A physical chemical study of the surface of Mycobacterium bovis BCG

Abstract

The electrokinetic properties of five sub-strains of Mycobacterium bovis BCG were studied in an attempt to explain inter-strain differences and the aggregation phenomena characteristic of all strains during growth. Cells of all strains behaved as a typical macropolyanion. Studies of the variation of charge with the pH and ionic strength of the suspension medium before and after chemical and enzymic treatments revealed that the surface charge of these organisms was due solely to ionizable phosphate groups. Anion adsorption on the surface was negligible and no carboxyl, amino or sulphydryl groups could be detected. The charge on cells of all strains was constant and independent of the growth medium, the age of the cells up to 28 days and storage of the washed cells at 4 °C. The large quantity of lipid material, known to be present in the cell walls of mycobacteria, was not detectable at either the inner or outer surfaces of the cell wall. It was concluded that the lipid must be located in the middle regions of the wall.

Cells of M. phlei, M. smegmatis and M. microti exhibited surface charge properties which were identical to those of all sub-strains of BCG. The results are discussed in the light of proposed cell wall structures of mycobacteria and it is concluded that the phosphate groups, which form the predominant charging species and which therefore occur at the surface, are the phosphodiester groups linking the peptidoglycan backbone with the arabinoglactan of the cell wall.

A "slime-like" amorphous material which coats aggregates of cells, but not individual cells, was revealed by stereoscan electron microscopy. This substance, which is easily removed by manual tissue-grinding, is believed to be responsible for the aggregation of mycobacterial cells during growth; physico-chemical, forces arising from the charged surface groups play no part in this aggregation phenomenon.