The cytology and life-history of bacteria

145

SUMMARY: The generation times of four species of organisms have been measured, each under several sets of conditions : Aerobacter cloacae, Serratia marcescens, Streptococcus f aecalis and Pseudomonas aeruginosa. Minor variations in the experimental conditions appear to affect the mean generation time less in large samples than in small. This can be explained as a result of association between the generation times of closely related organisms. Positive correlation between the generation times of sisters, cousins and perhaps second cousins shows that the influence of an ancestor is felt through two or three generations. The observed correlation between mothers and daughters is usually small, probably because of bias due to the interval between fission of cytoplasm and fission of cell wall. The coefficient of variation of generation time is not a constant for the species but it is stable under given circumstances. It is possibly related systematically to the chemical complexity of the growth medium. In unhampered growth, less than 1% of the organisms produced are non-viable. There is positive association between the viabilities of sisters, and between the viability of an organism and the generation time of its mother. The distribution of generation times can be represented by a Pearson Type I11 or else a Pearson Type V distribution ; both are convenient in applications.The generation time of an individual is considered to be determined partly by molecular accidents, partly by heredity.In a previous paper on this subject (Powell, 1955), I gave an account of some preliminary work designed to test and compare the hypotheses of Rahn (1932) and Kendall (1948, 1952) about the causes of variability in the generation times ( 7 ) of individual bacteria. It turned out that a comparison of the observed distributions of 7 with those predicted by the hypotheses was not altogether satisfactory, and I suggested that a further test might be made by determining the effect of alterations in temperature and growth medium on the coefficient of variation of 7. At the same time I concluded that such experiments should await improvements in observational technique. There appeared to be a possibility that the relevance of the observations was altogether vitiated by ' delayed fission ', that is, by the lapse of an appreciable and variable interval between the termination of the processes considered by Rahn and Kendall, and the overt occurrence of fission.The results which I now present consist of generation time measurements on unicellular organisms of four species-Aerobacter cloacae, Serratia marcescens, Streptococcus faeculis and Pseudomonas aeruginosa-each under several different growth conditions. The undertaking was originally encouraged by the development of an improved culture chamber (Powell, 1956a), which would permit much greater accuracy in the determination of times of fission. At first, I hoped that the results would be of enhanced diacritical value, but it soon became clear that the generation times of bacteria do not depend ...

Section: Methodsmentioning

confidence: 99%

An Outline of the Pattern of Bacterial Generation Times

Powell¹

1958

145

“…estimated because acid-Giesma staining (Bisset, 1950) of W organisms reveals 1-4 chromatinic bodies per organism. These low mutation rates are compatible with the fact that under conditions prevailing in continuous culture, W organisms are stable.…”

Section: J N Coetzee and T G Sacksmentioning

confidence: 99%

Morphological Variants of Proteus hauseri

Coetzee

Sacks

1960

SUMMARYCultures of Proteus hauseri may consist of one or more of five colonial variants. Population pressure experiments started with one variant eventually yielded all other variants. Y variants form raised non-swarming colonies on a MacConkey-type agar at 37O, but swarm in concentric step-like rings a t room temperature on this medium; they swarm in step-like concentric rings on nutrient agar a t room temperature and at 37'. This variant is stable under conditions of logarithmic growth and is regarded as the wild type. Variant X is a typical Rough form and does not swarm under any circumstances. In continuous culture it mutates to the Y type a t a high rate. The W variant forms colonies very similar to those of the Y variant, but like the X variant does not swarm. It is stable under conditions of continuous culture, but mutates on MacConkey agar to X and Y variants at low rates. On MacConkey agar at 37' the Z type appears as a flat amorphous colony surrounded by a continuous thin film of swarm. It also swarms at room temperature. This variant is replaced by a mixture of Y and W colonial forms in continuous culture experiments.Drop-like colonies, the ' Tropfchenform' previously described by Weil & Felix (1917) were also encountered. Drop-like colonies are unpredictable in appearance and were not studied in detail.Hanging-drop preparations of 4 hr. old broth cultures at 37" of either W or X colonial variants (which never swarm) reveal many motile forms. Young broth cultures of the other three variants consist of forms which are all actively motile.Lytic phage for a particular strain is active on all its variants except the X or Rough type which is also the only variant to form unstable suspension in saline and in broth. The Y, W and Z variants described here correspond reasonably well to the A, B and C phases of Belyavin (1951). The H, 0 terminology of Weil& Felix (1917) is not suitable and the nomenclature of Belyavin (1951) may be more satisfactory.

“…Even the very small swarm cells, with their single polar flagella, are capable of motility in the thin film of moisturg on an agar surface, where normal spirilla are usually completely immobilized. In appearance, these swarmers closely resembled vibrios, and although not strictly variants as in the case of the bacillary branched peritrichously-flagellate or hyphomicrobial forms, they afford further evidence of the ability of spirilla to produce or simulate most of the characters of the bacterial groups to which, on the hypothesis of Bisset (1950Bisset ( , 1955, they may be considered as ancestral. fig.…”

Section: Discussionmentioning

confidence: 99%

“…The theory of bacterial evolution proposed by Bisset (1950) and subsequently elaborated (Bisset, 1955), depends for its validity upon the potentialities of spirilla as forebears of all or most of the main groups of bacteria. The postulated development of different flagellar types from polar-flagellate aquatic ancestors appears to be acceptable to certain workers in this field (Leifson, 1960, p. 18) by whom the theory has been adopted.…”

Section: Discussionmentioning

confidence: 99%

See 1 more Smart Citation

Morphological Variation in Spirillum spp., with Observations upon the Origin of the Hyphomicrobia

Bisset

1961

SUMMARYPure cultures of Spirillum serpens grown on solid medium contained variants of very diverse morphological appearance ; these were relatively straight rods, and branched coccobacillary and coccal forms. Flagella were in the normal lophotrichous arrangement, peritrichous, single or absent. Some forms possessed characters resembling those of the hyphomicrobia. The development of Rhodomicrobium in cultures of photosynthetic spirilla was also observed. It is suggested, not only that the spirilla possess potentialities consistent with a suggested role as the ancestral bacterial type, but also that Rhodomicrobium may be a growth form of a photosynthetic spirillum. Appearances found in the parasitic Hyphomonas polymorpha and in certain pleuropneumonia-like organisms were considered to support this concept.