**6. Cellulose-degrading microorganisms**

Since cellulose is very difficult to degrade as a component of plant cell walls, only a few mi‐ croorganisms specialized for plant cell wall degradation can hydrolyse cellulose. Among these, anaerobic and aerobic genera of Domain Bacteria and fungi of Domain Eukarya are included.

Generally speaking, two types of systems occur in regards to plant cell wall degradation by microorganisms. In one type, the organism produces a set of free enzymes that act synergis‐ tically to degrade plant cell walls. In the second type, the degradative enzymes are organ‐ ized into an enzyme complex located in cellular surface called the cellulosome. This complex is very effective in degrading plant cell walls [52].

Anaerobic and aerobic bacteria have different strategies to degrade cellulolytic substrates; whereas anaerobic bacteria degrade cellulose using cellulosomes, aerobic bacteria secretes enzymes capable of degrading cellulose that freely diffuse to reach the substrate.

Anaerobic bacteria of the order *Clostridiales* (Phylum *Firmicutes*) are generally found in soils, decaying plant waste, the rumen of ruminant animals, compost, waste water, and wood processing plants; these bacteria have also been found in insects like termites (*Isopteran*), bookworm (*Lepidoptera*), and so, in a symbiotic relationship in their guts responsible for cel‐ lulosic feed digestion. Anaerobic hydrolysis represents 5% to 10% of global cellulose degra‐ dation [53-55].

Aerobic bacteria with cellulolytic activities of the order *Actinomycetales* (phylum *Actinobacte‐ ria*) have been found on soils, water, humus, agricultural waste (sugar cane) and decaying leaves, these bacteria excretes enzymes capable of degrading cellulose (cellulases) [52]. In aerobic bacteria *Pseudomonas fluorescens* subsp. cellulosa, *Streptomyces lividans* and *Cellulomo‐ nas fimi* cellulolytic systems of degradation have been reported [56-58].

Although considerable progress has been made in elucidating the crystal structures of cellu‐ lose in microfibrils, they are still not well understood, and a deeper understanding of cellu‐

126 Sustainable Degradation of Lignocellulosic Biomass - Techniques, Applications and Commercialization

**Figure 4.** Projections of the crystal structures of cellulose I (α,and β) II and III down the chain axes directions. C, O, and H atoms are represented as gray, red, and white balls, respectively. Covalent and hydrogen bonds are represented as full and dashed sticks, respectively. H atoms involved in hydrogen bonding are explicitly represented for only cellulose IIII. Only the major components of hydrogen bonds are represented. Adapted with permission from [45]. Copyright

Since cellulose is very difficult to degrade as a component of plant cell walls, only a few mi‐ croorganisms specialized for plant cell wall degradation can hydrolyse cellulose. Among these, anaerobic and aerobic genera of Domain Bacteria and fungi of Domain Eukarya are

Generally speaking, two types of systems occur in regards to plant cell wall degradation by microorganisms. In one type, the organism produces a set of free enzymes that act synergis‐ tically to degrade plant cell walls. In the second type, the degradative enzymes are organ‐ ized into an enzyme complex located in cellular surface called the cellulosome. This complex

Anaerobic and aerobic bacteria have different strategies to degrade cellulolytic substrates; whereas anaerobic bacteria degrade cellulose using cellulosomes, aerobic bacteria secretes

Anaerobic bacteria of the order *Clostridiales* (Phylum *Firmicutes*) are generally found in soils, decaying plant waste, the rumen of ruminant animals, compost, waste water, and wood processing plants; these bacteria have also been found in insects like termites (*Isopteran*), bookworm (*Lepidoptera*), and so, in a symbiotic relationship in their guts responsible for cel‐ lulosic feed digestion. Anaerobic hydrolysis represents 5% to 10% of global cellulose degra‐

enzymes capable of degrading cellulose that freely diffuse to reach the substrate.

lose structure is required [50-51].

(2012) American Chemical Society.

included.

dation [53-55].

**6. Cellulose-degrading microorganisms**

is very effective in degrading plant cell walls [52].

Some anaerobic bacteria with cellulolytic activity are Butyrivibrio fibrisolvens, Fibrobacter succinogenes, Ruminococcus flavefaciens, Clostridium cellulovorans, C. cellulolyticum and C. thermocellum [59-61].

Due to the significant diversity in the physiology of cellulolytic bacteria, sometimes is diffi‐ cult to classify bacteria as mentioned above, therefore, on this basis, they can be placed into three diverse physiological groups: (1) fermentative anaerobes, typically Gram-positive, (*Clostridium* and *Ruminococcus)*, but with a few Gram-negative species (*Butyvibrio* and *Aceti‐ vibrio*) that are phylogenetically related to the *Clostridium* assemblage (*Fibrobacter*); (2) aero‐ bic Gram-positive bacteria (*Cellulomonas* and *Thermobifida)* and (3) aerobic gliding bacteria, (*Cytophaga* and *Sporocytophaga)* [1, 53].

The ability to utilize lignocellulosic material is widely distributed among fungi, from chytri‐ diomycetes to basidiomycetes. Among fungi, the most efficient at using wood as substrate are the basidiomycetes, considered the principal taxonomic group involved in the aerobic degradation of wood with all its components, they are the main organic material decompo‐ sition agents. These aerobic fungi produce extracellular enzymes allowing lignocellulose degradation (lacasses, hemicellulases, and cellulases), although some Ascomycetes are able to degrade cellulosic compounds as well. Unlike aerobic fungi, some of the Chytridiomy‐ cetes anaerobic fungi, have multienzymatic complexes similar to cellulosomes of bacteria [1, 3, 62-63] some members are anaerobic species living in the gastrointestinal tract of rumi‐ nants such as *Anaeromyces, Caecomyces, Neocallimastix, Orpinomyces* and *Piromyces.*

Examining the taxonomic composition of cellulolytic fungi inhabiting the decaying leaves and rotting woods of forest soils, zygomycetes are represented by a single genus, *Mucor,* while ascomycetes and basidiomycetes are represented by genera such as *Chaetomium, Tri‐ choderma, Aspergillus, Penicillium*, *Fusarium, Coriolus, Phanerochaete, Schizophyllum, Volvariella, Pycnoporus* and *Bjerkandera.* Two of the most studied fungi, due to their industrial relevance, are *Trichoderma reesei* and *Phanerochaete chrysosporium*.

Nowadays, more than 14,000 fungi, which are active against cellulose and other insoluble fibres, are known [1, 24, 64-66]. A more detailed list of cellulose degrading bacteria and fun‐ gi is listed in Table 5.
