Asepsis in biotechnology means freedom from unwanted microorganisms, just as in clinical medicine it means freedom from pathogenic microorganisms (179). However, there do exist many fermentation industries e.g., ethanol, baker's yeast, and vinegar, where asepsis is not a matter of concern. Economic considerations suggest that a contamination probability of 1 in 100 is acceptable for batch fermentations, considering a contamination probability of 1 in 1000 taken into design calculations for a sterilization process (180). Recombinant microorganisms are at a greater risk of being overwhelmed by wild organisms. Cell culture fermentations are susceptible to microbial contamination due to a longer fermentation process and a slow growth rate. However, with use of aseptic procedures large scale animal cell cultures are operated with a contamination rate of about 2% (181).
The sources of contamination in a fermentation process could be attributed to inoculum, nutrient medium, bioreactor system, air or liquid transfer, and mutation. The aseptic procedures necessary to achieve a sterile fermentation process include:
1. A sensitive sterility assessment protocol: The results of conventional sterility checks in nutrient media incubated at optimum conditions may not be possible before sufficient damage is done to the production fermentor. There is a need for accelerated detection of contaminants like use of thioglycolate agar and oxidoreductive indicators (e.g., methylene blue or resazurin) for rapid detection of aerobes and anaerobes in the medium (182).
2. Development of a certified laboratory inoculum: The preparation and propagation of inoculum required for fermentation should be carried out in designated clean rooms. The procedures and equipment used for sterilization of the media and glassware for inoculum preparation should be strictly followed.
3. Efficient sterilization of bioreactor, medium, and air: Proper sterilization of bioreactor, medium, and air is crucial for a successful fermentation batch. Continuous sterilizers have been employed for sterilization of medium required for large scale bioreactors (183). After sterilization of the bioreactor, the system should be held under positive pressure with sterile air to avoid the possibility of external air being drawn due to vacuum formation (184).
4. Asepsis during fermentation: The most common cause of loss of asepsis during fermentation is the depressurization of bioreactor which could be averted by providing the air compressor with automatic change over to a captive power generator and automatic closure of inlet and outlet valves at a fall in the preset pressure or air flow. Foaming is a characteristic feature of gas and liquid operation in presence of surface active agents.
5. Containment of foam: Foaming is perceived as the most probable cause of contamination in fermentation processes, leading to overflows and consequently loss of broth and products. The addition of silicon based antifoaming agents leads to a change in the hydrodynamic and mass transfer characteristics of the bioreactor (185). However, the use of foam breakers has been applied to avoid the need for addition of antifoaming agents, and the necessity of antifoaming agent addition depends on the nature of fermentation (186).
6. Routine and preventive maintenance of bioreactor: The inspection of a bioreac-tor should cover the shell, dome, agitator seal, air exhaust line, nozzles, sight glasses, O rings, air filters, probes, shaft, impellers, coils, sparger, inoculum, feed, and sampling lines (187).
Asepsis of a bioreactor ultimately rests on the integration of adequate bioreactor design, efficient validation and operating protocols, routine maintenance, and operation by skilled personnel.
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