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Encapsulated
Gram-positive diplococci or short chains; Oval or lancet-shaped
cells (0.5-1.2um) (see WebLinked
image; see WebLinked
image; see WebLinked image) 
Capsular
antigen in virulent strains: Complex polysaccharide; Cross reactive Gram
positive but stain poorly and may appear gram negative No
Lancefield group-specific antigens Alpha
hemolysis (aerobically); beta hemolysis (possible anaerobically) Growth
on surface of blood agar is alpha-hemolytic when incubated aerobically Subsurface
or anaerobic growth may show beta-hemolysis due to production of pneumolysin
which is oxygen labile Autolysis
of colonies with aging Fastidious
Growth Requirements Enriched
media with 5% defibrinated blood (provides catalase) Increased
carbon dioxide for 5-10% of strains (capnophilic) Ferment
carbohydrates; Major endproduct is lactic acid; High concentrations of
glucose in medium is inhibitory as lactic acid levels become toxic (like
Grp A streptococci) Choline
requirement (phosphocholine in cell wall is unique to S. pneumoniae)
for cell wall hydrolysis; Amidase (autolytic enzyme) inactive without
choline and cell division stops Catalase
negative (unlike staphylococci); Accumulation of hydrogen peroxide inhibits
growth; Exogenous source of catalase needed for growth (e.g., blood) Two
forms of teichoic acid polymer (galactosamine and phosphocholine)
in cell wall Cell
surface exposed (C-substance): Precipitates a serum globulin fraction
known as C-reactive protein (CRP) in the presence of calcium Remains
a leading cause of morbidity and mortality Pneumonia
(Pneumococcal Pneumonia; Lobar Pneumonia) Aspiration
(inhalation) of endogenous
oral pneumococci into lung alveoli Bacteria
multiply rapidly in nutrient-rich edema Sudden
onset with shaking chills and sustained fever Erythrocytes
leak from capillaries Phagocytic
cells (neutrophils,
followed by alveolar macrophages)
migrate and phagocytose and destroy pneumococci Productive
cough with blood in sputum Pleurisy
common Generally
localized in lower lobes Children
and elderly may have diffuse bronchopneumonia Pneumococcal
lesion can be observed by
X-ray Radiological
resolution (clean chest X-ray) within 2-3 weeks with antimicrobial therapy Bacterial
Sinusitis (paranasal sinuses) Often
antecedent upper respiratory tract viral infection with PMN obstruction
of sinuses and ear canal All
age groups Otitis
Media (middle ear inflammation): Often
antecedent upper respiratory tract viral infection with PMN obstruction
of sinuses and ear canal Primarily
in young children Meningitis:
CNS (central nervous system) involvement after bacteremia, sinusitis or
otitis media, head trauma Primarily
in pediatric population Mortality
and severe neurological complications are 4-20 times more likely than
with meningitis caused by other organisms Bacteremia: 25-30%
with pneumonia 80%
with meningitis Endocarditis
possible Colonizes
oropharynx and/or nasopharynx; Spreads to normally sterile site Stimulates
local inflammatory response, but evades phagocytic killing Carriers
(5-75% Incidence): Transient colonization of throat and nasopharynx
by a succession of different serotypes; Duration decreases with each
successive serotype (serotype-specific immunity); Disease generally from
newly acquired strain Serious
disease most commonly in young children or elderly adults Often
preceded by (antecedent) respiratory viral infection (e.g.,
influenza, measles) Conditions
interfering with normal clearance of bacteria predisposes host to pneumococcal
infections
Chronic
pulmonary disease Alcoholism Congestive
heart failure Diabetes
mellitus Chronic
kidney disease Splenic
dysfunction or splenectomy: Decrease in clearance from blood and defective
early antibody production Pathogenesis
primarily due to host response to infection Three
Classes of Virulence Factors: Cellular
components: Complex
polysaccharide capsule: Antiphagocytic.gif){kind=small}
Encapsulated
(smooth) strains are virulent Nonencapsulated
(rough) strains are avirulent Type-specific
anticapsular antibodies are protective Capsular
polysaccharides are soluble (a.k.a., specific soluble substances) and
free polysaccharides can bind opsonins and further inhibit phagocytosis Protein
adhesin: Binds to oropharyngeal epithelium enabling initial colonization Teichoic
acid: Activate alternative complement pathway; C5a production mediates
inflammation Peptidoglycan
fragments: Activate alternative complement pathway; C5a production mediates
inflammation Phosphorylcholine:
Mediates tissue destruction, spreading, and protection from opsonization
and phagocytosis; Binds to host cell receptors for platelet-activating
factor allowing entrance into cells Extracellular
components: Pneumolysin
(pneumolysin O): Multifactorial cytotoxin Biochemically
similar to Streptolysin O (O2 labile) Assists
spreading by destroying ciliated epithelial cells by binding cholesterol
in host cell membranes and causing pore formation Tissue
destruction mediated by activation of classic complement pathway; C3a
and C5a production with activated leukocytes producing cytokines (e.g.,
IL-1, TNF-alpha) which result in enhanced inflammatory cell response,
fever, tissue damage, etc. Inhibits
phagocytic killing by suppressing oxidative burst Secretory
IgA protease: Assists spreading by enzymatically disrupting secretory
IgA (sIgA) clearance of bacteria Immune
Response: Naturally
acquired immunity: Type-specific anticapsular immunity Artificially
acquired immunity: Immunization via polyvalent vaccine prophylaxis Purified
capsular material from most common serotypes (23 different
polysaccharides covering 94% of clinically relevant serotypes);
Until recently this formulation was not effective in young children
or elderly adults or in other high risk groups In
1999, a vaccine formulation was shown to be effective and has
been approved by the FDA for use against otitis media in young children Quellung
(capsular precipitation; swelling) reaction: Anticapsular antibodies
cause an increase in refractivity of the capsule making it readily apparent
by microscopy Susceptibility
to optochin (ethylhydrocupriene dihydrochloride) Soluble
in bile: Colonies dissolve when exposed to drop of bile (differentiates
from alpha-hemolytic streptococci and Enterococcus); Autolysis after
bile activation of autolysins; Also performed in broth culture with clearing Drug
of Choice: Penicillin; Alternates: Cephalosporins, erythromycin, chloramphenicol
(meningitis); Resistant to tetracycline Multiple
Antibiotic Resistance (Multidrug Resistance): Recognized in 1977;
Including low level resistance to penicillin Dramatic
increase in frequency of moderate resistance and in the emergence of high
levels of resistance (High level resistance: MIC > 2.0 ug/ml)(1).gif)
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