Mixed microbial culture technology.

Micro-organisms in the environment usually occur in multispecies populations or communities; in contrast, monospecies populations typical of most industrial fermentation processes and research activities can be thought of as artificial and atypical of the situations in which microorganisms usually grow. Although 19th century microbiologists appreciated the existence of mixed populations, the main driving force in the development of the emerging science was the pure-culture technique with which to investigate, unequivocally, causal agents of infectious disease and spoilage. Intensive studies of mixed microbial cultures in consequence have been neglected and among the contributory reasons may be identified : (1) a firm belief that the pure culture provided the most ?ppropriate means for studying micro-organisms; (2) experimental analysis of complex communities was, by definition, difficult; (3) community complexity was synonymous with community instability; (4) ecology for a long time did not attract the widespread attention of microbiologists; and (5 ) that the exploitation of mixed cultures was entirely dependent on chance and empiricism. In recent years, however, microbiology has come full circle with respect to its concern with mixed cultures and the stimuli for research and exploitation have been very diverse (Bull & Slater, 1982). Indicated below are some of the areas of microbiology that first encouraged the move towards mixed-culture systems but first it is useful to survey some of the perceived advantages of working with mixed cultures. (a) Mixed substrate utilization. The greater metabolic capacity of mixed cultures enables a greater range and multiplicity of substrates to be used. (b) Mineralization of complex chemicals. This is especially true of xenobiotics whose degradation may necessitate the combined attack of several species. (c) Multi-step transformations involving, for example, the sequential modification of steroids or the conversion of starch to ethanol. (d) Resistance to toxic materials. For example, the resistance to heavy metals and the ability to accumulate metals may be increased substantially in microbial communities compared with their constituent member species (Dunn & Bull, 1983). (e) Enhanced biomass yield. (f) Increased maximum specific growth rate. (g) Protection against contamination due to the fact that potential niches for contaminant organisms are occupied already by members of the mixed community. This advantage has been demonstrated with respect to microbial protein production (Rokem et al., 1980). (h) Resistance to environmental perturbations. Mixed cultures for the production of microbial protein from nalkanes and methanol show greater resistance than monocultures to fluctuations in pH, temperature and ammonia (Linton & Drozd, 1982). (i) An alternative, in certain circumstances, to genetic manipulation of single species. For example, production of novel antibiotics by mixed cultures rather than by protoplast fusion, and production of ethanol from starch via mixed cultures rather than by cloning of amylases. Within biotechnology, the development of mixed-culture systems and the necessity to understand mixed-population dynamics is relevant to at least four areas : biodegradation, environmental management, infectious disease, and industrial fermentation. The utility of mixed-culture technology in these sectors is uneven and may rely on undefiried (spontaneous) cultures or defined mixed populations. (1) Biodegradation. There are very serious drawbacks in using the pure-culture approach to biodegradative studies of natural materials like lignocellulose and xenobiotics. The biodegradative capacity of communities is often far greater, qualitatively and quantitatively, than pure culture and many of these effects are interpretable in biochemical terms (e.g. commensal and synergistic interactions, relief of substrate inhibition, co-metabolism; see Bull, 1980). Similarly as our understanding at the biochemical level of wellestablished mixed activated sludge and anaerobic digester cultures improves, the means of rational process improvement is provided (Mah, 1982; Wolin, 1982). (2) Environmental management. Under this broad heading can be placed diverse activities like soil inoculant technology (Rhizobia, mycorrhizae), pest and weed control (microbial insecticides, mycoherbicides), crop protection (ice nucleation-active bacteria), and addition of a single species or defined mixtures to the environment, be that environment soil, water, root or leaf surface, and their effectiveness is determined largely by the competitiveness of the introduced organism with the indigenous microflora. (3) Infectious disease. Given the importance of pureculture techniques for the progress of medical bacteriology during the last century, it is somewhat ironic that now it is one of the stimuli for mixed-culture studies (Smith, 1982). Thus, interaction of potential pathogens with the indigenous microflora may determine whether infection occurs or is prevented, while some pathogens potentiate the effects of others and disease conditions may be caused by mild or non-pathogenic species acting in concert. (4) Industrial fermentation. Numerous traditional fermented foods and beverages are the products of spontaneous mixed microbial action (Hesseltine, 1983). Recent interest, however, has centered increasingly on defined mixed cultures and on a wide range of products and processes including microbial protein, biofuels, chemical transforma-