7.3.1 Distinguishing Features
The alicyclobacilli are Gram-positive, spore-forming bacteria having thermoacidophilic characteristics. Key diagnostic traits of the alicyclobacillus phenotype include the presence of ffl-alicyclic fatty acids in cell membranes, growth and endospore production under aerobic to facultative conditions at 45°C and pH 3.0, limited, if any, spore production under anaerobic conditions, and, to differentiate from sulfobacillus, no utilization of ferrous iron, sulfide, or sulfur as energy sources under any conditions .
It should be noted that some publications refer to the alicyclobacilli as Gram variable. This is due to their propensity to destain very rapidly during the Gram stain procedure, which yields a visual appearance that can be interpreted as indeterminate.
The acidophilic and thermophilic nature of this genus is generally well characterized although specific growth conditions appear to be species and strain dependent [5,12,13]. Farrand et al.  conducted response surface analyses to investigate growth of B. acidocaldarius over a range of temperatures and pH, thereby establishing the extremes at which growth can occur, and confirming their thermoacidophilic character. Growth optima and upper/lower limits for temperature and pH are shown in Table 7.1. In general, A. acido-caldarius is the most tolerant of very high temperatures with optimum growth at 60 to 65°C. A. herbarius has a slightly lower optimum growth temperature (55 to 60° C) while all other species have temperature optima generally in the 45 to 50°C range. Most species have similar pH optima for growth (pH 3.0 to 4.5) although A. herbarius and A. sendaiensis optima are higher at 4.5 to 5.0 and 5.5, respectively.
Darland and Brock  described the strain-dependent nature for growth requirements for 14 strains of A. acidocaldarius. Two of the 14 strains had a minimum pH for growth of 3.0 while the remaining 12 were capable of growth down to pH 2.0. The lower temperature limit for all 14 strains was 45°C while the upper limit was 65 and 70°C for eight and six strains, respectively. Sinigaglia et al.  modeled the effects of temperature, water activity, and pH on germination of A. acidoterrestris spores. Their results support the thermoacidophilic characteristics of these organisms. Confirmation of their model by other laboratories is needed.
In all alicyclobacillus species except one, alicyclic fatty acids are the major lipid components in the cell membrane. While this is a key distinguishing feature, it should be noted that all species of sulfobacillus, as well as Curtobacterium pusillum and Propionibacterium cyclohexanicum, also contain significant quantities of these fatty acids [25—27]. Additionally, one recently named species, A. pomorum, does not contain significant quantities of «-alicyclic fatty acids .
Since there is no consistent correlation between the thermophilic/ acidophilic phenotype and the presence of «-alicyclic fatty acids, the purpose of these lipids in cell membranes is not clearly understood and requires further study.
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