Chapter Summary


  • Deep-branching thermophiles such as Aquifex and Thermotoga species share traits and habitats with thermophilic archaea. They show extensive transfer of archaeal genes.
  • Deinococcus-Thermus bacteria have ornithine in their peptidoglycan cross-bridges. Deinococcus bacteria are highly resistant to ionizing radiation, and they can be isolated from irradiated foods. Thermus bacteria are moderate thermophiles isolated from hotwater taps.
  • Chloroflexi bacteria are deep-branching thermophilic photoheterotrophs, occurring as filamentous mats in association with thermophilic cyanobacteria in hot springs.


  • Cyanobacteria are the only oxygenic prokaryotes. They conduct photosynthesis in thylakoids, fix CO2 in carboxysomes, and maintain buoyancy using gas vesicles. They exhibit gliding motility.
  • Single-celled cyanobacteria such as Prochlorococcus are among the smallest and most abundant phototrophic producers in the oceans.
  • Filamentous cyanobacteria such as Nostoc and Oscillatoria are common in freshwater lakes. Filaments form heterocysts to fix nitrogen, and reproduce by hormogonia or by akinetes.
  • Colonial cyanobacteria such as Myxosarcina produce large cell aggregates with an anaerobic core for nitrogen fixation. The colonies reproduce through baeocytes.
  • Symbiotic associations of cyanobacteria are formed with animals, fungi, and plants.


  • Firmicutes, the low-GC Gram-positive bacteria, include endospore-forming genera such as Bacillus and Clostridium. The cycle of endospore formation was probably present in the common ancestor of this phylum.
  • Nonsporulating firmicutes include pathogenic rods such as Listeria, as well as food-producing bacteria such as Lactobacillus and Lactococcus.
  • Staphylococcus and Streptococcus are Grampositive cocci that include normal human flora, as well as serious pathogens causing toxic shock syndrome, pneumonia, and scarlet fever.
  • Mycoplasmas belong phylogenetically to Firmicutes but lack the cell wall and S-layer. They have flexible cytoskeletons and show ameboid motility. Species cause diseases such as meningitis and pneumonia.
  • Actinomycetes (order Actinomycetales) include mycelial spore-forming soil bacteria, such as Streptomyces. Other actinomycetes have irregularly shaped cells, such as Clostridium species that cause tetanus and botulism. Actinomycetes stain acid-fast.
  • Mycobacteria are actinomycete rods whose cell envelope contains a diverse assemblage of complex mycolic acids. Mycobacterial species cause tuberculosis and leprosy. They stain acid-fast.


  • Proteobacteria stain Gram-negative, with a thin cell wall and an LPS outer membrane. They show wide diversity of form and metabolism, including phototrophy, lithotrophy, and heterotrophy on diverse organic substrates.
  • Alphaproteobacteria include photoheterotrophs (such as Rhodospirillum) and heterotrophs, as well as methylotrophs. They include intracellular mutualists such as rhizobia, and pathogens such as the rickettsias. Rickettsias share ancestry with mitochondria.
  • Betaproteobacteria include photoheterotrophs (Rhodocyclus), as well as nitrifiers (Nitrosomonas) and iron-sulfur oxidizers (Thiobacillus). Pathogenic diplococci include Neisseria gonorrhoeae, the cause of gonorrhea.
  • Gammaproteobacteria include sulfur and iron photolithotrophs (Chromatium), as well as members of Enterobacteriaceae found in the human colon. Intracellular pathogens include Salmonella and Legionella. The pseudomonads, aerobic rods, can respire on a wide range of complex organic substrates.
  • Deltaproteobacteria include sulfur and iron reducers (Geobacter), fruiting-body bacteria (Myxobacteria), and bacterial predators (Bdellovibrio).
  • Epsilonproteobacteria are spirillar pathogens such as Helicobacter pylori, the cause of gastritis.
  • Nitrospirae are Gram-negative spiral bacteria that oxidize nitrite to nitrate (Nitrospira).


  • Bacteroides bacteria are anaerobes that ferment complex plant materials in the human colon. They may enter body tissues through wounds and cause abscesses.
  • Chlorobium bacteria are green sulfur phototrophs, obligate anaerobes incapable of heterotrophy.


  • The spirochete cell is a tight coil, surrounded by a sheath and periplasmic space containing periplasmic flagella.
  • Spirochete motility occurs by a flexing motion caused by rotation of the periplasmic flagella, propagated the length of the coil.
  • Spirochetes grow in diverse habitats. Some are free-living fermenters in water or soil. Others are pathogens, such as Treponema pallidum, the cause of syphilis. Still others are endosymbionts of an animal digestive tract, such as the termite gut.


  • Chlamydiae are obligate intracellular parasites that undergo a complex developmental progression, culminating in a spore-like form called an elementary body that can be transmitted outside the host cell. Chlamydiae lack cell walls.
  • Planctomycetes lack cell walls, and they have evolved a membrane enclosing the nucleoid, analogous to the eukaryotic nuclear membrane.
  • Verrucomicrobia have cell projections containing tubulin. Their tubulin genes are believed to have arisen through horizontal transfer from a eukaryote.