the largest and most metabolically diverse phylum of Bacteria

leads in numbers of cultivated and well characterized species which accounts for > 33% of characterized species of bacteria

all gram-negative bacteria

are divided into six classes, the Alpha- , Beta- Gamma- Delta- , Epsilon-, and Zetaproteobacteria

second largest class of Proteobacteria

most species are obligate aerobes or facultative aerobes and many are oligotrophic

has 10 well-characterized orders but vast majority of species fall within the six orders

the largest and most metabolically diverse order of Alphaproteobacteria

contain ▪ phototrophs (e.g., Rhodopseudomonas) ▪ chemolithotrophs (e.g., Nitrobacter) ▪ symbionts (e.g., rhizobia)

▪ free-living nitrogen-fixing bacteria (e.g., Beijerinckia) ▪ plant and animal pathogens (e.g., Agrobacterium tumefaciens also known as Rhizobium radiobacter) ▪ diverse chemoorganotrophs

are all obligate intracellular parasites or mutualists of animals

No species have not yet been cultivated in the absence of host cells

contain metabolically diverse, including ▪ purple nonsulfur bacteria e.g., Rhodobacter and Rhodospirillum

▪ aerobic anoxygenic phototrophs, e.g., Roseobacter ▪ nitrogen-fixing bacteria, e.g., Azospirillum ▪ denitrifiers, e.g., Paracoccus ▪ magnetotactic bacteria, e.g., Magnetospirillum

are typically oligotrophic and strictly aerobic chemoorganotrophs

typically form prosthecae or stalks and many species display asymmetric forms of cell division

include diverse aerobic and facultatively aerobic chemoorganotrophs, aerobic anoxygenic phototrophs and a few obligate anaerobes

widespread in aquatic and terrestrial environments

metabolize a wide range of organic compounds including many aromatic compounds, potential agents of bioremediation

Obligate intracellular parasites, transmitted by a r t h r o p o d s

Form root nodules with soybean and other l e g u m e s

F a c u l t a t i v e i n t r a c e l l u l a r parasites o f animals, zoonotic p a t h o g e n