Streptococci Flashcards
Characteristics of Streptococci
- facultative anaerobes
- Catalase negative
- normal members of body flora
- non motile
- non spore forming
Growth medium of streptococci
Blood agar
Hemolysis on blood agar
Alpha hemolytic -> incomplete hemolysis (oxidizes haemoglobin to biliverdin) -> greenish discoloration
Beta hemolytic -> complete hemolysis -> clear zone (no RBC, we only see the agar)
Gamma (non) hemolysis -> some bacteria grow, but no change in RBC
Structure of Streptococcal cell from inside to outside
- cytoplasmic membrane
- peptidoglycan
- group carbohydrate
- protein antigens
- capsule
Serologic classification is based on
Antigenic differences in cell wall carbohydrates
(-> in beta hemolytic streptococci)
Which streptococci has no group specific antigen?
Viridans streptococci & s. Pneumonia
Who is the most important human pathogen in Group A Streptococci?
S. Pyogenes
Susceptibility of s. Pyogenes to
Bacitracin
S. Pyogenes causes disease by three mechanisms
- pyogenic inflammation
- exotoxin production
- immunologic
Name the virulence factors of s. Pyogenes
- capsular polysaccharide -> hyaluronic acid
- M Protein -> inhibits alternate C pathway, opsonization, phagocytosis
- Lipoteichoic acid -> cytotoxic
- protein F -> binds fibronectin (attachment)
- protein G -> binds Fc portion of ABs
Enzymes in S. pyogenes
- streptokinase (digests fibrin clots)
- nuclease
- hyaluronidase
- proteinase
Exotoxins of s. Pyogenes
Streptolysins
Streptococcal pyrogenic exotoxins
Suppurative diseases of S. pyogenes
- pharyngitis
- scarlet fever (strawberry tongue)
- cutaneous & soft tissue infection (Impetigo, Erysipelas, necrotizing fascitis)
- systemic diseases (Toxic schock syndrome)
S. Pyogenes appearance on blood agar
Small colony, large zone of hemolysis
Member of group B Streptococci
S. Agalactiae
How does S. agalactiae reacts to bacitracin?
Resistant
Virulence factors of S. agalactiae
- polysaccharide capsule
- beta hemolysin
- C5a peptidase
Which test is used to detect s. Agalactiae?
CAMP test
Which test is done?
CAMP test -> s. Agalactiae
Infections of S. agalactiae common in ?
Pregnant & newborn infants or adults
-> pneumonia, sepsis, meningitis
Hippurase test is positive in which bacteria?
S. Agalactiae
Group D streptococci member
S. Bovis
S. Bovis can cause
Bacteremia
Abdominal abscesses (associated with carcinoma of colon)
Wound infections
Group D enterococci
Enterococcus faecalis
Lab identification of Enterococci
Black colonies on bile esculin agar
Enterococci treatment
Intrinsic resistance to cephalosporins
Inhibited but not killed by penicillin -> combined with aminoglycosides for invasive infections
Characteristics of S. pneumonia
- diplococci
- encapsulated
Virulence factors of S. pneumonia
- capsule
- pneumolysin O
- leukocidin
- neuraminidase
- hyaluronidase
- IgA protease
S. Pneumoniae infections
Pneumonia
Meningitis
Otitis media
How to differentiate S pneumonia from other alpha hemolytic streptococci?
S pneumoniae is Optochin sensitive
Optochin test
