WHY DO BACTERIAL PROTOPLASTS BURST IN HYPOTONIC SOLUTIONS

A study of the relative effectiveness of sugars, of short peptides and of amino acids as osmotic stabilizers for nonrespiring Bacillus megaterium protoplasts indicated that the protoplast membrane could act as a porous differential dialysis membrane in much the same manner as cellophane membranes do and that its effective porosity increased when it was stretched during osmotic swelling. The protoplast membrane also behaved as a highly extensible structure, in contrast to membranes such as those of erythrocytes and enormous protoplasts could be prepared by slowly dialyzing stabilizing solutes from protoplast suspensions. In effect, osmotic bursting of bacterial protoplasts could not be related to the ultimate tensile strength of maximally extended membranes. Rather, it appeared that when protoplasts swelled in hypotonic solutions, their surface membranes might become sufficiently stretched so that they admit stabilizing solutes. There would be then a rapid influx of solutes and water resulting in rapid stretching of the membrane and rupture due to a process of brittle fracture. Thus, bursting generally occurs without the membrane becoming fully extended. We also found that urea and glycerol induced mechanical relaxation of protoplast membranes. This finding, together with our other findings concerning mechanical properties of protoplast membranes and the strengthening effect of formaldehyde treatment, suggests that the mechanically important membrane component is protein, possibly in the form of a microfibrillar array. © 1969.