What is the most common organism that can cause preterm birth?

Group B Streptococcus (GBS) is the Most Common Organism Causing Preterm Birth

Group B Streptococcus (Streptococcus agalactiae) is the leading infectious organism associated with preterm birth and represents the most common cause of early-onset neonatal sepsis in the United States. 1, 2

Epidemiology and Clinical Significance

• GBS colonizes 10-30% of pregnant women in the vaginal or rectal tract, with the gastrointestinal tract serving as the primary reservoir 1, 2
• GBS is directly associated with preterm birth through ascending infection from the vagina to the amniotic fluid after onset of labor or rupture of membranes 2
• Mortality is substantially higher among preterm infants, with case-fatality rates of approximately 20% and as high as 30% among those ≤33 weeks' gestation, compared with 2-3% among full-term infants 1

Mechanism of Preterm Birth Induction

• GBS ascends from the vagina to the amniotic fluid after onset of labor or rupture of membranes, though it can also invade through intact membranes 1, 2
• The organism's hemolytic pigment toxin induces neutrophil cell death and circumvents neutrophil extracellular traps (NETs) in placental membranes, facilitating microbial invasion of the amniotic cavity 3
• Major virulence factors include β-hemolysin, outer membrane vesicles, and hyaluronidase, which drive inflammation through NLRP3 activation and elevated IL-1β production 4

Risk Stratification

High-risk indicators for GBS-associated preterm birth include:

• GBS bacteriuria during pregnancy (marker for heavy genital tract colonization and increased preterm birth risk) 2
• Gestational age <37 weeks at onset of labor or membrane rupture 1, 2
• Membrane rupture >12 hours before delivery 1
• Intrapartum fever (temperature >99.5°F or >37.5°C) 1
• Previous delivery of an infant with invasive GBS disease 1

Clinical Management Algorithm

For women presenting with preterm labor or rupture of membranes <37 weeks:

1. Obtain vagino-rectal GBS culture immediately if not performed in the previous 5 weeks 5
2. Initiate intrapartum antibiotic prophylaxis immediately while awaiting culture results 2, 5
3. Continue prophylaxis until delivery if culture is positive or unknown 5
4. Intrapartum antibiotic prophylaxis administered ≥4 hours before delivery is 78% effective (95% CI: 44-91%) in preventing early-onset GBS disease 2

Important Clinical Caveats

• Infection may be less clinically evident at early gestational ages (<24 weeks), and diagnosis should not be delayed due to absence of maternal fever 5
• The median latency between rupture of membranes and first signs of infection is only 5 days, but once infection is identified, the median time to maternal death is only 18 hours 5
• Heavy GBS colonization (culture from direct plating rather than selective broth only) is associated with higher risk for early-onset disease 1
• Women with GBS colonization are >25 times more likely to deliver infants with early-onset GBS disease compared to those with negative cultures 1