Disclosed and claimed are a mutant of a gram negative bacterium, wherein said bacterium has at least one mutation in a nucleotide sequence which codes for a polypeptide having an identity which is equal or more than 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% with an amino acid sequence coded by a nucleotide sequence selected from the group consisting of nucleotide sequences identified SEQ ID NO: 2, 6, 9, 12, 16, 19, 22, 25, 28, 31, 34, 37, 40, 43, 46, 49, 52, 55, 58, 61, 64, 67, 70, 75, 78, 81, 84, 87, 90, 93; said mutation resulting in attenuated virulence of the bacterium. Immunogenic compositions and vaccines containing such a mutant are also disclosed and claimed.