Bacterial and other cell suspensions

The contamination of pharmaceutical suspensions for oral use has recently caused concern. The behaviour of organisms in such suspensions is therefore of some interest before interpretation of experimental results can be undertaken. Under some conditions, bacteria may be strongly adsorbed and therefore more resistant to the effects of preservatives in other cases, the bacteria may be free in suspension. 

Two distinct phases of bacterial sorption on to glass have been observed [78] the first reversible phase may be interpreted in terms of DLVO theory. Reversible sorption of a non-mobile strain (Achromobacter) decreased to zero as the electrolyte concentration of the media was increased, as would be expected. The second irreversible phase is probably the result of polymeric bridging between bacterial cell and the surface in contact with it. It is obviously not easy to apply colloid theory directly but the influence of factors such as ij/o, pH and additives can be predicted and experimentally confirmed. 

The flexibility of surfaces and the formation in the cells of pseudopodia with small terminal radii of curvature will obviously complicate the application of theory. In the agglutination of erythrocytes and the adsorption of erythrocytes to virus-infected cells, projections of small radius of curvature have been observed. Such highly curved regions could well account for local penetration of the energy barrier and strong adhesion at the primary minimum. 

Sorption of microbial cells is selective and there is no obvious relation between gram-staining characteristics and attachment. 

Lysis of the cells at higher CTAB concentrations increased the positive zeta potential of the cell wall. 

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