Gram-positive cocci

Name the three important pathogenic genera of Gram positive cocci.

Staphylococcus, Streptococcus, Enterococcus.

Describe the role of the catalase and coagulase tests in distinguishing staphylococci and streptococci 

The catalase test can be used to distinguish Staphylococcus from other Gram-positive cocci. Staphylococcus is catalase-positive, whereas Streptococcus and Enterococcus are catalase-negative.

The coagulase test can be used to distinguish Staphylococcus aureus from other types of Staphylococcus. Staphylococcus aureus is coagulase-positive, whereas most other strains of Staphylococcus (at least out of those that are involved in human disease) are coagulase-negative.

Name clinically important species within these genera and describe diseases they typically produce

Staphylococcus

One of the most important Staphylococcus species is the coagulase-positive S. aureus. It is often carried in the nasal area and can cause a range of infections, ranging from minor wound infections to serious, fatal illnesses. S. aureus is a common cause of a range of skin lesions, ranging from boils, to cellulitis (subcutaneous infection in which the skin is inflamed), to hidradenitis suppurativa (recurrent boils in the axillary region). It can cause pyogenic (pus-forming) infections in almost any organ and is also the commonest cause of osteomyelitis (bone infection) and septic arthritis (joint infection). Some strains of S. aureus can also release toxins, which can cause conditions such as Staphylococcal scalded skin syndrome (SSSS), Staphylococcal toxic shock syndrome (STSS), and Staphylococcal food poisoning.

There are also several important coagulase-negative Staphylococcus species. The most common coagulase-negative Staphylococcus is S. epidermidis, which typically lives on the skin as part of the commensal flora. It is low-virulence, but is associated with infections related to foreign devices, such as prosthetic heart valves and prosthetic joints. Its main virulence factor is an extracellular polysaccharide ("slime"), which can cause biofilms on foreign devices.

Staphylococcus saprophyticus is another common coagulase-negative Staphylococcus. It is the second-most common cause of urinary tract infections in healthy, sexually-active, young females. (The most common cause is E. coli).

Streptococcus

Streptococcus species are usually classified according to how they break down blood agar. α-haemolytic Streptococci leave a greenish tinge, whereas β-haemolytic Streptococci completely break down the blood agar, leaving a clear area. Sreptococcus can also be classified based on the Lancefield grouping system, which looks at antigenic carbohydrates in the bacterial cell wall and assigns a letter accordingly (group A, group B, etc.).

α-haemolytic Streptococci can be further broken down into species groups. The main species groups are Streptococcus bovis, Streptococcus anginosis, Streptococcus mitis, Streptococcus mutans, and Streptococcus salivarius. α-haemolytic Streptococci can get into the blood and cause infective endocarditis as well as abscesses. S. bovis is also associated with colonic malignancy. S. mutans is important in dentistry, as it can cause dental caries. S. mutans has glucosyltransferases, which converts sucrose into glucans, which are insoluble and adhere to dental enamel, forming a plaque (an example of a biofilm).

Another important α-haemolytic Streptococcus is S. pneumoniae. Around 5-10% of the population is a carrier for S. pneumoniae, which can cause pneumonia and/or meningitis. There are over 80 distinct serotypes of S. pneumoniae, some of which can be vaccinated against.

The most important Lancefield groupings for β-haemolytic Streptococci are Lancefield group A and Lancefield group B. First, we will talk about Lancefield group A β-haemolytic Streptococci, particularly S. pyogenes. S. pyogenes usually causes a primary infection with suppurative (pus-forming) or non-suppurative complications. Non-suppurative complications are usually caused by toxins or the immune response.

S. pyogenes is the most common cause of pharyngitis and tonsillitis. Suppurative complications of S. pyogenes pharyngo-tonsillitis include abscesses and mastoiditis (inflammation within the mastoid process). Non-suppurative complications include scarlet fever, rheumatic fever, and post-streptococcal glomerulonephritis, though those complications will only occur if the strain of S. pyogenes has the toxin required to cause the complication. S. pyogenes can also cause a range of skin conditions, including impetigo (school sores), erysipelas (a raised reddish-orange rash with sharp borders), and cellulitis. Some of these skin infections may have further complications, such as necrotising fasciitis ("flesh-eating disease").

Enterococcus

Enterococci are usually commensals in the human bowel, but can cause infection. Enterococci are Lancefield group D. The main species are E. faecalis and, to a lesser extent, E. faecium. They can cause a range of infections in the abdomen, urinary tract, soft tissues, and so on.