Invited Review: Adhesion Mechanisms of Rumen Cellulolytic Bacteria

摘要

We divided the adhesion process of the predominant cellulolytic rumen bacteria Fibrobacter succinogenes, Ruminococcus fiavefaciens, and Ruminococcus albus into four phases: 1) transport of the nonmotile bacteria to the substrate; 2) initial nonspecific adhesion of bacte- ria to unprotected sites of the substrate that is domi- nated by constitutive elements of bacterial glycocalyx; 3) specific adhesion via adhesins or ligands formation with the substrate, which can be dominated by several bacterial organelles including cellulosome complexes, fimbriae connections, glycosylated epitopes of cellulose- binding protein (CBP) or glycocalyx, and cellulose-bind- ing domain (CBD) of enzymes; 4) proliferation of the attached bacteria on potentially digestible tissues of the substrate. Each of the phases and its significance in the adhesion process are described. Factors affecting bacterial adhesion are described including: 1) factors related to bacterial age, glycocalyx condition, and mi- crobial competition; 2) factors related to the nature of substrate including, cuticle protection, surface area, hy- dration, and ionic charge; and 3) environmental factors including pH, temperature, and presence of cations and soluble carbohydrate. Based on the information avail- able from the literature, it appears that each of the predominant rumen bacteriang succinogenes, R. fia- vefaciens, and R. albusthas a specific mechanism of adhesion to cellulose. In F succinogenes, both the glyco- sidic residues of the outer membrane CBP and espe- cially of the 180-kDa CBP, and the distinct CBD of EG2 EGF and Cl-stimulated cellobiosidase, may play a role in the adhesion to cellulose. No direct evidence, except scanning electron microscopy observations, yet sup- ports the existence of either cellulosome complex or fimbriae structures involved in the adhesion mecha- nism of F succinogenes. At least two mechanisms, cellu- losome-like complexes and carbohydrate epitopes of the glycocalyx layer are involved in the specific adhesion of R. fiavefaciens to cellulose. Ruminococcus albus pos- sesses at least two mechanisms for specific adhesion to cellulose: a cellulosomal-like mechanism, and a CbpC (Pil)-protein mechanism that probably involves the pro- duction of fimbrial-like structures. Indirect and direct studies suggested that carbohydrate epitopes of CBPs and CBD epitope of cellulases may also be involved mostly in the nonspecific phase of adhesion of R. albus.