Cell envelope composition and organisation in the genus Rhodococcus.

A knowledge of the organisation of the rhodococcal cell envelope is of fundamental importance if the environmental and biotechnological significance of these bacteria are to be understood and successfully exploited. The genus Rhodococcus belongs to a distinctive suprageneric taxon, the mycolata, which includes among others the genera Corynebacterium, Mycobacterium and Nocardia. Members of this taxon exhibit an unusual complexity in their cell envelope composition and organisation compared to other Gram-positive bacteria. Models that describe the architecture of the mycobacterial cell envelope are extrapolated here to provide a model of the rhodococcal cell envelope. The rhodococcal cell envelope is dominated by the presence of an arabinogalactan cell wall polysaccharide and large 2-alkyl 3-hydroxy branched-chain fatty acids, the mycolic acids, which are covalently assembled into a peptidoglycan-arabinogalactan-mycolic acid matrix. This review further emphasises that the mycolic acids in this complex form the basis of an outer lipid permeability barrier. The localisation and roles of other cell envelope components, notably complex free lipids, lipoglycans, proteins and lipoproteins are also considered.