Table 6. Typical features of reference taxa

Cell shape

straight rods, 4–5 µm long

rods, 1.5–6.3 µm long

rods, normally up to 5 µm long

Gram stain

positive

positive

positive

Oxygen requirement

aerobic, facultative anaerobic, but not with nitrate

aerobic, facultative anaerobic

aerobic, facultative anaerobic, a few species are strictly anaerobic

Endospores

subterminal, oval

terminal up to subterminal

ellipsoidal, central

sspE genes for acid-soluble spore proteins

missing

missing

frequent

growth temperature

>47°C to <67°C, opt. 55°C

4–70°C, opt. 35–65°C

10–60°C

pH optimum

4.2–4.8

< 4.5, (range 1.5–5)

5–10

Phenotype

thermoacidophile

thermoacidophiles

some obligate or facultative thermophiles

Habitat

ponds in solfatara of geothermal area

acidic geothermal sites (soil and water), fruit juices, ores

often saprophytes in decaying organic matter

Flagellation

single lateral flagellum

motility reported for some species

motile or nonmotile; lateral

Menaquinones

MK-7 (personal communication Brian J. Tindall)

MK-7

MK-7

Major membranous lipid components

no ω-alicyclic fatty acids (personal communication Brian J. Tindall)

Large amounts (>80%) of ω-alicyclic fatty acids with six- or seven carbon rings, such as ω-cyclohexane undecanoic acid-C_17:0 and ω-cyclohexane tridecanoic acid-C_19:0. Three species do not possess these fatty acids. Some strains are known to form hopanoids.

cellular fatty acids: ai-C_15:0, i-C_15:0, ai-C_15:0; no ω-alicyclic fatty acids

Oxidase/catalase

weak/weak

±/±

±/±

Inclusion bodies

poly-β-hydroxybutyric acid when grown autotrophically under ammonium starvation

not specified

some species

Energy metabolism

grows best under autotrophic conditions, chemolithoautotrophic with H₂ and CO₂, but also chemoorganoheterotrophic; does not metabolize sugars

chemoorganotrophic or mixotrophic; carbohydrates, organic acids and amino acids can be utilized. Mixotrophic species utilize Fe²⁺ and S⁰

chemoorganotrophic