Streptococci (1/9) Flashcards
What are streptococci/enterococci? What type of pathogens?
Pyogenic pathogens: nonmotile, catalase negative, gram positive cocci in chains
Enterococci used to be steptococci but based on DNA studies were classified as separate
What are the 6 species of streptococci? What are the sites of colonization/infection
What is the hemolytic classificaiton system for streptococci?
Beta hemolytic: complete = S. pyogenes
Alpha hemolytic: incomplete = viridans streptococci
Gamma hemolytic: no hemolysis = enterococci
What are the structural components of Group A Streptococcus? (GAS)
Structural components: hyaluronic acid capsule that interferes with phagocytosis. Cell wall with antigenic molecules/surface proteins that atach to ECM on host
Expresses M protein, virulence factor
Secretes enzymes including the one responsible from hemolysis- streptolysins
Secrete pyrogenic exotoxins that can also cause TSS & Scarlet fever
What is the importance of M protein?
It’s the major virulence factor of GAS - interferes with phagocytosis & promote colonization
Strains wtihout M protein are avirulent
M protein = target for vaccines
Also used to classify GAS based on variation in structure and sequence
What diseases can GAS cause?
Infections: pharyngitis (most common), impetigo, erysipelas, puerperal infections, sepsis, myonecrosis
Toxin mediated: scarlet fever, streptococcal TSS
Post-streptococal diseases: glomerulonephritis, rheumatic fever (carditis, polyarthritis, subcuteneous nodules, chorea, erythema marginatum- pink rings on trunk; Jones criteria diagnose)
How would you diagnose GAS pharyngitis?
Clinical presentation: fever, lymphadenopathy, swollen/red tonsils with exudate
Difficult to distinguish from pharyngitis due to other pathogens (i.e. viral, group C strep) & is a small % of cases for pharyngitis seen by physicians
There are findings suggestive of GpA strep: sore throat, sudden fever, pain with swallowing, headache, lymphadenitis, tonsillar exudates, soft palate petechiae. But not all of these symptoms will be present!!
Not suggestive of GpA strep: conjunctivitis, coryza, cough, diarrhea
Obtaining a culture is the next step in diagnosis: gold standard
Rapid streptococcal antigen detective kit: highly specific
Streptococcal antibody test: ASO to diagnose recurrent infections
What is the natural reservoir for GAS? How is it spread?
Humans = natural reservoir
Asymptomatic nasal carriage in nares, pharynx (and rectum) is common
Spread = through droplets in sneezing/coughing, or direct contact with secretions
Minor trauma to colonized skin –> skin/soft tissue infections
What factors lead to pathogenesis of GAS?
Strains rich in M protein & hyaluronate = easier to transmit
Streptococci adhere to epithelial cells using adhesins
Colonization of oropharynx involves formation of bacterial aggreggates/micro-colonies
Type specific antibody to M protein determins susceptibility to infection
What is streptococcal pyoderma?
Pyoderma = pyogenic skin condition (pus)
Caused by cutaneous colonization of streptococcus –> autoinoculation at sites of injury
Strains differ from those that cause pharyngitis
Complications are rare: lymphadenitis, immune-complex glomerulonephritis
How is streptococcal TSS different from S. aureus TSS?
Differs from S. aureus TSS because of the frequent presence of infection, which is not common in S. aureus
Necrotizing fasciitis linked with specific M types
How do you treat S. pyogenes infection?
Sensitive to penicillin
In invasive infections, helps to use antibiotics that are protein synthesis inhibitors (i.e. clindamycin)
Soft tissue infection - surgical debridement
IV immunoglobin can help
Prophylactic antibiotics
Vaccines- being researched
What is a classical clinical presentation of streptococcus pneumoniae?
Sudden onset of fever with peluritic chest pain and cough
Rusty sputum with gram stain of neutrophil diplococci
X ray lobar pneumonia
Smoking is a big risk factor
What are characteristic features of S. pneumoniae?
Gram positive, lancet-shaped diplococci
Alpha hemolytic colonies on blood agar plate
Encapsulated - covalently bound to peptidoglycan; 90 serotypes, basis for type-specific immunity
Teichoic acid containing phosphorylcholine C - polysaccharide = unique to pneumococci
Who tends to get pneumococcal disease? How is it transmitted? What increases risks of infection?
Causes disease at extremes of age
Colonizes nasopharynx in 10% of adults, 40% children
Close contact = increased risk of transmission– i.e. in daycare, homeless shelter
Invasive disease in adults increased in winter
Defective antibody/complement formation, antecendent viral respiratory infection, smoking, HIV infection, COPD all increase risk
Mortality is high despite effective antibiotics
