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Genital specimens and group B streptococcus

16 min

Skills you'll build3 skills
  • Identify bacterial vaginosis by Nugent score, Amsel criteria, or a validated NAAT
  • Decide when gonococcal culture is needed in addition to NAAT
  • Process an antepartum group B streptococcus screen through enrichment broth

Try first

Try first

A Nugent-scored vaginal Gram stain totals 5. How is it reported?

Keep going to see the reasoning.

Keep going to see the reasoning.

Right. Keep going to follow the reasoning.

Keep going to see the reasoning.

Get the idea

Use the explanation to check your first decision.

Bacterial vaginosis

In bacterial vaginosis (BV), the vaginal Lactobacillus species are replaced by Gardnerella, anaerobes and other bacteria. The reference laboratory method is a Gram stain scored by the Nugent system.1,2 Representative oil-immersion fields are graded for three morphotypes.3

Morphotypes per oil-immersion fieldNoneBelow 11–45–30Above 30
Large gram-positive rods (Lactobacillus)43210
Small gram-variable rods (Gardnerella and Bacteroides)01234
Curved gram-variable rods (Mobiluncus)01122

The three scores add to a total from 0 to 10. A total of 0 to 3 is normal, 4 to 6 is intermediate and 7 to 10 is consistent with BV.3

The Amsel criteria need 3 of 4 clinical findings, including clue cells on a wet mount. Validated multi-organism NAAT panels are used for symptomatic patients.1,2 Gardnerella grows from about half of women without BV, so a culture for it is not a test for BV. A request for one is redirected to a Nugent stain, Amsel criteria or a BV NAAT.2

Gonorrhea NAAT and culture

NAAT is the recommended test for Neisseria gonorrhoeae, because it is more sensitive than culture. A NAAT gives no isolate and no susceptibility result. Culture is reserved for suspected treatment failure, disseminated infection, conjunctivitis, medicolegal cases and surveillance.4,2 The swab for culture is plated promptly on a selective medium such as modified Thayer-Martin. It is transported at room temperature and is never refrigerated.4

The antepartum GBS screen

A vaginal-rectal swab is collected at 36 0/7 to 37 6/7 weeks of gestation.5 The laboratory places it in a selective enrichment broth, such as Lim or Trans-Vag broth, for 18 to 24 hours. It then subcultures the broth or runs a NAAT on it. Enrichment roughly doubles recovery, and direct plating alone is unacceptable. About 5% to 6% of screening isolates are nonhemolytic, so plates are read at 24 and 48 hours.6 For a patient with a high-risk penicillin allergy, the isolate is tested for clindamycin, with erythromycin as the screen for inducible clindamycin resistance.6,5

Follow the bench demonstration

Sources for this step
References
  1. Workowski KA, Bachmann LH, Chan PA, et al. Sexually transmitted infections treatment guidelines, 2021. MMWR Recomm Rep. 2021;70(4):1-187. Bacterial vaginosis. Accessed September 27, 2026. https://www.cdc.gov/std/treatment-guidelines/bv.htm
  2. Miller JM, Binnicker MJ, Campbell S, et al. Guide to utilization of the microbiology laboratory for diagnosis of infectious diseases: 2024 update by the Infectious Diseases Society of America and the American Society for Microbiology. Clin Infect Dis. Published online March 5, 2024. doi:10.1093/cid/ciae104
  3. Nugent RP, Krohn MA, Hillier SL. Reliability of diagnosing bacterial vaginosis is improved by a standardized method of gram stain interpretation. J Clin Microbiol. 1991;29(2):297-301. doi:10.1128/jcm.29.2.297-301.1991
  4. Workowski KA, Bachmann LH, Chan PA, et al. Sexually transmitted infections treatment guidelines, 2021. MMWR Recomm Rep. 2021;70(4):1-187. Chlamydial infections, gonococcal infections, and mycoplasma genitalium. Accessed September 27, 2026. https://www.cdc.gov/std/treatment-guidelines/chlamydia.htm
  5. American College of Obstetricians and Gynecologists. Prevention of group B streptococcal early-onset disease in newborns. ACOG Committee Opinion No. 797. Obstet Gynecol. 2020;135(2):e51-e72. doi:10.1097/AOG.0000000000003668
  6. Filkins L, Hauser J, Robinson-Dunn B, et al. Guidelines for the detection and identification of group B Streptococcus. American Society for Microbiology; updated July 23, 2021. Accessed September 27, 2026. https://asm.org/getcontentasset/87c90ce7-5cef-4191-9c36-529b1212242a/dfc3d011-8f63-43f6-9ed8-4b444333a1d0/asm-gbs-guideline.pdf

Watch one

Read each decision and its explanation.

A vaginal Gram stain is sent for BV. Across representative oil-immersion fields you see:

  • large gram-positive rods: fewer than 1 per field
  • small gram-variable rods and coccobacilli: more than 30 per field, many coating epithelial cells
  • curved gram-variable rods: about 2 per field

What is the Nugent result?

  1. Confirm the smear is scored under oil immersion across several representative fields.

    A score is valid only when representative fields are scored.

  2. Score the large gram-positive rods: fewer than 1 per field scores 3.

    Lactobacillus is scored in reverse, because BV is marked by its loss.

  3. Score the small gram-variable rods: more than 30 per field scores 4.

    The Gardnerella and Bacteroides score increases with their number.

  4. Score the curved gram-variable rods: 1 to 4 per field scores 1.

    Curved rods add at most 2 points.

  5. Add the scores: 3 + 4 + 1 = 8, which falls in the 7 to 10 range.

    The category comes from the total.

The Nugent score is 8, consistent with bacterial vaginosis.

Practice this skill

Your turn

Problem 1 of 3 · Hints available

A clinician orders a vaginal culture for Gardnerella to diagnose bacterial vaginosis (BV). What does the laboratory do?

Incorrect. Gardnerella is recoverable from about half of asymptomatic women, so its growth in culture is not a diagnostic test for BV.

Correct. Gardnerella culture is not a diagnostic test for BV. Nugent scoring is the reference laboratory method (0 to 3 normal, 4 to 6 intermediate, 7 to 10 consistent with BV), a complete Amsel assessment needs 3 of 4 criteria, and FDA-cleared multi-organism panels are intended for symptomatic patients.

Incorrect. No CLSI method or breakpoint exists for the genus, so routine susceptibility testing of a vaginal isolate is not indicated, and culture does not diagnose BV.

Hint
  1. Ask how often Gardnerella grows from women without BV.
  2. BV is a shift in the whole vaginal flora. Choose the test that measures that shift.

Review Vaginal specimens: bacterial vaginosis and vulvovaginitis

Problem 2 of 3 · Hints available

A vaginal-rectal swab for antepartum group B streptococcus (GBS) screening arrives. Which processing is acceptable?

Incorrect. Enrichment roughly doubles recovery, and ASM 2021 guidance considers direct plating alone unacceptable for the antepartum screen.

Incorrect. Direct NAAT has reported false-negative rates of 6.3% to 22% for antepartum screening and yields no isolate for susceptibility testing when it is needed.

Correct. Enrichment in selective broth such as Lim or Trans-Vag roughly doubles recovery compared with direct plating, and reading the subculture at 24 and 48 hours detects the 5% to 6% of screening isolates that are nonhemolytic.

Hint
  1. Think about how many carriers a single plate misses.
  2. Think about how carriers with few organisms are recovered.

Review Group B Streptococcus screening

Problem 3 of 3 · On your own

A vaginal Gram stain shows, per oil-immersion field, about 8 large gram-positive rods, about 3 small gram-variable rods and about 3 curved gram-variable rods. What is the Nugent score?

Show the answer

4

Large gram-positive rods at 5 to 30 per field score 1. Small gram-variable rods at 1 to 4 per field score 2. Curved rods at 1 to 4 per field score 1. The total is 1 + 2 + 1 = 4, which is intermediate flora.

Review Vaginal specimens: bacterial vaginosis and vulvovaginitis

Use it

Read the case evidence, then make each decision. The step is complete without it.

  • Lucía Pelletier, 29, is 36 weeks pregnant and has had anaphylaxis to penicillin.
  • Her vaginal-rectal swab for GBS screening grows Streptococcus agalactiae after enrichment.
  • Three weeks ago her gonorrhea NAAT was positive, and she was treated. Her symptoms have not cleared.
  • Her clinician asks for "the resistance result from the NAAT" and sends a new endocervical swab.
Decision 1 of 3

What susceptibility testing does the GBS isolate need?

GBS is predictably susceptible to penicillin. Her high-risk penicillin allergy is the reason the isolate is tested for clindamycin.

GBS remains predictably susceptible to both, so they are not tested routinely. Her allergy calls for clindamycin testing.

A high-risk penicillin allergy calls for clindamycin testing. If erythromycin is resistant and clindamycin susceptible, a D-zone test is run, and clindamycin is reported resistant when the D-zone is present.

Review Vaginal specimens: bacterial vaginosis and vulvovaginitis

Decision 2 of 3

How does the laboratory respond to the request for a resistance result?

A NAAT detects gonococcal nucleic acid without a viable isolate. It gives no phenotypic susceptibility result.

Expected a routine NAAT to provide phenotypic susceptibility

A NAAT detects gonococcal nucleic acid without a viable isolate, so it supplies no susceptibility result. After treatment failure, culture with susceptibility testing is needed. The swab for gonococcal culture stays at room temperature.

Symptoms after treatment raise the question of treatment failure. Culture on a selective medium gives the isolate that susceptibility testing needs.

N. gonorrhoeae is fragile and is never refrigerated in transit. Overnight refrigeration can kill the organism.

Review Endocervical and urethral specimens, and the NAAT-versus-culture decision

Decision 3 of 3

The culture grows N. gonorrhoeae, and its ceftriaxone MIC is 0.5 µg/mL, resistant under the laboratory's breakpoints. What else does the laboratory do?

The isolate is the evidence of a possible treatment failure. Public health needs it for confirmation and surveillance.

Suspected cephalosporin treatment failure is reported to public health within 24 hours. The preserved isolate goes to the public health laboratory.

The clinician needs the result, and the suspected treatment failure and its isolate also go to public health.

Review Public-health reporting and referral

Both decisions depend on the need for a viable isolate. Her penicillin allergy means the GBS isolate needs a clindamycin result. Her treatment failure means gonococcal culture is the only way to get a susceptibility result, and the isolate goes to public health.

For reference

Sources checked