Atypical bacteria are bacteria that do not color with gram-staining but rather remain colorless, they are neither Gram-positive nor Gram-negative (they’re atypical! 😛). These include Chlamydiaceae, Legionella, and Mycoplasma; though Rickettsiaceae are also often considered atypical bacteria.
Antibiotics can be bacteriostatic (static=unmoving) meaning that the agent prevents the growth of bacteria or bactericidal (rhymes with suicidal) meaning that it kills bacteria. Keep in mind that bacteriostatic and bactericidal categorizations in clinical practice are not absolute and can vary depending on the dose and what is being treated.