Graduate Thesis Or Dissertation
 

Fresh water myxobacteria : a taxonomic study

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  • Myxobacteria are known to occur in the aquatic environment, however, little information is available regarding the taxonomy of these organisms. This investigation was initiated in an attempt to classify a group of fresh water cytophagas on the basis of biochemical and physiological as well as cultural and morphological characteristics. The 35 isolates used in this investigation were determined to be myxobacteria on the basis of their cellular morphology. Vegetative cells were slender, gram negative, weakly refractile rods characterized by a marked flexibility. Examination of young cells revealed typical gliding motility. Cells became shorter and thicker with age and involution forms occurred. The thin, spreading, yellow to yellow-orange colonies produced by the organisms were typical of myxobacteria. Fruiting bodies were never observed although microcyst-like structures were formed by two isolates. As a result these two organisms are considered to be members of the genus Sporocytophaga. The remaining 33 isolates can be classified in the genus Cytophaga. The isolates were non-halophilic mesophiles which grew best at pH 7.2. All grew anaerobically under defined conditions and most were moderately thermostable. All but one of the isolates were resistant to neomycin and most were sensitive to tetracycline, streptomycin, erythromycin and novobiocin. The results of the physiological studies showed that the myxobacteria studied in the present investigation have the capacity to degrade macromolecules. All of the organisms were lipolytic and most were proteolytic and amylolytic. Cellulolytic activity as demonstrated by the utilization of carboxymethyl cellulose was exhibited by over 70% of the isolates. All of the organisms lysed dead cells of Aerobacter aerogenes and the majority were capable of lysing dead cells of a variety of other bacteria and yeast. A small portion also lysed similar preparations of algal and protozoan cells. Simple carbohydrates were oxidized by most of the strains, however, very few of the organisms were able to ferment these substances. Carbohydrates oxidized by the majority of the isolates include glucose, galactose, maltose and cellobiose. Of the fermentative strains, only one required substrate amounts of CO₂ for glucose fermentation. This isolate may have a metabolic pathway similar to Cytophaga succinicans, a facultative anaerobic myxobacterium which carries out a CO₂-dependent fermentation of glucose. The nutritional requirements of the organisms were relatively simple. The nitrogen requirement could be readily satisfied by either casein hydrolysate or KNO₃ and starch served as a sole source of carbon for most of the isolates. However, only 12 of the 35 strains were able to utilize glucose as a sole source of carbon. Simple amino acids did not support the growth of any of the isolates when used as a sole source of carbon, The DNA base composition and carotenoid pigments of six of the isolates also were studied. Five of the organisms were found to have a DNA base composition between 34.88 and 38.54% GC. A value of 53.44% GC was found for one isolate identified as a Sporocytophaga. The results of the pigment analysis indicated that carotenoid pigments spectrally similar to lutein, alpha-carotene-5, 6-epoxide and rhodopin were present in the organisms. It appears from these studies that pigment analyses of the myxobacteria could contribute significantly to the taxonomy of this group of organisms. Based on the results of this investigation, an improved taxonomic scheme for the genus Cytophaga has been proposed. Unlike the taxonomic keys which are currently available, the major subdivisions of this scheme are based on biochemical and physiological characteristics. Key features for distinguishing members of the genus Cytophaga according to the proposed scheme include use of carboxymethyl cellulose, chitin, citrate and carbohydrates as well as nitrate reduction.
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