Streptococcus agalactiae (group B strep) - MegaMicro

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Streptococcus agalactiae (group B strep)

Microbiology > Bacteriology > Gram-positive bacteria > Gram-Positive Cocci > Streptococci
a. Structure
i. Gram positive
ii. Coccoid
iii. Form chains
iv. Catalase-negative
v. β-hemolytic
vi. Lancefield Group B (and only species with Group B antigen)
vii. Polysaccharide capsule

b. Pathobiology
i. Facultative anaerobe
ii. Primary virulence factor: polysaccharide capsule
1. Interferes with phagocytosis until development of antibodies
a. This is the reason why neonates are particularly at-risk – they are no longer under the protection of maternal antibodies.
iii. Colonizes the lower gastrointestinal tract, genitourinary tract, and upper respiratory tract

c. Epidemiology
i. Most common cause of bacterial septicemia and meningitis in newborns
ii. Transient vaginal carriage in 10-30% of pregnant women
iii. 60% of infants born to colonized mothers become colonized
iv. Other associations for neonatal colonization:
1. Premature delivery
2. Prolonged membrane rupture
3. Intrapartum fever
4. Mother is without type-specific antibodies
5. Mother has low complement levels
v. Risk of invasive disease in adults greater in pregnant women than in men and non-pregnant women
vi. Conditions that predispose to development of disease in men and non-pregnant women are: diabetes mellitus, chronic liver or renal disease, cancer, and HIV infection.

d. Disease manifestations
1. Early-onset neonatal disease
a. Disease in infants <7 days
b. Acquired in utero or at birth
c. Bacteremia, pneumonia, meningitis
d. Mortality rate <5% due to early diagnosis and treatment
i. Severe neurologic sequelae in 15-30% of infants
2. Late-onset neonatal disease
a. Disease in infants between 1 week and 3 months
b. Acquired from an exogenous source (e.g. mother or another infant)
c. Bacteremia with meningitis
d. Mortality rate 3%
i. Neurologic complications 25-50% in infants with meningitis
3. Infection in pregnant women
a. Postpartum endometritis, wound infection, and urinary tract infections
b. Excellent prognosis
4. Infection in men and non-pregnant women
a. Generally older and have debilitating underlying conditions
b. Bacteremia, pneumonia, bone and joint infections, and skin and soft-tissue infections

e. Diagnostic methods
i. Culture
1. Most sensitive test
2. Produces large colonies after 24 hours of incubation
3. Selective broth (i.e. LIM) with antibiotics added to suppress growth of other organisms needed for optimal detection of vaginal carriage
ii. Microscopy
1. Useful for meningitis (CSF), pneumonia (respiratory tract secretions), wound infections (exudates)
iii. Antigen detection tests
1. Available but too insensitive for screening
iv. PCR-based nucleic acid amplification tests
1. Approved for rectal/vaginal swabs of pregnant women
2. Relatively insensitive, so also requires selective enrichment broth (i.e. LIM)
v. CSF analysis by microscopy required in suspected early-onset neonatal disease (because meningeal involvement may not be initially apparent)

f. Differentiating from other streptococci
i. Colonies are larger than those of S. pyogenes.
ii. Colonies have smaller zones of β-hemolysis than those of S. pyogenes.
iii. Early-onset neonatal disease is indistinguishable from sepsis caused by other organisms.

g. Therapy
i. Penicillin – drug of choice
ii. Cephalosporins and vancomycin may be used if penicillin alllergy
iii. Macrolides, clindamycin, tetracyclines – should generally be avoided because resistance is common

h. Prevention and control
i. Screening of all pregnant women at 35-37 weeks gestation
ii. Chemoprophylaxis for all women who are either colonized or at high risk (ensures high protective antibiotic levels in infant at time of birth)
1. High-risk classification:
a. Previous birth to an infant with disease
b. Risk factors present at birth:
i. Intrapartum temperature of >38ᵒC
ii. Membrane rupture >18 hours before delivery
iii. Positive vaginal or rectal culture at 35-37 weeks gestation
2. Chemoprophylaxis:
a. IV penicillin G or ampicillin administered at least 4 hours before delivery
i. In penicillin-allergic women, cefazolin, clindamycin (if susceptible), or vancoymycin (for mothers at high risk of anaphylaxis) used
iii. Vaccines: none currently, but in development

Related concepts
1. Group B Streptococcus
2. Neonatal meningitis
3. Neonatal bacteremia
4. Pregnancy
5. Neonate
6. Meningitis
7. Premature delivery
8. Prolonged membrane rupture
9. Intrapartum fever
10. Bacitracin Resistant

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