The streptococcal plasmid ERL1 determining inducible resistance to erythromycin, lincomycin, and staphylomycin S was isolated by dye-buoyant density centrifugation and shown to have a molecular weight of about 17.5 Mdal, as revealed by sedimentation through neutral sucrose gradients. In SM60 cells entering the stationary phase its covalently closed circular form was present to the extent of 5 copies per chromosomal genome equivalent, ERL1 was subject to the DNA restriction and modification mechanism discovered in strain 56188. It did not appear to exercise restriction of phage DNA but mediated a partial release of the restricted growth of A25.
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.
The inactivation of yeast cells in different growth phases by an electric field pulse was investigated. Cells of Saccharomyces cerevisiae in the logarithmic growth phase were found t o be much more sensitive with respect to a n electric discharge than those in the stationary phase. The influence of the electric field pulse characteristics on the inactivation as well as possible secondary effects were studied. The polyene antibiotic perhydrohexafungin (PHI?) is used as a tool to sense defects in the yeast cell envelope brought about by electric field action. The repair kinetics of these defects was followed after the impulse. At least two repair stages can be distinguished, a fast one in the second range and a slower one which takes place after plating the cells on a nutrient medium. The obtained results are discussed in connection with current theories of reversible dielectric breakdown in biological membrane systems.
The aim of this study was to check the action of electric field pulses (1) on the survival of intact cells and protoplasts of Bacillus cereus and ( 2 ) on the transformation frequency of these protoplasts with plasmid DNA from Bacillus thuringiensis transformants. -B. cerew cells and protoplasts are very resistant to high electric field pulses in the microsecond range. The transformation frequency of B. cereus protoplasts with plasmids from B. thuringiensis (PUB 110) can be increased by about one order of magnitude with the electric field pulse technique. The transformants are stable and PUB 110 is preserved as an extrachromosomal element.
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