ABSTRACT
INTRODUCTION Filamentous fungi are extensively used as cell factories for different biotechnological products such as enzymes, pharmaceuticals, as well as primary and secondary metabolites. The two most eminent biotechnological processes are citrate production by Aspergillus niger and penicillin production by P. chrysogenum, which will be the focus of this chapter. In terms of industrial production, these two metabolite hyperproducers represent billions of U.S.$ in annual revenues. Hyperproduction can be defined as the ability of an organism to obtain very high yields, in some cases close to stoichiometric conversion of carbon into the product of interest in a relatively short period of time (generally a few days). Such intensive production can only be obtained with a few selected isolates of filamentous fungal species under specific nutrient and morphological conditions. The original natural isolates would be considered poor producers by modern standards. Intensive efforts have been applied to the improvement of these original isolates, using mostly classical random mutation strain improvement techniques, and more recently genetic and metabolic engineering techniques. Concurrent to the advances in fungal genome manipulation techniques, great strides forward in the development of comprehensive metabolite detection, large scale mRNA transcript quantification and other so-called “omic” techniques have been made. The application of these techniques to strain improvement is
already starting to revolutionize the way in which we improve and develop current or novel processes. In this chapter we discuss these developments, but we start with a discussion on the historical development of citrate and penicillin production, two examples where hyperproducing filamentous fungi have been developed. HISTORY From a historic perspective, citrate production by A. niger, fumarate production by Rhizopus oryzae, and penicillin production by Penicillium chrysogenum have great importance as the first true examples of industrial scale biotechnological processes. These fungal fermentation processes have led the way towards further industrial applications of biotechnology. Particularly the development of citrate and penicillin production processes resulted in mutual advantages, and led to rapid progress in the field of biotechnology as a whole. In the course of developing these processes many similar problems arose, and hence many of the techniques developed for one process could rapidly be transferred to the other process (see Figure 1 for a timeline overview of key historical developments), e.g., large-scale fermentation technology. Citrate was first discovered by Scheele in 1784 and was produced mainly from lemons during the nineteenth century. In 1880 it was attempted to produce citrate from glycerol, but this was not economically competitive.
