Further Experiments to Determine the Organisms Responsible for Decomposition of Cellulose in Soils

It is well known that the ability to decompose cellulose is common to a number of soil microorganisms. Species of a half dozen or so genera of true bacteria, most soil molds except the Mucorales, and a considerable proportion of the actinomycetes of the soil (Streptonmyces spp. Waksman and Henrici, '43) are able to use cellulose as the sole source of energy. However, it is not entirely established which organisms in nature are responsible for decomposing the immense amounts of cellulose plant residues that become incorporated in the soil. Waksman and Skinner ('26) several years ago found that when finely divided filter paper was mixed with soil, the numbers of molds, but not of bacteria, increased, as determined by the plate method. This was true so long as the soil was not waterlogged and anaerobic. However, in soils saturated with water, the only organisms found to increase were anaerobic bacteria. Both these findings were confirmed by the Conn-Winogradsky direct microscopic technique. It was concluded that only bacteria are instrumental in decomposing cellulose in water-logged soils, but that molds are primarily, probably solely, concerned in the decomposition of cellulose in aerated soils. While these workers amassed many data, their conclusions were not universally accepted. Winogradsky ('27) pointed out that the medium used for counting the bacteria was not suitable for the growth of cellulose-decomposing bacteria. This is true as far as "obligate" cellulose decomposers, such as Cytophaga, are concerned. These bacteria require cellulose, compounds of cellulose, or unautoclaved glucose, and hence will not grow on the medium used by Waksman and Skinner. The "facultative" cellulose decomposers, such as Cellulomonas, grow well on the medium used. However, it is admitted that there might have been a large number of cellulose decomposers present under aerobic conditions, along with the molds, and the technique used would not have detected them. Dubos ('28), working in the same laboratory as Waksman and Skinner, attempted to determine whether bacteria as well as molds increased in soils to which filter paper was added. He counted cellulose-decomposing bacteria by the dilution-extinction method and the molds by the usual plating method. Briefly, he found that as far as soils under anaerobic conditions, and non-water-logged soils under distinctly acid conditions were concerned, the conclusions of Waksman and Skinner held, but that in nonacid aerated soils, both molds and cellulose-decomposing bacteria greatly increased in numbers when paper was added, over soils to which no paper was added. He concluded that in these soils both molds and bacteria were active in the decomposition of cellulose. This was the natural conclusion. In recent years the authors have .become more and more dubious of the reliability of the conclusions of Dubos, as they have become more and more aware of the unreliability of the dilution-extinction method of counting when a limited number of replicate tubes is used. Halvorson and Ziegler ('33) showed that when 10 tubes of each of three dilutions are used, 97 per cent of all possible values will be found within a range which varies from 0.44 to 2.5 times the most probable number; and if 5 tubes are used per dilution, the range is from 0.28 to 3.33 times the most probable number.