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

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Introduction0:00–0:37

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.

Antepartum0:37–4:05

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.

Intrapartum4:05–7:41

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.
Your next step is to assess their mode of delivery. If the patient is undergoing vaginal delivery, then they will need IV intrapartum antibiotics to prevent vertical transmission of GBS to the newborn.
Now, if your patient is undergoing a c-section, start by assessing for the onset of labor and rupture of membranes. If labor has not started and membranes are intact, then standard preoperative antibiotics are administered.
This is because the opportunity for a neonate to be exposed to maternal vaginal-rectal colonization is minimal, so the rate of neonatal GBS EOD is very low.
However, if your patient is undergoing a c-section and their labor has started or their membranes are ruptured, they require a single dose or combination of antibiotics that provide both GBS and surgical prophylaxis.
Here’s a clinical pearl. Preoperative antibiotics are administered to prevent surgical site infections, or SSIs, with the most commonly used ones being cephalosporins, like cefazolin.
You might recall from earlier that cefazolin actually covers GBS! Clinically this means that all patients undergoing a c-section will be covered for GBS and SSI via standard preoperative antibiotics.
However, it’s still important to distinguish if labor has started or if the membranes have ruptured prior to a c-section because, in addition to standard preoperative antibiotics, you may also consider giving IV antibiotics on presentation for GBS colonization, especially if their c-section is not imminent.
Finally, let’s look at intrapartum patients that have unknown GBS status. This means they either did not have the antepartum testing done or the results are not available.
The next step is to assess for GBS risk factors. These include GBS colonization in a previous pregnancy; preterm labor prior to 37 weeks and 0 days; preterm prelabor rupture of membranes, prior to 37 weeks and 0 days; ruptured amniotic membranes greater than 18 hours; or maternal intrapartum temperature of 100.4 degrees Fahrenheit or 38.0 degrees Celsius or more.
You may also consider intrapartum nucleic acid amplification testing, or NAAT, if available. If there are no risk factors present, then no intrapartum antibiotics are needed.
If your patient has one or more of these risk factors, or if your patient has a positive NAAT, then IV intrapartum antibiotics are indicated.
Alright, as a quick recap… GBS colonization in pregnancy is a risk factor for neonatal GBS EOD. When assessing patients in the antepartum phase, start with history.

Review7:41–8:39

All patients with a history of a previous GBS-infected newborn will require IV intrapartum antibiotics. For antepartum patients without this history, screening starts with a routine urine culture followed by a vaginal-rectal swab.
If either is positive for GBS then your patient will need IV intrapartum antibiotics. Intrapartum patients require a focused history to assess the need for intrapartum GBS prophylaxis.
If the antepartum screening is negative, no antibiotics are needed. If it’s positive then your patient requires treatment depending on their route of delivery.
Finally, if GBS status is unknown, check if they have any risk factors. If present, they need intrapartum IV antibiotics.