The Selection Of Morphologically Favourable Strains

The morphology of a micro-organism in submerged culture frequently has an effect on the economics or ease of operation of a process. The morphological form of a filamentous micro-organism will affect both the aeration of the system and the ease of filtration of the fermentation broth. As discussed in Chapter 6, the morphology of a fungus in submerged culture may be controlled by the level of a spore inoculum and by the medium, but it is also possible to influence the organism's morphology by altering its genotype. Backus and Stauffer (1955) recognized the influence of the genetics of a strain on the morphology of P. chrysogenum in submerged culture and its role in controlling foaming and broth filtration characteristics.

Bartholomew et al. (1977) selected strains of P. chrysogenum (for penicillin production) which gave a lower viscosity broth which increased the oxygen-transport ability of their fermentation plant equipment. The use of mutation and screening compared with recombination for the production of strains of a particular morphological type has been discussed by Ball (1978). Ball claimed that recombination techniques would be at an advantage over mutation and selection because of the large number of mutants that would have to be screened. A recombination programme would involve the crossing of a strain of the required morphology with a commercial producer, which should involve the screening of relatively few progeny. Hamlyn and Ball (1979) applied the technique of protoplast fusion to the construction of a desirable stain of C. acremonium for the production of cephalosporin, as discussed in an earlier section. A cross between an asporulating, slow-growing strain with a sporulating, fast-growing strain which only produced one third of the cephalosporin level of the first strain eventually resulted in the isolation of a recombinant which displayed the desirable properties of both strains, i.e. a strain which demonstrated good sporulation, a high growth rate and produced 40% more antibiotic than the higher-yielding parent. The ability of a strain to sporulate profusely is a very useful characteristic in the development of inoculum, as discussed in Chapter 6.

Rowlands (1992) reported that genetically altering P. chrysogenum strains to produce a pelletted rather than a filamentous broth was an important feature in the Panlabs penicillin improvement programme. The pelletted form gives rise to a much lower broth viscosity, resulting in lower power consumption. Although Rowlands did not indicate which genetic technique was used (mutant selection or protoplast fusion), protoplast fusion was certainly used to improve the sporulation and growth of the Panlabs strain, as discussed in a previous section (Table 3.7).

Flocculation of yeasts is the adherence of cells in clumps resulting in the separation of the cells from the liquid in which they were suspended. Thus, the flocculating property of yeasts may be described as a morphological characteristic. When flocculation occurs in a beer fermentation the yeast will rise to the top of the vessel if it is a 'top-fermenting' yeast or drop to the bottom of the vessel if it is a 'bottom-fermenting' yeast. Thus, the flocculence of a yeast will determine the time of contact of the yeast with the wort (and hence the conversion of the wort to alcohol) and the ease of clarification of the beer. Although mutant selection is rarely applied, the selection of natural variants is a very common practice in many breweries. As discussed in Chapter 6, the yeast produced from one beer fermentation may be used to inoculate a new fermentation, a practice which is very rarely employed in the rest of the fermentation industry. By the careful selection of the yeast produced during a fermentation, strains showing the desired degree of flocculence may be selected. The first cells to rise to the top of a top fermentation, or drop to the bottom of a bottom fermentation, are the most flocculent cells whereas the last to rise or drop (depending on the type of fermentation) are the least flocculent cells. Thus, by selecting those cells that flocculate at an intermediate time the brewer isolates the natural variants which have the most desirable morphological properties for the inoculation of the next fermentation.

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