B4M1C3: Cardiovascular System Flashcards
MORPHOLOGY OF STREPTOCOCCUS PYOGENES (GROUP A Β-HEMOLYTIC STREP)
- spherical to ovoid organisms
- 0.5 to 1.0 μm in diameter
- gram-positive cocci that grows in pairs or chains
What are the cultural characteristics of S. pyogenes (GABHS)?
Preferred culture media for primary isolation of Group A streptococci contains blood or blood products.
○ Sheep blood agar is preferred.
○ The optimal pH for growth is 7.4 to 7.6 at 37 C.
● Most group A streptococci are β-hemolytic on sheep blood agar.
○ Hemolysis is enhanced under anaerobic conditions, so it is recommended that the agar be slashed by the loop at the primary site of inoculation to ensure subsurface growth.
● Streptococci are gram positive and catalase negative.
After 18 to 24 hours of growth on agar, S. pyogenes colonies are:
0.5mm in diameter
doomed, grayish to opalescent, and surrounded by a zone of β-hemolysis several times greater than the diameter of the colony
Group A streptococci is distinguished from β-hemolytic streptococci by a variety of techniques such as
Lancefield extraction and precipitation, fluorescent antibody or coagglutination
ANTIGENIC STRUCTURES AND VIRULENCE FACTORS:
○ C-polysaccharide
○ Proteins
○ M protein
What is specific to the species, and is composed of a branched polymer of L-rhamnose and N-acetyl-D-glucosamine in 2:1 ratio?
C-polysaccharide
What is the antigenic determinant of C-polysaccharide?
N-acetyl-D-glucosamine
The polysaccharide is linked by the phosphate-containing bridges to the peptidoglycan, which consists of:
● N-acetyl-D-glucosamine
● N-acetyl-D-muramic acid
● D-glutamic acid
● L-lysine
● D- and L-alanine.
Group A streptococci produce two major classes of protein antigens, the _________, that are responsible for type specificity in the group.
M and T antigens
○ Both antigens are sufficiently stable and immunologically distinct to provide useful serologic methods of typing. More than 90% of all group A strains may be typed by use of these antigens.
What are resistant to heat and acid but are destroyed by trypsin?
M antigens
What are heat and acid stable but are resistant to trypsin?
T antigens
What is performed by capillary tube precipitin tests using hydrochloric acid extract to harvested cells as antigens against absorbed rabbit-type specific hyperimmune sera?
Routine M typing
For expression of the M protein, organisms should be grown on media containing what?
Peptides
To avoid destruction of the M protein by proteinase activity, the pH should not be allowed to fall below
6.5
What is an antiphagocytic fibrillar molecule located on the surface of group A organisms?
Streptococcal M protein
PATHOGENESIS
● The different species of streptococci produce many virulence factors and toxins.
● Many streptococci, including S. pyogenes also express M protein, that resist phagocytosis. S. pyogenes also expresses M protein, a surface protein that prevents bacteria from being phagocytosed, and a complement C5a peptidase, which degrades this chemotactic peptide.
● Poststreptococcal acute rheumatic fever is probably an autoimmune disease caused by antistreptococcal M protein antibodies that cross-react with cardiac myosin.
● Virulent S. pyogenes have been referred to as flesh-eating bacteria because they cause rapidly progressive necrotizing fasciitis.
● Pneumolysin is a cytosolic bacterial protein released on disruption of S. pneumoniae.
○ Pneumolysin inserts into target cell membranes and lyses them, greatly increasing tissue damage.
○ This toxin also activates the classical pathway of complement, reducing the complement available for opsonization of bacteria.
● Streptococci secrete a phage-encoded pyrogenic exotoxin that causes fever and rash in scarlet fever.
● S. mutans produces caries by metabolizing sucrose to lactic acid (which causes demineralization of tooth enamel) and by secreting high-molecular-weight glucans that promote aggregation of bacteria and plaque formation.
What can resist phagocytosis?
M protein
What is a surface protein that prevents bacteria from being phagocytosed that also can be expressed by S. pyogenes?
M protein
degrades this chemotactic peptide
complement C5a peptidase
What is probably an autoimmune disease caused by antistreptococcal M protein antibodies that cross-react with cardiac myosin?
Poststreptococcal acute rheumatic fever
Virulent S. pyogenes have been referred to as flesh-eating bacteria because they cause
rapidly progressive necrotizing fasciitis
What is a cytosolic bacterial protein released on disruption of S. pneumoniae?
Pneumolysin
This inserts into target cell membranes and lyses them, greatly increasing tissue damage.
○ This toxin also activates the classical pathway of complement, reducing the complement available for opsonization of bacteria.
Pneumolysin
What does streptococci secrete that causes fever and rash in scarlet fever?
phage-encoded pyrogenic exotoxin