The quinolones are a set of broad-spectrum antibiotics, with notable activity against Gram-negative bacteria. The class has been researched for over 50 years. Alterations of the functionality about the quinolone core have led to the development of several generations of analogues. Later generations have expanded Gram-positive coverage to include penicillin resistant and sensitive Streptococcus pneumoniae as well as atypical pathogens such as Mycoplasma pneumoniae and Chlamydia pneumoniae, and broad anaerobic coverage.The Quinoline Metabolite Set 2 contains important examples of third and fourth generation quinolones. Levofloxacin and moxifloxacin are listed as Essential Medicines by the WHO. Levofloxacin, one of the first Gen 3 quinolones, was developed by Daichi Sankyo in the early 1990s and approved by the US FDA in 1996.