Sentence:  Generally, ionophore antibiotics are highly effective against <WSD>Gram-positive</WSD> bacteria but exhibit little or no activity against Gram-negative bacteria (Chen and Wolin, 1979; Watanabe "et al.", 1981). 
Meaning of the word marked with <WSD> tags:  That stains dark blue or violet after Gram staining, due to large quantities of peptidoglycan in the cell wall.