What is the treatment for a pregnant woman with a urine culture showing less than 10,000 colony-forming units (CFU) of Group B Streptococcus (GBS) per milliliter?

Treatment for GBS Bacteriuria <10,000 CFU/mL in Pregnancy

Pregnant women with any concentration of Group B Streptococcus in urine, including <10,000 CFU/mL, must receive intrapartum IV antibiotic prophylaxis during labor to prevent early-onset neonatal GBS disease, regardless of whether the bacteriuria is treated earlier in pregnancy. 1, 2

Understanding the Clinical Context

The key principle is that GBS bacteriuria at any concentration is a marker for heavy genital tract colonization and significantly increases the risk of early-onset neonatal GBS disease. 3, 1 This is why the management differs dramatically from non-pregnant patients, where low colony counts would typically be ignored.

Evidence on Low Colony Count GBS Bacteriuria

While the CDC 2010 guidelines acknowledge that most data on early-onset GBS disease risk comes from studies of significant bacteriuria (≥10⁵ CFU/mL), they explicitly note that:

• Lower concentrations (<10⁴ CFU/mL) of GBS in urine can be associated with vaginal-rectal colonization 3
• One Utah study found elevated risk for early-onset GBS disease among infants born to women with low colony-count GBS bacteriuria compared to those without GBS bacteriuria 3
• Research demonstrates that 13 of 16 women with GBS bacteriuria and intrapartum colonization had urinary colony counts <10⁴ CFU/mL 4

Treatment Algorithm

Step 1: Immediate Management During Pregnancy

Do NOT treat asymptomatic low colony-count GBS bacteriuria with oral antibiotics during pregnancy. 5 This approach:

• Does not eliminate GBS colonization from the genitourinary tract 1, 2
• Recolonization after oral antibiotics is typical 3, 1
• May promote antibiotic resistance without clinical benefit 1

Exception: If the patient has symptomatic UTI, treat the acute infection immediately according to standard pregnancy UTI protocols, but this does not replace the need for intrapartum prophylaxis. 1, 2

Step 2: Intrapartum Antibiotic Prophylaxis (The Critical Intervention)

All pregnant women with GBS bacteriuria at any concentration during any trimester must receive IV antibiotic prophylaxis during labor. 1, 2, 5

First-Line Regimen (No Penicillin Allergy):

• Penicillin G: 5 million units IV initially, then 2.5 million units IV every 4 hours until delivery 1
• Alternative: Ampicillin 2 g IV initially, then 1 g IV every 4 hours until delivery 1

Penicillin-Allergic Patients:

For patients NOT at high risk for anaphylaxis:

• Cefazolin: 2 g IV initially, then 1 g IV every 8 hours until delivery 1

For patients at HIGH risk for anaphylaxis (history of anaphylaxis, angioedema, urticaria, or asthma):

• Clindamycin: 900 mg IV every 8 hours (if isolate is susceptible) 1
• Vancomycin: 1 g IV every 12 hours (if clindamycin-resistant or susceptibility unknown) 1

Step 3: Timing and Effectiveness

• Intrapartum prophylaxis must be administered ≥4 hours before delivery for maximum effectiveness 1, 2
• When given ≥4 hours before delivery, prophylaxis is 78% effective in preventing early-onset neonatal GBS disease 3, 1

Critical Clinical Pitfalls to Avoid

Pitfall #1: Treating with Oral Antibiotics and Assuming No Further Action Needed

This is the most dangerous error. Oral antibiotics during pregnancy do not eliminate GBS colonization, and recolonization occurs rapidly. 1, 2 Only IV antibiotics given during labor are effective in preventing neonatal disease.

Pitfall #2: Assuming Low Colony Counts Don't Require Prophylaxis

The CDC guidelines from 2010 acknowledge uncertainty about low colony-count bacteriuria but recommend reporting any GBS in urine. 3 More recent evidence shows that low colony-count bacteriuria is associated with intrapartum colonization and increased risk. 4 The safest approach is to provide intrapartum prophylaxis for any GBS bacteriuria. 1, 2

Pitfall #3: Performing Repeat Screening at 35-37 Weeks

Women with documented GBS bacteriuria at any point in pregnancy should NOT be re-screened with vaginal-rectal cultures in the third trimester, as they are presumed to be GBS colonized. 5

Pitfall #4: Underdosing or Premature Discontinuation

Ensure the full dosing regimen is continued until delivery to prevent treatment failure. 1

Special Considerations

• Cesarean delivery before labor with intact membranes: While the risk of transmission is extremely low in this scenario, the guidelines do not explicitly exempt women with GBS bacteriuria from prophylaxis 3
• Preterm labor or PPROM: These patients should receive GBS prophylaxis immediately at hospital admission 1
• Laboratory reporting: Laboratories should be informed when urine specimens are from pregnant women so they report GBS at concentrations ≥10,000 CFU/mL 1

Nuances in the Evidence

There is some divergence in the literature regarding the clinical significance of low colony-count GBS bacteriuria. The 2010 CDC guidelines express uncertainty about whether screening for low colony-count bacteriuria is cost-effective in the context of universal late antenatal screening. 3 However, given that the intervention (intrapartum IV antibiotics) is highly effective, safe, and the potential outcome (neonatal sepsis) is devastating, the prudent approach is to provide prophylaxis for any detected GBS bacteriuria. 1, 2, 4