Cellulose extracted from wheat straw, Avicel and CF11 cellulose powder contained 90% (w/w) glucose, whereas filter paper and carboxymethylcellulose contained 18% and 28% (w/w) hemicelluloses, respectively. The straw cellulose was used as the sole carbon source to enrich for a mixed culture of micro-organisms in a stirred liquid medium and on a perfusion column containing glass beads. Amongst the micro-organisms isolated only fungi had cellulase activity and this activity was greatest in cultures of Cladosporium cladosporoides. The remainder of the community consisted of non-cellulolytic secondary organisms. The interactions of cellulolytic fungi with other micro-organisms found in the enrichment mixed culture are discussed.
A multistage continuous culture system is described in which solutes are transferred between vessels in opposite directions simultaneously. The system, called a gradostat, produces opposing solute gradients and is a good laboratory model of many natural microbial ecosystems in which solute gradients are important. Theoretical predictions concerning solute transfer were confirmed under steady-state and non steady-state conditions, using a coloured dye. Paracoccus denitrificans grew anaerobically in the gradostat at the intersection between opposing gradients of succinate and nitrate. Opposing gradients of glucose and oxygen separated the growth of a Bacillus sp. (a facultative anaerobe) and Clostridium butyricum (an obligate anaerobe). Viable counts for both species fell exponentially away from their growth positions at the ends of the gradostat. The potential value of the gradostat and possible alternative conformations are discussed.
Gradients of nutrients are extremely common in nature, and this paper decribes changes in the physiology of Escherichia coli grown in the gradostat, a series of five linked vessels with opposing gradients of glucose and of oxygen plus nitrate. Most growth occurred at the aerobic and anaerobic ends of the system. High rates of respiration, high energy charge and high activities of various oxidative enzymes were seen in the two most aerobic vessels; however, oxygen provision was presumably poor, because nitrate reductase activities were also high in this region. Vessels 3 and 4 showed the lowest values for respiration rate, enzyme activity and energy charge, and cells here were both nutrient starved and possibly inhibited by nitrite. Vessel 5 was highly anaerobic, resulting in the presence of hydrogenase activity. It was concluded that cells found in different regions of the gradostat had undergone biochemical differentiation in spatial gradients of electron donors and acceptors.
The structured nature of microbial ecosystems makes their study difficult, and simple laboratory analogues are needed. Two gel-stabilized gradient systems are described in which solute transfer is by diffusion alone. In the first, organisms grow in a semi-solid agar gel located above a source layer of full-strength agar containing the diffusible solute, which was glucose in the experiments reported here. Changes in physicochemical parameters, various solutes and cell concentration have been monitored in cultures of Bacillus megaterium NCTC 10342 and Lactobacillus confusus NCIB 4037 grown in this system. In experiments with a range of bacteria, banded growth was noted for several strains. The microbiology of the water at the base of an oil storage tank was investigated in the second model, in which gas oil was poured over a semi-solid layer containing agar, a basal salts medium, cells and a steel plate. After incubation for up to 90 d the system had differentiated into aerobic and anaerobic regions, and activities included hydrocarbon catabolism, oxygen removal, sulphate reduction, and the growth of a large population of aerobic and anaerobic heterotrophs. The value of these models is discussed with reference to microbial ecology.