A. xylinium
A. xylinum is an obligate aerobe, chemotrophic, ellipsoidal to rod
shaped, straight or slightly curved organism belonging to the family of
Acetobacteraccae. It can occur single, in pairs or as a chain. A. xylinum
is gram negative and in a few cases gram variable (e.g., old cells, Bergey's
Manual). A. xylinum is treated as a species in scientific
literature. For classification, however, it has been considered a subspecies
of A. aceti. The vast majority of cells in a static culture lie
immobilized within the tough cellulose pellicle that covers the surface
of the liquid, a fact which hinders the study of their metabolism and obstructs
the transport of nutrients and oxygen to the cells. On the other hand,
the cellulose pellicle offers a competitive advantage to A. xylinum
as it constitutes an effective barrier for oxygen for other organisms living
in the culture. Since A. xylinum is primarily found on decaying
fruits, the pellicle also helps to retain the water for the biochemical
degradation of substrate for nutrition and offers protection against the
lethal effect of UV light. The optimal temperature is 25 to 30 C and the
optimal pH is in the range 5.4 to 6.3 (Bergey's Manual). Hestrin et al.,
(1947) determined pH 5.5 as the optimum for the cellulose production. They
also reported that at a temperature of 37 C A. xylinum failed completely
to multiply even in an optimal medium. In the studies conducted by Schramm
et al., (1954) and 15 % than under atmospheric conditions. In contrast
to this it was found that the cell growth remained constant under a variety
of oxygen tensions. A more recent publication by Maccormick et al., (1996)
stated that Acetobacter bacteria can be grown over a range of pH of 3 to
8, temperature of 12 to 35 C (at pH 6) and show an ethanol tolerance up
to 10%.
An important aspect of A. xylinum is its conversion of glucose
to gluconic acid. This leads to a significant pH decrease of 1 to 2 units
depending on the initial pH. Embuscado et al., (1994) found that fructose
gave the best yield of cellulose and related this partly to the fact that
the production of gluconic acid removes glucose from the medium at the
expense of cellulose production. The choice of the substrates is mainly
determined by economic factors. The protection against contamination by
a lower pH can be valuable in large scale production.
The doubling time for A. xylinum in static cultures is in the
range of 8 to 10 hours and 4 to 6 hours in agitated and aerated systems
(Cannon et al, 1991). Valla et al., (1982) on the other hand reported doubling
times in shaken cultures as low as 2 hours.