Streptococcus agalactiae is known for being resistant to which antibiotic?

Streptococcus agalactiae, also known as Group B Streptococcus (GBS), is particularly noted for its resistance to bacitracin. This bacterium is used as a marker in differentiating it from Streptococcus pyogenes (Group A Streptococcus), which is sensitive to bacitracin. When testing for the presence of these bacteria, the growth or inhibition of growth on bacitracin-containing media helps to confirm the identity of Streptococcus agalactiae; its resistance to bacitracin supports the diagnosis.

In contrast, Group B Streptococcus is generally susceptible to the other antibiotics listed, including penicillin, cephalosporins like ceftriaxone, and amoxicillin, making them effective treatment options for infections caused by this organism. Thus, when considering the antibiotics listed, bacitracin stands out as the one to which Streptococcus agalactiae is resistant, while the others are typically used for treatment against this pathogen. This characteristic has important implications in clinical settings, especially in managing GBS in pregnant women and newborns.