S11: streptococcus, pharyngitis & EBV Flashcards
Describe the gram stain of streptococci
Gram positive chains of cocci
Describe the classification of streptococci based on haemolysis
Alpha haemolysis – oxidise iron in Hb so appear green on the plate
-strep pneumoniae, viridans’ streptococci
Beta haemolysis – cause complete rupture of blood cells which appear white/clear on the plate
-group A (strep pyogenes) & group B strep
Gamma haemolysis – no effect on the blood agar
Describe Lancefield grouping & Sherman classification
Lancefield – serological method for classifying streptococci into one of 20 groups based on the presence of polysaccharide antigens in the bacterial cell wall
Sherman – classified streptococci into four groups
1) Pyogenic (pus-forming)
2) Viridans
3) Enterococcal – bile tolerant & penicillin resistant
4) Lactic – bacteria from diary sources
Describe different virulence factors found in streptococcus pyogenes
Hyaluronic acid capsule – inhibits phagocytosis by neutrophils & macrophages
M protein – resistance to phagocytosis by inhibiting activation of alternative complement pathway on bacterial cell surface
Streptolysins O and S – lysis of RBCs, neutrophils & platelets
Exotoxins – cleaves IgG bound to group A step
Describe the microbiological features and clinical presentation of streptococcal pharyngitis
Strep pyogenes
Peak incidence 5-15 years old
Droplet spread -> associated with over-crowding
Untreated patients develop M protein specific antibody
Clinical features: abrupt onset sore throat, malaise, fever, headache, lymphoid hyperplasia & tonsillopharyngeal exudates
Describe the complications of streptococcal pharyngitis
Scarlet fever – high fever, sepsis, arthritis & jaundice
Suppurative complications – peritonsillar cellulitis/abscess, retropharyngeal abscess, meningitis
Acute rheumatic fever (10-14 days later) – inflammation of heart, joints, CNS which follows on from pharyngitis
Acute post-streptococcal glomerulonephritis – acute inflammation of renal glomerulus
List the different skin infections caused by strep pyogenes
Impetigo – childhood infection, most common cause of glomerulonephritis
Erysipelas – dermis infection with lymphatic involvement
Cellulitis – skin & subcutaneous tissue infection
Necrotising fasciitis – infection of deeper subcutaneous tissues & fascia, rapid & extensive necrosis, usually secondary to skin break
Describe streptococcal toxic shock syndrome
Deep tissue infection with strep pyogenes ‘health to death in hours’
Entry of group A strep into deeper tissues & bloodstream
Bacteraemia, vascular collapse & organ failure
Outline common causes of pharyngitis
Viral – influenza, EBV
Bacteria – strep pyogenes
Why is the feverPAIN score used?
Used to differentiate between viral & bacterial causes of pharyngitis & to help guide decision making
Higher scores = more severe symptoms & higher chance of streptococcus infection
Outline management strategies for pharyngitis
Use feverPAIN scoring system
Usually mild illness – plenty of fluids & rest (usually caused by viral infections so antibiotics are ineffective)
Come back to the doctor if illness gets worse
Describe features of EBV
Member of the herpesvirus family
Double-stranded DNA virus
Causes infectious mononucleosis/glandular fever
Describe the microbe-host interaction for EBV
EBV infects its target cell, B cell, for life & it establishes a latent infection Acute EBV is characterised by a massive increase in the number of CD8+ T cells in the blood -> antibody response is mounted After a few months, Ig class switching takes place -> become IgG Combination of antibodies & cytotoxic T cells reduces but does not eliminate the infection -> limits the infection but cannot accomplish ‘sterilizing immunity’
Which cancers does EBV infection increase the risk of?
Nasopharyngeal cancer
Burkitt’s lymphoma
Describe the main clinical features of EBV infection
Sore throat
Malaise
Lymphadenopathy
Enlarged spleen (rare)
