CORYNEFORM BACTERIA
CELLULOMONAS
INTRODUCTION TO CELLULOMONAS
The classification of Cellulomonas is often based on 16S rRNA sequencing. However, phenotypic characteristics such as cell or colony morphology, Gram stain, motility, metabolism, electrophoretic mobility (indicating the size and charge) of L- lactate dehydrogenase and cell wall composition can also be used to identify species, (35).
​
Whilst lacking spores and retaining a Gram-positive cell wall and pleomorphic rod-shaped cell morphology similar to other cornyeforms, the genus can be distinguished from them in a number of ways:
-
High G + C content ranging from 68.5%-76%, (44).
-
Some species possess motile or non motile flagella, (45).
-
The ability to hydrolyse cellulose, for which they are named.
Many species are also facultatively anaerobic, enabling them to thrive in a diverse range of environments.
CELL MORPHOLOGY
Cells in a young colony undergo snapping division, and, like many coryneforms, are found in V-shaped pairs. However, as the population density grows, the morphology of many species changes. Cells may become shorter rods or even appear almost coccoid, (38).
Fig 15: SEM of C. flavigena
HABITAT
Most species of Cellulomonas bacteria are found in soil, (40) or on decaying plants where cellulose from decomposing debris supply a carbon source for this genus. However, they are more ecologically diverse than this, one species being discovered at the bottom of the Indian Ocean (C. marina), (40) and another within human cerebrospinal fluid (C. hominis), (46). The latter not does not produce cellulolytic enzymes but is classified as Cellulomonas due to its similar 16S rRNA and its phenotypic traits. The cellulolytic action of Cellulomonas combined with its ability to be facultatively anaerobic makes the gut or rumen of various animals and termites an ecological niche. Cells are sporadically detected in the sludge of ruminant livestock such as cows and sheep, and in the gut of termites. The relationship between these organisms is symbiotic, where in the Cellulomonas bacteria provides the cellulolytic enzymes that the ruminant or termite does not possess, and the host supplies a constant source of plant matter for fermentation, (39).
(Hover mouse left or right to move through the images)
DEGRADATION OF CELLULOSE
Production of cellulases (in the form of a variety of different glucanases, (41), (42) and xylanases) induces the cellulolytic action of Cellulomonas bacteria.
Cellulose is firstly hydrolysed by endoglucanases and release α-cellodextrin, which is then hydrolysed by exoglucanase to produce cellobiose. With the mediation of β-glucosidases, the
Fig 23: Mechanism of cellulose degradation
cellobiose is hydrolysed to glucose, (43).
​
A notable example from the Cellulomonas genus is Cellulomonas fimi that produces a unique, semi-processive enzyme that has partial endo- and exo-glucanase activities (coded by the CenC gene), (47). CenC works in combination with other enzymes held together by a cluster of scaffolding proteins to hydrolyse cellulose into glucose, which is used by Cellulomonas bacteria for respiration, (43). There is particular interest from companies producing biofuels to study species from Cellulomonas because this diversity of enzymes is likely to provide more efficient enzymes than other species. This feature also gives the organisms a competitive edge in environments populated by other bacteria. It has been found that many strains of Cellulomonas bacteria produce greater yields of cellulases when not in the presence of xylan, galactomannan, starch, or sugars, (48). This suggests that hydrolysis of cellulose is a mechanism to survive in low nutrient environments during times of food stress.
