- Proteobacteria are further classified into the classes alpha-, beta-, gamma-, delta- and epsilonproteobacteria,
each class having separate orders, families, genera, and species. - Alphaproteobacteriaare oligotrophs. The taxa chlamydias and rickettsias areobligate intracellular
pathogens,feeding on cells of host organisms; they are metabolically inactive outside of the host cell. Some
Alphaproteobacteria can convert atmospheric nitrogen to nitrites, making nitrogen usable by other forms of
life. - Betaproteobacteriaareeutrophs. They include human pathogens of the genusNeisseriaand the species
Bordetella pertussis. - Gammaproteobacteriaare the largest and the most diverse group of Proteobacteria. Many are human
pathogens that are aerobes or facultative anaerobes. Some Gammaproteobacteria areentericbacteria that may
be coliform or noncoliform.Escherichia coli, a member of Gammaproteobacteria, is perhaps the most studied
bacterium. - Deltaproteobacteriamake up a small group able to reduce sulfate or elemental sulfur. Some are scavengers
and form myxospores, with multicellular fruiting bodies. - Epsilonproteobacteriamake up the smallest group of Proteobacteria. The generaCampylobacterand
Helicobacterare human pathogens.
4.3 Nonproteobacteria Gram-Negative Bacteria and Phototrophic Bacteria
- Gram-negative nonproteobacteria include the taxaspirochetes; theCytophaga,Fusobacterium,Bacteroides
group; Planctomycetes; and many representatives ofphototrophic bacteria. - Spirochetes are motile, spiral bacteria with a long, narrow body; they are difficult or impossible to culture.
- Several genera of spirochetes contain human pathogens that cause such diseases as syphilis and Lyme disease.
- Cytophaga,Fusobacterium, andBacteroidesare classified together as a phylum called theCFB group. They
are rod-shaped anaerobic organoheterotrophs and avid fermenters.Cytophagaare aquatic bacteria with the
gliding motility.Fusobacteriainhabit the human mouth and may cause severe infectious diseases.Bacteroides
are present in vast numbers in the human gut, most of them being mutualistic but some are pathogenic. - Planctomycetes are aquatic bacteria that reproduce by budding; they may form large colonies, and develop a
holdfast. - Phototrophic bacteria are not a taxon but, rather, a group categorized by their ability to use the energy of
sunlight. They include Proteobacteria and nonproteobacteria, as well as sulfur and nonsulfur bacteria colored
purple or green. - Sulfur bacteria perform anoxygenic photosynthesis, using sulfur compounds as donors of electrons, whereas
nonsulfur bacteria use organic compounds (succinate, malate) as donors of electrons. - Some phototrophic bacteria are able to fix nitrogen, providing the usable forms of nitrogen to other organisms.
- Cyanobacteriaare oxygen-producing bacteria thought to have played a critical role in the forming of the
earth’s atmosphere.
4.4 Gram-Positive Bacteria
- Gram-positive bacteria are a very large and diverse group of microorganisms. Understanding their taxonomy
and knowing their unique features is important for diagnostics and treatment of infectious diseases. - Gram-positive bacteria are classified intohigh G+C gram-positiveandlow G+C gram-positivebacteria,
based on the prevalence of guanine and cytosine nucleotides in their genome - Actinobacteria is the taxonomic name of the class of high G+C gram-positive bacteria. This class includes the
generaActinomyces, Arthrobacter, Corynebacterium, Frankia, Gardnerella, Micrococcus, Mycobacterium,
Nocardia,Propionibacterium, Rhodococcus,andStreptomyces. Some representatives of these genera are used
in industry; others are human or animal pathogens. - Examples of high G+C gram-positive bacteria that are human pathogens includeMycobacterium tuberculosis,
which causes tuberculosis;M. leprae, which causes leprosy (Hansen’s disease); andCorynebacterium
diphtheriae, which causes diphtheria.
178 Chapter 4 | Prokaryotic Diversity
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