Which antibiotic is effective mainly against gram-positive bacteria?

Bacitracin is an antibiotic that is primarily effective against gram-positive bacteria. This effectiveness is due to its mechanism of action, which inhibits cell wall synthesis in bacteria. Gram-positive bacteria have a thick peptidoglycan layer in their cell walls, making them more susceptible to bacitracin.

In contrast, other antibiotics listed have broader spectrums or target different types of bacteria. For instance, polymyxin B primarily targets gram-negative bacteria by disrupting their cell membranes, which is quite different from the action of bacitracin. Fluoroquinolones are broad-spectrum antibiotics that affect both gram-positive and gram-negative bacteria by inhibiting bacterial DNA synthesis, while tetracyclines also have a broad spectrum of activity, targeting both types but being particularly effective on a range of bacteria including some gram-negative species.

The specificity of bacitracin for gram-positive bacteria makes it a useful treatment option for infections caused by such organisms, further distinguishing it from the other antibiotics listed.