SUMMARY: The variations in Vibrio cholerae studied included changes from Ogawa to Inaba type, smooth+rough, and motile-+non-motile. With a sloppy-agar technique it was possible to estimate the rate of formation of thcse variants (i.e. probable mutation rate 1 in 1 0 5 and 1 in lo4, respectively). It was possible to show that the action of antiserum in promoting the change from Ogawa to Inaba was selective rather than mutagenic. With the other two variations studied no selective process had to be considered owing to their high spontaneous rate of appearance. It may be said that all three variations studied were due to spontaneous mutants in the parent cultures giving rise to these forms.The studies of Gardner & Venkatraman (1935) showed that vibrio species could be separated on the basis of their somatic '0' antigens, while the flagellar 'H' antigens were widely shared. Since then the differentiation of Vibrio cholerae, the pathogen of cholera, from similar cholera-like vibrios, rests on its agglutinability with specific '0 ' antisera. V . cholerae constituted 0 group I, in this scheme of classification, along with certain haemolytic 'El Tor' vibrios. The definition of V. cholerae as a non-haemolytic vibrio belonging to 0 group I (Gardner & Venkatraman) is now generally accepted, and 0 antisera prepared with heat-killed suspensions are almost universally used for the identification of V . cholerae, in the absence of any reliable biochemical critera. Gardner & Venkatraman (1935) also confirmed the observations of early Japanese workers (Kabeshima, 1918; Nobechi, 1923) when they defined subtypes Inaba, Ogawa and Hikojima in V. cholerae which, while sharing group-specific 0 antigens, differed in minor type-specific 0 antigens. Types Inaba and Ogawa were the 'end' types, usually designated by the antigenic symbols AC and AB respectively, while Hikojima formed the intermediate type ARC. I n view of the poor content of antigen C in Hikojima strains, as well as its presence in Ogawa type cultures when grown a t ZOO, Kauffmann (1950) suggested that Ogawa-Hikojima types constituted one form of V . cholerae, as distinct from the Inaba form.Cholera outbreaks in India are usually confined to a single type of Vibrio cholerae in localized areas, though the type may vary from place to place and epidemic to epidemic. Hikojima-type strains seem to be of rare occurrence while Inaba and Ogawa types have contributed to most epidemic outbreaks. The shift between these two types, over several years, in the cholera endemic
SUMMARY : Young actively dividing cultures of Pseudomnaspyocyanea (aeruginosa) at 37" were killed when diluted into cold liquids which were without effect when used above 18". In general the simpler the composition of the diluent the more lethal it was ; distilled water was the most active. The cooling had to be rapid to be effective. Old cultures were only slightly sensitive to this effect. The killed organisms appeared normal when examined under the microscope. Under similar conditions Staphylococcus aureus was resistant t o cold shock.The killing effect of cold diluent on young Escherichia coli was shown by Sherman & Albus (1923) who suggested that the organisms passed through a phase of 'rejuvenation ' before multiplication. Hegarty & Weeks (1940) made an extensive investigation into the sensitivity of E. coli to cold shock and showed that low osmotic pressure and cold were both necessary in order to kill organisms exposed to these conditions. Overnight broth cultures became sensitive within 20 min. of their addition to a suitable medium, i.e. before the first bacterial division took place. The organisms showed maximum sensitivity during the log phase of growth. No other report of this phenomenon appeared until that of Meynell (1958) who showed, with E. coli, that the number of organisms which appeared to be killed by cold shock depended upon the medium on which they were subsequently plated. The importance of the composition of the diluent and the speed of cooling were also emphasized. Cultures which had suffered cold shock showed no decrease in numbers on direct counting and the bacteria looked normal microscopically. The present investigation stemmed from the observation that direct viable counts of young shaken cultures of Pseudomonas pyo yanea (aeruginosa) diverged from those expected from the turbidity of the cultures. This paper reports work on the sensitivity of P. pyo yanea to cold shock. METHODS
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