▪ third largest class of Proteobacteria ▪ has a total of six (6) orders - Burkholderiales - Hydrogenophilales - Methylophilales - Neisseriales - Nitrosomonadales - Rhodocyclales

strictly aerobic, facultatively aerobic, and obligately anaerobic chemoorganotrophs, anoxygenic phototrophs, obligate and facultative chemolithotrophs, free-living nitrogen fixers, and pathogens of plants, animals, and humans.

one of the genera of this order ▪ All species can grow aerobically, some also grow anaerobically with nitrate as the electron acceptor

include strains which are able to fix N2, metabolize organic and aromatic compounds, promote plant growth and cause diseases in plants and animals

- often colonize the rhizosphere of plants and produce both antifungal and antinematodal compounds that protect and promote plant growth - is a major cause of soft rot in onions - emerged as an opportunistic hospital acquired infection in humans - has natural resistance to many antibiotics making it dangerous for patients with cystic fibrosis

➢ its member species are metabolically and ecologically diverse

▪ An important genus of this order ▪ a purple non-sulfur bacterium that grow best as photoheterotrophs can but can also grow as photoautotrophs with H2 as electron acceptor ▪ commonly grow in illuminated anoxic environments such as sulfur springs and deep ocean sediment

▪ another important genus of this order ▪ contain species known for producing a thick gelatinous capsule that binds cells together into a complex matrix with branching, fingerlike projections which causes flocculation of pollutants during wastewater treatment

➢ contains at least 29 genera of diverse chemoorganotrophs ➢ Neisseria and Chromobacterium are the key genera of this order

▪ are cocci which are commonly isolated from animals particularly in the oral cavity and moist areas of the body ▪ some are pathogenic

cause a potentially fatal inflammation of the membranes lining the brain

causative agent of the sexually transmitted disease gonorrhea

close phylogenetic relative of Neisseria but rod in shape

- a purple-pigmented organism (see picture below) - commonly found in soil and water; sometimes isolated from pus-forming wounds of humans and other animals - the purple pigment violacein is a water-insoluble pigment with both antimicrobial and antioxidant properties

➢ contain species with fairly specialized metabolic capabilities

▪ an important genus of Hydrogenophilales ▪ contain species that oxidize reduced sulfur compounds as electron donors, grow by aerobic respiration or denitrification, fix CO2 using the Calvin cycle ▪ commonly found in soils, sulfur springs and marine habitats

▪ genus under Methylophilales ▪ Its species are obligate and facultative methylotrophs that grow on methanol and other C1 compounds except on CH4

▪ key genera of Nitrosomonadales ▪ contain species that are capable of oxidizing ammonia

▪ the largest and most diverse class of Proteobacteria ▪ contains more than 1500 characterized species and at least 15 well-characterized orders

▪ Include many well-known human pathogens ▪ members of this group often grow rapidly in laboratory media and can be isolated from a wide diversity of habitats