PLATES XV AND XVIB A c T ER I o c I N ES represent a unique class of proteinaceous bactericidal substances that are produced by certain strains of bacteria and exhibit a narrow range of effectiveness against other strains of the same or closely related species (Nomura, 1967;Brandis and Smarda, 1971 ;Reeves, 1972). Besides those on which a great amount of work has been done, there are others that are still poorly characterised ; and there may even exist inhibitory phenomena that have mistakenly been attributed to the action of bacteriocines. The latter seems to be true of some antagonistic phenomena observed among group-A streptococci.Sherwood et al. (1949) were among the first to report on the inhibitory action of certain streptococcal strains of various serological groups on other strains of streptococci. They named the inhibitory substance " streptostasin " without relating it to the class of substances later called bacteriocines. More recently, several investigators (Kuttner, 1966 ;Kolesnichenko, 1967 ;Overturf and Mortimer, 1970;Kolesnichenko and Totolyan, 1971 ; Totolyan and Kolesnichenko, 197 1) have studied the production of antibacterial substances by group-A streptococci from the epidemiological point of view. These authors claimed that the inhibitory agents were bacteriocines, despite the fact that the respective substances had rather unusual properties. Thus, inhibitory activity was demonstrable on agar only when the producing and the indicator strain were growing actively. Cell-free filtrates or supernatant fluids from liquid cultures, as well as bacteria disintegrated by various means, were inactive; likewise, chloroformed cultures showed no activity. The inhibitory substances were insensitive to trypsin and were active against the homologous producer strain (Kuttner, 1966).In this paper we present evidence to show that there exist inhibitory phenomena among group-A streptococci that are attributable to the toxic action of hydrogen peroxide produced by the test strains rather than to bacteriocine production. MATERIALS AND METHODS Strains.Representative group-A streptococcal strains of many different serotypes and from various sources were used in this study.

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