Cocos Gram-positivos

Streptococcus agalactiae (grupo B)

Gram-positive cocci in chains

Identification & Growth

Identification

Catalase-negative, CAMP test-positive, hippurate-positive
Resistant to bacitracin, Lancefield group B latex-positive

Growth Conditions

Blood agar: greyish colonies with a narrow zone of beta-hemolysis
Selective Todd-Hewitt broth for antenatal screening; 35–37 °C, CO₂

Clinical Significance

Vaginal/rectal colonization in pregnant women, early-onset neonatal sepsis, urinary tract infection

Intrinsic Resistance

Aztreonam, colistin and low-level aminoglycosides have no activity

Bench Alerts

Antenatal screening at 35–37 weeks requires enrichment broth
In penicillin-allergic patients, test clindamycin with a D-test

Clinical Notes

Clinical presentations

Causes early-onset neonatal sepsis (first 24–48h of life) and late-onset disease (up to 3 months), including meningitis.
In adults, causes urinary tract infections, skin and soft tissue infections, bacteremia, and osteomyelitis, especially in diabetics and the elderly.
Asymptomatically colonizes the female genital tract, the main obstetric concern for vertical transmission.

Specimens and collection

Prenatal screening (35–37 weeks) uses a combined vaginal-rectal swab in transport medium, plated onto selective broth (Todd-Hewitt or LIM).
Blood culture and CSF culture are mandatory in a neonate with signs of sepsis or meningitis.
Wound and urine specimens in adults should follow standard aseptic technique to avoid contamination by perineal flora.

Epidemiology and at-risk populations

Approximately 10–30% of pregnant women are colonized in the genital or gastrointestinal tract, varying by population.
Intrapartum prophylaxis has drastically reduced early-onset neonatal disease incidence in countries with universal screening.
Adults with diabetes, obesity, cancer, or liver failure have increased risk of invasive infection.

Resistance and therapeutic implications

Remains universally penicillin-susceptible, which is the antimicrobial of choice for prophylaxis and treatment.
Clindamycin and erythromycin resistance is common and increasing, requiring susceptibility testing and D-test in penicillin-allergic pregnant patients.
In severely penicillin-allergic patients with a clindamycin-resistant isolate, vancomycin is the recommended alternative for intrapartum prophylaxis.

Bench and reporting notes

Narrow beta-hemolysis on blood agar and a positive CAMP test (synergy with staphylococcal toxin) support presumptive identification.
Latex agglutination for Lancefield group B antigen confirms the species when available.
Prenatal screening reports should include clindamycin/erythromycin susceptibility testing with D-test whenever the patient is penicillin-allergic.

Sources

Educational decision-support content. It does not replace laboratory validation, current guidelines or review by the responsible professional.

FAQ: Frequently Asked Questions

How to identify Streptococcus agalactiae (grupo B) in the lab?

Streptococcus agalactiae (grupo B) is identified through catalase-negative, camp test-positive, hippurate-positive, resistant to bacitracin, lancefield group b latex-positive.

What are the intrinsic resistances of Streptococcus agalactiae (grupo B)?

This organism is naturally resistant to aztreonam, colistin and low-level aminoglycosides have no activity. These drugs should not be reported as susceptible.

Where is Streptococcus agalactiae (grupo B) commonly found?

It is typically associated with vaginal/rectal colonization in pregnant women, early-onset neonatal sepsis, urinary tract infection.

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