Graduate Thesis Or Dissertation

Ecological study of aquatic myxobacteria in a woodland stream

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  • Most studies of bacteria in water have concerned public health; little attention has been given to organisms which are indigenous to the aquatic environment. Myxobacteria are known to occur in soil, in the marine environment, and several have been studied in relation to diseases of salmonid fishes. However, little information is available regarding the role of myxobacteria in the freshwater environment. The major purpose of this research was to obtain data on the occurrence, distribution, and activities of aquatic myxobacteria in a woodland stream. As a preliminary step for studying the ecology of myxobacteria in the freshwater stream, a culture medium and procedure were developed to provide a means for isolation and enumeration of these organisms. Enumeration of myxobacteria was based on the morphology of the colonies growing on the dilute nutrient medium, cytophaga peptonized milk agar (CPM). The low nutrient concentration of this medium favored spreading of the myxobacterial colony and the production of rhizoid edge patterns which are typical of myxobacteria and distinguishable from eubacterial colonies. The enumeration method was used to obtain data on the occurrence and distribution of myxobacteria as compared to the total bacteria population in Berry Creek. Results of the ecological study conducted over a two and a half year period indicate that myxobacteria are present in this aquatic environment all during the year with highest levels obtained in the fall (October and November) and lowest levels during the summer (July and August). Seasonal variations also occurred in the types of myxobacteria comprising the population of the stream. It is interesting to note that peaks in the myxobacterial and the total bacterial populations occurred in advance of the peak in flow rate. Water temperature and flow rate did not seem to influence the population levels as might be expected if the myxobacteria were transient members of the microbial flora. Additional surveys have shown that the myxobacteria are widespread in fresh water; they have been found in abundance in bottom sediments and surface films as well as in the flowing waters. Myxobacteria also appear to be well adapted to the aquatic environment. It was shown that they are able to utilize the dilute nutrients present in water for their growth. In an attempt to determine the possible role myxobacteria play in the aquatic environment, the predominant myxobacterial types were isolated and studied morphologically and biochemically. All the organisms studied corresponded to the classical definition of myxobacterial cells: gram negative, slender, weakly refractile rodshaped bacteria which exhibit gliding motility. Colony morphology of these myxobacteria plated on CPM has been studied in detail and found to be a constant characteristic of the particular type of myxobacteria isolate. Photographs of several of the predominant forms present in the stream samples illustrate the distinguishable morphology of the myxobacterial colonies. Six morphological groups have been arbitrarily designated on the basis of colony morphology; these morphological groups also show similarities in their biochemical capabilities. Biochemical studies on the myxobacterial isolates indicated that most of the strains were able to utilize simple carbohydrates. All of the isolates were capable of degrading various macromolecules, such as chitin, starch, aesculin, caesin, gelatin, and carboxy methyl cellulose. One of the isolates obtained was strictly proteolytic. The ability to hydrolyze macromolecules appears to be characteristic of aquatic isolates as well as of other myxobacteria. One myxobacterium isolated was believed to be intimately associated with the sheathed bacterium, Sphaerotilus natans. The latter organism was abundant in the sucrose and urea enriched section of Berry Creek. This myxobacterium could not utilize sucrose or urea and occurred only in the enriched section of the stream when Sphaerotilus was present. The fish pathogen, Chondrococcus columnaris was also isolated from Berry Creek water. This myxobacterium can be distinguished from the other aquatic myxobacteria on the basis of its unique colony morphology. This strain of C. columnaris proved to be one of the common serological types found in the Pacific Northwest. Based on the results obtained thus far, it is possible to speculate on the role of myxobacteria in the freshwater environment. All of the myxobacteria isolated in this study are capable of decomposing complex materials, it seems likely therefore, that these organisms may be active in the decomposition of such complex organic compounds, including the remains of other bacterial cells, which are present in the aquatic habitat. Since the isolates studied are also able to utilize the nutrients present at low levels in the stream water, these myxobacteria are not dependent on macromolecular substrates. Myxobacteria with these abilities are apparently well adapted to the aquatic environment.
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