Heterologous protein secretion and fungal morphology in chemostat cultures of a recombinant Aspergillus niger (B1-D)

Oxidative Stress in Submerged Cultures of Fungi

It has been known for many years that oxygen (O2) may have toxic effects on aerobically growing microorganisms, mainly due to the threat arising from reactive oxygen species (ROS). In submerged culture industrial fermentation processes, maintenance of adequate levels of O2 (usually measured as dissolved oxygen tension (DOT)) can often be critical to the success of the manufacturing process. In viscous cultures of filamentous cultures, actively respiring, supplying adequate levels of O2 to the cultures by conventional air sparging is difficult and various strategies have been adopted to improve or enhance O2 transfer. However, adoption of those strategies to maintain adequate levels of DOT, that is, to avoid O2 limitation, may expose the fungi to potential oxidative damage caused by enhanced flux through the respiratory system. In the past, there have been numerous studies investigating the effects of DOT on fungal bioprocesses. Generally, in these studies moderately enhanced levels of O2 supply resulted in improvement in growth, product formation and acceptable morphological changes, while the negative impact of higher levels of DOT on morphology and product synthesis were generally assumed to be a consequence of "oxidative stress." However, very little research has actually been focused on investigation of this implicit link, and the mechanisms by which such effects might be mediated within industrial fungal processes. To elucidate this neglected topic, this review first surveys the basic knowledge of the chemistry of ROS, defensive systems in fungi and the effects of DOT on fungal growth, metabolism and morphology. The physiological responses of fungal cells to oxidative stress imposed by artificial and endogenous stressors are then critically reviewed. It is clear that fungi have a range of methods available to minimize the negative impacts of elevated ROS, but also that development of the various defensive systems or responses, can itself have profound consequences upon many process-related parameters. It is also clear that many of the practically convenient and widely used experimental methods of simulating oxidative stress, for example, addition of exogenous menadione or hydrogen peroxide, have effects on fungal cultures quite distinct from the effects of elevated levels of O2, and care must thus be exercised in the interpretation of results from such studies. The review critically evaluates our current understanding of the responses of fungal cultures to elevated O2 levels, and highlights key areas requiring further research to remedy gaps in knowledge.

Early phase process scale-up challenges for fungal and filamentous bacterial cultures

Culture pelleting and morphology has a strong influence on process productivity and success for fungal and filamentous bacterial cultures. This impact is particularly evident with early phase secondary metabolite processes with limited process definition. A compilation of factors affecting filamentous or pelleting morphology described in the literature indicates potential leads for developing process-specific control methodologies. An evaluation of the factors mediating citric acid production is one example of an industrially important application of these techniques. For five model fungal and filamentous bacterial processes in an industrial fermentation pilot plant, process development strategies were developed and effectively implemented with the goal of achieving reasonable fermentation titers early in the process development cycle. Examples of approaches included the use of additives to minimize pelleting in inoculum shake flasks, the use of large-volume frozen bagged inoculum obtained from agitated seed fermentors, and variations in production medium composition and fermentor operating conditions. Results were evaluated with respect to productivity of desired secondary metabolites as well as process scalability. On-line measurements were utilized to indirectly evaluate the cultivation impact of changes in medium and process development. Key laboratory to pilot plant scale-up issues also were identified and often addressed in subsequent cultivations.

Succinate synthesis and excretion by Penicillium simplicissimum under aerobic and anaerobic conditions

Succinate is an interesting chemical for industries producing food and pharmaceutical products, surfactants, detergents and biodegradable plastics. Succinate is produced mainly by a mixed-acid fermentation process using anaerobically growing bacteria. However, succinate excretion is also widespread among fungi. In this article we report results on the intracellular concentration and the excretion of succinate by Penicillium simplicissimum under aerobic and anaerobic conditions. The intracellular concentration of succinate increased slightly with the specific growth rate and strongly if the respiratory chain was inhibited by sodium azide or anaerobic conditions (N(2)). A strong increase of succinate excretion was observed if the respiratory chain was inhibited. It is suggested that succinate synthesis under functional (sodium azide) or environmental (N(2)) anaerobic conditions occurs via the reductive part of the tricarboxylic acid cycle. Succinate is then excreted because the oxidative part of the tricarboxylic acid cycle is inactive. A possible role of succinate synthesis in the regeneration of NAD ('fumarate respiration') is discussed.

Use of the chemiluminescent probe lucigenin to monitor the production of the superoxide anion radical in a recombinant Aspergillus niger (B1-D)

Direct detection of intracellular superoxide anion radical (O(2)(.-)) production is of critical importance for investigating the responses of filamentous fungi to oxidative stress in bioprocesses. The purpose of this study is to establish a reliable method to monitor the O(2)(.-) production within pellets of Aspergillus niger. Addition of pure oxygen and the redox cycling agent paraquat to fungal pellet suspensions resulted in a considerable increase in lucigenin-derived chemiluminescence (LDCL). In the presence of exogenous superoxide dismutase (SOD), the LDCL of a disrupted cell solution was inhibited. In contrast, with addition of diethyldithiocarbamate and sodium azide, respectively, the inhibitors of Cu, Zn-SOD and Mn-SOD, an increased LDCL was observed. Further, as a probe, lucigenin can be absorbed and accumulated in fungal pellet within a few minutes. Various pretreatments of the bioreactor sample for the measurement of LDCL, were also investigated in the present study, and the use of intact pellets was adopted here rather than disrupting cells because the latter treatment led to difficulties in LDCL measurement. These results show that lucigenin may be used as a convenient chemiluminescent probe to monitor intracellular production of O(2)(.-) in filamentous fungi, and thus to follow changes in the level of this stressor within fungi

Growth of filamentous fungi in submerged culture: problems and possible solutions

Filamentous fungi are important organisms industrially and continue to attract research interest as microbiologists attempt to overcome the problems associated with their behavior in submerged culture. This review critically examines the literature describing these problems and where available suggests possible solutions to them. The influence of the chemical and physical environment on culture morphology, the process engineering challenges presented by different fungal morphologies, and the relationship between fungal morphology and metabolite production are all discussed.