Is Group B strep (Streptococcus agalactiae) a gram-positive aerobe?

Group B Streptococcus (Streptococcus agalactiae) Classification

Yes, Group B Streptococcus (Streptococcus agalactiae) is a gram-positive bacterium that is facultatively anaerobic, meaning it can grow in both aerobic and anaerobic conditions. 1, 2

Microbiological Characteristics

• Group B Streptococcus (GBS) is definitively classified as a gram-positive coccus that can survive and grow in both oxygen-rich and oxygen-poor environments (facultative anaerobe) 1, 2
• GBS is beta-hemolytic, producing a clear zone of hemolysis when grown on blood agar plates 2
• It is encapsulated with a polysaccharide capsule, which serves as a major virulence factor, with 10 known serotypes based on immunological activity 2
• GBS differs from Group A Streptococcus, which is more commonly associated with pharyngitis and skin infections 3

Clinical Significance

• GBS is the leading infectious cause of morbidity and mortality among infants in the United States 1
• It colonizes the lower genital tract of approximately 18% of women globally as an asymptomatic member of the gastrointestinal and/or vaginal flora 4
• The gastrointestinal tract is considered the most likely human reservoir of GBS, with the genitourinary tract being the most common site of secondary spread 1
• In pregnant women, colonization rates range from 10% to 30% in the vaginal or rectal areas 1

Laboratory Identification

• Culture of GBS on sheep blood agar is the standard method for identification 1
• The CDC drug label confirms that Streptococcus agalactiae is among the gram-positive aerobes susceptible to certain antibiotics like cefazolin 5
• For optimal detection, specimens should be collected from both the anorectum and vaginal introitus, which increases the likelihood of GBS isolation by 5%-27% over vaginal culture alone 1
• The evidence for the benefit of anaerobic incubation in GBS culture is not clearly established according to CDC guidelines 3

Disease Prevention

• Screening pregnant women at 35-37 weeks gestation with both rectal and vaginal swabs is recommended to identify carriers 3
• Intrapartum antibiotic prophylaxis is the primary strategy for preventing early-onset GBS disease in newborns 1
• The incidence of early-onset GBS disease has declined dramatically from 1.7 cases per 1,000 live births in the early 1990s to 0.34-0.37 cases per 1,000 live births in recent years due to prevention efforts 1

Transmission and Colonization

• GBS colonization is dynamic, with a 3-week incidence of 11.3% among women and 8.8% among men in a dormitory setting 6
• The estimated average duration of GBS colonization is longer for women (13.7 weeks) than men (8.5 weeks) 6
• Colonization with more than one serotype occurs significantly less than would be expected by chance 6
• GBS is not transmitted by casual contact, as transmission between roommate pairs occurs at the expected rate based on overall incidence 6

Understanding the gram-positive, facultatively anaerobic nature of GBS is essential for proper laboratory identification, clinical management, and prevention strategies for GBS-related diseases.