Two Gram-negative bacteria were found to produce the new antibacterial antibiotics TAN-1057 A, B, C and D. The producing bacteria were characterized and designated as Flexibacter sp. PK-74 and PK-176. These antibiotics were active against Gram-negative and Gram-positive bacteria, including methicillin-resistant Staphylococcus aureus. TAN-1057 A inhibited protein biosynthesis in Escherichia coli and S. aureus. It showed excellent protective effects against an experimental methicillin-resistant S. aureus infection in mice.