Population balance modeling of the conidial aggregation of Aspergillus niger.

Numerous biotechnological production processes are based on the submerse cultivation of filamentous fungi. Process design, however, is often hampered by the complex growth pattern of these organisms. In the morphologic development of coagulating filamentous fungi, like Aspergillus niger, conidial aggregation is the first step of filamentous morphogenesis. For a proper description of this phenomenon it is necessary to characterize conidial populations. Kinetic studies performed with an in-line particle size analyzer suggested that two distinct aggregation steps have to be considered. The first step of conidial aggregation starts immediately after inoculation. Both the rate constants of formation and disintegration of aggregates have been determined by measuring the concentration of conidia at the beginning of the cultivation and the concentration of particles at steady state during the first hours of cultivation. In contrast to the first aggregation step, where the collision of conidia is presumed to be responsible for the process, the second aggregation step is thought to be initiated by germination of conidia. Growing hyphae provide additional surface for the attachment of non- germinated conidia, which leads to a strong decrease in particle concentration. The specific hyphal length growth rate and the ratio of particle concentration to the growing adhesion hyphal surface are decisive matters of the second aggregation step. Both aggregation steps can be described by population dynamics and simulated using the program package PARSIVAL (PARticle SIze eVALution) for the treatment of general particle population balances.

Morphology and productivity of filamentous fungi.

Cultivation processes involving filamentous fungi have been optimised for decades to obtain high product yields. Several bulk chemicals like citric acid and penicillin are produced this way. A simple adaptation of cultivation parameters for new production processes is not possible though. Models explaining the correlation between process-dependent growth behaviour and productivity are therefore necessary to prevent long-lasting empiric test series. Yet, filamentous growth consists of a complex microscopic differentiation process from conidia to hyphae resulting in various macroscopically visible appearances. Early approaches to model this morphologic development are recapitulated in this review to explain current trends in this area of research. Tailoring morphology by adjusting process parameters is one side of the coin, but an ideal morphology has not even been found. This article reviews several reasons for this fact starting with nutrient supply in a fungal culture and presents recent advances in the investigation of fungal metabolism. It illustrates the challenge to unfold the relationship between morphology and productivity.

Influence of mechanical stress and surface interaction on the aggregation of Aspergillus niger conidia.

Productivity of fungal cultures is closely linked with their morphologic development. Morphogenesis of coagulating filamentous fungi, like Aspergillus niger, starts with aggregation of conidia, also denominated as spores. Several parameters are presumed to control this event, but little is known about their mode of action. Rational process optimization requires models that mirror the underlying reaction mechanisms. An approach in this regard is suggested and supported by experimental data. Aggregation kinetics was examined for the first 15 h of cultivation under different cultivation conditions. Mechanical stress was considered as well as pH-dependent surface interaction. Deliberations were based on a two-step aggregation mechanism. The first aggregation step is only affected by the pH-value, not by the fluid dynamic conditions in the bioreactor. The second aggregation step, in contrast, depends on the pH-value as well as on agitation and aeration induced power input. For the given experimental set-up, agitation had much more influence than aeration. In addition, hyphal growth rate was determined to be the driving force for the second aggregation step.

Kinetic studies on the aggregation of Aspergillus niger conidia.

Morphology has a crucial effect on productivity and the supply of substrate for cultures of filamentous fungi. However, cultivation parameters leading to the desired morphology are often chosen empirically as the mechanisms governing the processes involved are usually unknown. For coagulating microorganisms like Aspergillus niger the morphological development is considered to start with the aggregation of conidia right after inoculation. To elucidate the mechanism of this process, kinetic studies were carried out using an in-line particle size analyzer. Based on the data obtained from these experiments a model for conidial aggregation is proposed in this article. It consists of two separate aggregation steps. The first one takes place immediately after inoculation, but only leads to a small decrease of total particle concentration. Most suspended conidia aggregate after a second aggregation step triggered by germination and hyphal growth. Aggregation velocity of this second phase is linearly dependent on the particle growth rate.