Mim - Lecture 22

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22.1

streptococcus

Back 1

- gram positive cocci
- catalase - negative (important test for identifying strep vs. staph bacteria. catalase is an enzyme that decomposes H2O2 -> H2O and O2)
- widely distributed in nature: milk, water, dust, vegetation, soil
- used commercially in fermenting diary products; saprophytic.
- normal flora of mucosal surfaces, including upper respiratory and intestinal tracts (transiently colonize skin as well)
- majority of streptococci are nonpathegenic, some are opportunistic or overt pathogens

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22.2

saprophyte

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an organism that obtains its nutrients from non-living organic matter, usually dead and decaysing plant or animal matter by absorbing soluble organic compounds.

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22.3

streptococcus culture

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- spherical, sometimes elliptical (bullet or rod-shaped)
- if grown in a liquid medium (blood, CSF, peritoneal exudate, broth culture), colonies will appear as chains of varyig length
- 0.5-1 um in diameter
- can ferment a variety of carbs (lactic acid is a major end product)
- hemolactate fermenters: will ferment even in oxygen-rich environments

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22.4

growth of streptococcal on blood agar:

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- incomplete hemolysis: alpha-hemolysis
- complete hemolysis: beta-hemolysis
- no hemolysis: gamma-hemolysis

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22.5

classification of streptococcal

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- progenic group, the 'viridans' group, the enterococcus group (G.I. residing) and the lactic group (lactic acid producing, important commericial applications
- hemolysis on blood agar is a better classification (alpha, beta, gamma)
- lancefield system (most reliable) is based on the antigenic characteristics of the group-specific C polysaccharide substance (Lancefield group A, B, C, D etc)

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22.6

types of hemolysis

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=> alpha-hemolysis
- P-disk infused with optochin, a type of detergent
- S. pneumoniae will not grow near P disk, cells will burst open. Other types of streptococcus that are not optochin sensitive, such as the viridans and pyogenes, will survive
=> beta hemolysis
- A-disk infused with the enzyme bacitracin
- selective for Group A streptococcus: S. pyrogenes (bacitracin sensitive)

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22.7

S. pyrogenes

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- sore throat
confirmation
- streak for isolation on sheep blood agar plate (BAP)
- capnophilic: prefers growth in 5% CO2
- after 24h incubation at 37C in 5% CO2 in air, the colonies are small (0.5 mm), domed, grayish, opalescent and surrounded by a large zone of beta-hemolysis (clear)
- susceptible to the antibiotic bacitracin
- gram +, catalase-negative cocci growing in chains

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22.8

Lancefield Group A Streptococcus (GAS)

S. Pyogenes

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- latex agglutination: rapid strep testL isolates cell wall antigen and reacts with antibody. Ag-Ab complex is linked to an enzyme for visualization
- if test result if negative, proceed by plating on blood agar, incubate overnight 37C -> identify growth for confirmation

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22.9

beta hemolysis colonies

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- bacteria is incapable of growing near the anti-microbial disk

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22.10

commerical kits for Lancefield serotyping (A-G)

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- add a drop of sera (coated on latex beads) to a sample of bacteria, mix together, and place on a microscope slide: clumping = positive, non-agglutination = negative

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22.11

envelop composition

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- hyaluronic acid is a major component of eukaryote intracellular matrix
ex: molecular mimicry

most importnt features:
1) C-polysaccharide
2) M protein: extends into environment like fibrils. Virulence determinant

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22.12

Lipoteichoic acid:

virulence factor

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- surface-associated adhesion molecule, contains fibronectin; is an immune stimulating molecule
- bind to epithelial cells

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22.13

M protein:

virulence factor

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- 100+ types
- adhesin, antiphagocytic, degrades complement compnent C3b
- C3b is a principal opsonin in the immune system (binds to receptors of phagocytic cells)
- no cross-protection between infections m1-m6; immunologically distinct molecule

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22.14

Pyrogenic exotoxins:

virulence factor

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- induce fever by activating the release of TNFalpha and IL-1
- mediate pyrogenicity
- enhance delayed hypersensitivity and susceptibility to endotoxin, cytotoxicity, nonspecific mitogenicity for T cells, immunosuppression of B-cell function, and production of scarlatiniform rash

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22.15

streptolysin S and O:

virulence factor

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0 cytotoxins

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22.16

streptokinase:

virulence factor

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- spreading factor

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22.17

C5a peptidase:

virulence factor

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- degrades complement component C5a (primary chemotoxic factor)

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22.18

pathogenesis of S. pyogenes

disease resulting from local infections with GAS and their products

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- strep sore throat scarlet fever rash from erythrogenic toxin (bacteriophage encoded)
i. petechial hemorrhages (minute, round, non-raised hemorrhage in the skin, mucous, or serous membrane)
ii. scarletiniform rash
iii. strawberry tongue
iv. circumoral pallor (circular white region around the mouth)

s. pyoderma (impetigo). Commonly M types. May precede glomerulonephritis. usually a combination of GAS and S. aureus infection

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22.19

disease resulting from invasion of GAS

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- erysipelas - infection that penetrates skin to the dermis
- puerperal fever - infection of the uterus as placenta comes away from the wall. 19th century -> group A, now -> group B
- sepsis - infection resulting from traumatic or surgical wounds
- steptococcal toxic-shock syndrome (TSS) and necrotizing infections (caused by M protein 1 and 3): purple coloration, blisters, fever, shock. flesh-eating bacteria

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22.20

infective endocarditis

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acute endocarditis

subactue endocarditis

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22.21

acute endocarditis

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bacteremia (caused group A strep in the blood stream) leading to colonization and destruction of normal/deformed heart valves.

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22.22

subacute endocarditis

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- abnormal heart valves, typically caused by streptococcus viridans.
- GAS attacks previously damaged heart valves. Infection can crop up
following dental procedures => dentist must take
history to note any increased susceptibility to infection due to compromised
valves.

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22.23

post-streptococcal disease:

acute rheumatic fever

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i. Caused by β-hemolytic streptococci pharyngeal infection, only (not skin
infect.).
ii. Immune-mediated injury: occurs weeks after the infecting bacteria have
cleared.
iii. Type II tissue injury: epitopes on M protein are closely related to antigens on
cells in the myocardium and CNS (Syndenham’s chorea).
- Can cause Sydenham’s chorea (St. Vitus Dance)

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22.24

post-streptococcal disease:

acute rheumatic fever: symptoms:

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1. valvulitis
2. pancarditis – all the cells of the heart
3. sterile vegetations on the heart valves (inflammatory infections – no bacteria)
4. erythemia marginatum - rash
5. subcutaneous nodules – appear on skin
6. arthritis/arthralgias – joint pain and swelling
7. sore throat may be subclinical – physician must look at serology:
antibody production.

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22.25

post-streptococcal disease:

acute glomerulonephritis

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i. caused by pharyngeal or skin infection.
ii. Onset ~10 days following a skin infection
iii. Symptoms: edema, hypertension, hematuria, proteinuria.
iv. Spontaneous healing
v. fully resolves in children.
vi. Progressive course may lead to renal failure, congestive heart failure, and
death.
vii. Deposition of immune complexes in renal glomeruli (Type III immunopathy)

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22.26

Immunity

Back 26

-> Type specific to GAS M protein
-> Anti-m antibodies are opsonic (i.e. promote phagocytosis)
-> Erythrogenic toxin neutralized by antitoxin (i.e. inconsequential to infection, only produces rash).
-> Antibody to streptolysin O follows infection: positive agglutination test = recent GAS infection.
-> ASO titer >250 units indicates recent or repeated infections.
-> Anti-DNase antibodies also diagnostic: this test correlates better to glomerulonephritis.

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22.27

The Pneumococcus – S. pneumoniae

identification

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-> 24 hr. incubation @ 37ºC in 5% CO2: colonies small (0.5 mm diameter), domed, grayish, mucoid, zone of (green) hemolysis.
-> Gram-positive
-> Catalase-negative
-> Bullet-shaped diplococci
-> Determinative test: lysis by the detergent optochin and bile.
o Pyogenes resistant to optochin lysis = negative test
o Pneumoniae optochin sensitive = positive test
-> Quellung reaction: capsular swelling with homologous antiserum. If bacterium lacks
capsule = avirulent.

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22.28

envelope composition of S. pneumoniae

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-> Capsule – virulence determinant
-> C polysaccharide
species specific teichoic acid (TA)
antiphagocytic
-> F antigen (Forssman antigen) – (Lipoteichoic acid - LTA)
-> Both LA & LTA contain phosphocholine
Adherence – attachment to epithelial cell surface glycolipids
-> Pneumococcal surface protein A (PspA)
-> Enzymes
Neuraminidase
IgA1 and Ig proteases: cleaves Igs and contributes to invasive properties of the
bacterium.
-> Toxins: pneumolysin O, a hemolysin similar to streptolysin O.

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22.29

S. Pneumoniae clinical infections

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- most ocmmon of bacterial pneumonia
- mobile form in the elderly. Not so in hospital acquired infections (nosocomial)
- Nasopharynx is the reservoir.
- Carrier rate may be as high as 60%.
- Not all capsule types are equally invasive.
- Rarely a primary infection. Usually a secondary infection.
- affects very young and old, usually follow a respiratory infection, needs primary infection to soften the innate immune system
- PMN is major line of defense. Aided by antibiodies and C3b
- Opsonic antibodies important.
- Neonates are protected by passive immunity for 3-4 months postpartum.

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22.30

S. Pneumoniae disease

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- pneumonia - pleural cavity opacity, as seen in x-ray imaging
- ottis media - middle ear infection
ear drum becomes pus filled
common in day care centers
can infect meninges
- meningitis: 3 major causes:
Hemophyllus influenzae type B
Neisseria meningitinus
Streptococcus pneumoniae
- cause retardation and deafness

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22.31

S. pneumoniae immunity

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-> We are all naturally, highly resistant due to opsonic antibodies.
-> S. pneumoniae requires previous infection to “soften up” immune system.
-> Antibodies are capsule type specific.
-> Maternal transfer of IgG antibody for 3-5 weeks postpartum.
-> Artificially acquired immunity – polyvalent capsular polysaccharide vaccine.
-> 23 capsule types protect against 90% of pneumococcal infections.