Aerobic bacteria require oxygen for survival. They are present in aerated moist soil containing organic carbon sources. There are two main types of aerobic bacteria:
1. The obligate aerobes that compulsorily require oxygen for deriving energy, growth, reproduction, and cellular respiration. These organisms do not survive in the absence of oxygen or flooding.
2. Facultative and Microaerophile aerobes: Facultative bacteria behave both aerobically and anaerobically, according to the prevailing conditions. In reduced environments, they acquire energy via anaerobic pathways, whereas in oxidative environments, they develop aerobic pathways. The microaerophilic bacteria require oxygen but in very low concentrations.
Figure 1 Aerobic bacteria examples
The physiology of the microorganisms and the relevant metabolic pathways must be well understood, and the nutrient (and air in the case of aerobic cultures) requirements of the microorganism must be satisfied. These needs often change as the microbial biomass increases in concentration and the environmental conditions (e.g., nutrient composition, temperature, pH) are altered. What leads to a better oxygen supply of the culture and to a higher product formation compared to the reference strain, whether the selected strain has an improved pathway for product formation or not, these problems may occur in medium development.
Screening for improved culture conditions plays a decisive role in bioindustry. Selection of specific strains, medium development, and optimization of growth and product formation are only a few examples that require experimental investigations, which are usually done in shake flasks. Defined and reproducible experimental conditions are absolutely necessary requirements for successful screening experiments. Especially, a sufficient oxygen supply is crucial for growth and product formation of most industrially used aerobic cultures.
Atmospheric condition is generally satisfactory for culture of aerobes or facultative anaerobes but for the growth of many aerobes, it is necessary to provide extensive aeration. Forced aeration of cultures is therefore frequently desirable and can be achieved either by vigorously shaking the flask or tube on a shaker or by bubbling sterilized air into the medium. When aerobic organisms are to be grown in large quantities, it is advantageous to increase the exposure of the medium to the atmosphere. This can be accomplished by dispensing the medium in shallow layers or by providing aeration by constantly shaking the inoculated liquid cultures.
The need for carbon and molecular O2 sources is different at different stages of batch fermentation. Most industrial fermentation processes are fed-batch processes, in which nutrients are added to the fermenter, either intermittently or continuously in a variety of ways. To ensure that the requirements of the culture are met as the fermentation progresses, the environmental conditions, including the concentration of nutrients, must be controlled. Oxygen availability is considered to be the most critical parameter for microbial growth in various bioreactors. A change in oxygen availability to microorganisms has drastic effect on product formation in the bioreactors. Oxygen transfer rate (OTR) is a frequently employed parameter for quantifying the physiological state of an aerobic culture, and for the design and operation of the bioreactors.
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