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THE more detailed work on the quantitative estimation of bacteriostatic substances has been concerned with the assay of penicillin, and the most generally favoured method is the 'cylinder' method as given by Abraham et al.1, amplified by Heatley2 and discussed by Foster and Woodruff3. On beginning our investigation in 1941, the above method was tried but, though apparently satisfactory for its original specific purpose, we found it to be less so for the widespread estimation of the bacteriostatic effect of fungus extracts and metabolism solutions than a method devised by ourselves4. Briefly, this method consists of putting a few drops of the liquid to be tested into a circular hole cut in the centre of a plate of bulk-seeded agar medium, with the subsequent production, after incubation at 37° C., of a zone of bacterial inhibition the width of which varies in proportion to the concentration of the bacteriostatic substance. This method only differs from somewhat similar methods mentioned by other workers in that the standardized technique enables it to give a relatively quantitative estimation.

WHEN an avirulent fungus infects resistant plant tissue it often kills only a small number of cells and remains confined to the dead cells. This acute, very localized reaction is referred to as a hypersensitive response. When the pathogen is not an obligate biotroph, death of these cells need not explain why it does not continue to grow. In certain diseases caused by such pathogens growth is prevented because substances toxic to the pathogen accumulate at the locus of infection. Such substances are now commonly referred to as phytoalexins; in diseases of parenchyma there is some evidence that they are largely confined to the killed cells or those immediately adjacent1.

A NUMBER of polypeptides composed of various basic amino-acids have been prepared by Katchalski et al., and it had also been stated that agglutination of Grain-positive and Gram-negative bacteria was caused by these polypeptides, and bacteriostatic or bactericidal activity was observed depending on their concentration1. Stahmann and his co-workers2 have reported that the multiplication of some viruses was restrained by poly-L-lysine.

RECENTLY1 we published a method which was specially designed to estimate the anti-bacterial activity of certain fungi, for example, the Basidiomycetes, which are difficult to grow on liquid media. Briefly, the method consisted in cutting a disk from a growing fungus colony and dropping it into a plate of warm agar which had been bulk-seeded with bacteria. In case of a positive result a zone of bacterial inhibition was produced round the disk. The method stated that "all disks are cut at approximately the same distance from the edge of the colony". An attempt to determine what distance from the edge of the colony would give the best result brought out certain objections to the method and led to the present modification.

The Battle Against Bacteria: A Fresh Look. By P. Baldry. Pp. 179. (Cambridge University: Cambridge, London, New York and Melbourne, 1976). £4.50.