulinum, the cause of botulism ; C. tetani, the cause of tetanus ; C. perfringens, the cause of food-borne illness and gas gangrene; and C. difficile, causing antibiotic-associated colitis. Corynebacterium diphtheriae causes diphtheria, and Bacillus anthracis causes anthrax.