Group B streptococcus (GBS) colonization in pregnancy: Clinical sciences

Group B streptococcus, or GBS, is a normal flora commensal found in gastrointestinal and vaginal microbiomes. Pregnant patients who are colonized with GBS can transmit the bacteria to their newborns during labor or after the rupture of membranes, so it is a significant cause of neonatal infection including pneumonia, meningitis, and sepsis. For that reason, universal GBS screening for vaginal-rectal colonization in pregnancy is recommended to prevent neonatal GBS early-onset disease, or EOD.

Screening a patient for GBS colonization will vary depending on the time of presentation, so let's start with antepartum patients. Assessment begins with obtaining a focused history to see if they have any risk factors that increase the chance of neonatal GBS EOD. The most significant risk factor is a history of a previous GBS-infected newborn. These patients are presumed to be colonized with GBS and will require IV intrapartum antibiotics. The preferred antibiotic is penicillin, though ampicillin is an acceptable alternative. However, if a patient has an allergy to penicillin, you should use alternative antibiotics. When the allergy is low-risk for anaphylaxis, use a first-generation cephalosporin like cefazolin. On the other hand, if the allergy is high-risk for anaphylaxis, you’ll need to have the lab assess the GBS isolate for clindamycin resistance. If the GBS is sensitive to clindamycin, you can use that, but if it’s resistant, then vancomycin is used instead.

Here’s a clinical pearl. Many patients are unsure of their allergic reactions, especially when it comes to penicillin. In pregnant patients who report a history of penicillin allergy but are not sure of the severity of their reaction, penicillin allergy testing can be beneficial.

Next, let’s talk about screening for patients without a history of a GBS-infected newborn. Typically, at the first prenatal appointment, a urine culture is obtained to check for asymptomatic bacteriuria. This is because pregnant patients are at a higher risk of asymptomatic bacteriuria, which can then lead to an ascending infection and pyelonephritis. If urine culture is positive for GBS, your patient is considered to have a GBS colonization.

Pay attention to the colony count. If it’s greater than 100,000, start them on oral antibiotics at the time of diagnosis to prevent an acute ascending infection. Additionally, all patients with GBS present in their urine, regardless of the colony count on culture, will require IV intrapartum antibiotics.

Here’s a clinical pearl! Even though a screening urine culture is done at the first prenatal visit, you may encounter GBS in a urine specimen in any trimester. If it’s present at any time throughout the pregnancy, that means your patient is colonized with GBS and will need IV intrapartum antibiotics.

Okay, back to our cultures! If the initial urine culture is negative for GBS and no other urine cultures show GBS is present, then your next step will be to complete a routine vaginal-rectal swab between 36 weeks and 0 days and 37 weeks and 6 days of gestation. To obtain the GBS swab for culture, collect a sample from the lower vagina, near the introitus, and then from the rectum, through the anal sphincter. If the culture results are negative for GBS, then no antibiotics are needed. However, if the swab is positive for GBS, then your patient is colonized and will need IV intrapartum antibiotics. Keep in mind that the results of a vaginal-rectal swab are valid for 5 weeks, so if your patient remains pregnant past their due date you may need to repeat the swab.

Alright, that completes antepartum management, so let’s move on to your intrapartum patients. Your first step is to obtain a focused history to determine the need for intrapartum antibiotic prophylaxis. Check their records for three important things; one, if they have a history of a prior GBS-infected newborn; two, if they had GBS present in any antepartum urine culture; and three, if they had a positive vaginal-rectal GBS culture. If the answer to all three of these is negative, you can consider their GBS antepartum screen negative and they will not need intrapartum antibiotics. On the other hand, if any of the three scenarios are positive, then your patient has a positive GBS colonization antepartum screen and needs treatment.