A fermentation product is produced by the culture of a certain organism, or organisms, in a nutrient medium. M the fermentation is invaded by a foreign microorganism then the following consequences may occur:

(i) The medium would have to support the growth of both the production organism and the contaminant, resulting in a loss of productivity, (ii) If the fermentation is a continuous one then the contaminant may 'outgrow' the production organism and displace it from the fermentation.

(iii) The foreign organism may contaminate the final product, e.g. single-cell protein where the cells, separated from the broth, constitute the product.

(iv) The contaminant may produce compounds which make subsequent extraction of the final product difficult.

(v) The contaminant may degrade the desired product; this is common in bacterial contamination of antibiotic fermentations where the contaminant would have to be resistant to the normal inhibitory effects of the antibiotic and degradation of the antibiotic is a common resistance mechanism, e.g. the degradation of ¡3-lactam antibiotics by ¡8-lactamase-producing bacteria.

(vi) Contamination of a bacterial fermentation with phage could result in the lysis of the culture.

Avoidance of contamination may be achieved by:

(i) Using a pure inoculum to start the fermentation, as discussed in Chapter 6.

(ii) Sterilizing the medium to be employed.

(iii) Sterilizing the fermenter vessel.

(iv) Sterilizing all materials to be added to the fermentation during the process.

(v) Maintaining aseptic conditions during the fermentation.

The extent to which these procedures are adopted is determined by the likely probability of contamination and the nature of its consequences. Some fermentations are described as 'protected' — that is, the medium may be utilized by only a very limited range of microorganisms, or the growth of the process organism may result in the development of selective growth conditions, such as a reduction in pH. The brewing of beer falls into this category; hop resins tend to inhibit the growth of many micro-organisms and the growth of brewing yeasts tends to decrease the pH of the medium. Thus, brewing worts are boiled, but not necessarily sterilized, and the fermenters are thoroughly cleaned with disinfectant solution but are not necessarily sterile. Also, the precautions used in the development of inoculum for brewing are far less stringent than, for example, in an antibiotic fermentation. However, the vast majority of fermentations are not 'protected' and, if contaminated, would suffer some of the consequences previously listed. The approaches adopted to avoid contamination will be discussed in more detail, apart from the development of aseptic inocula which is considered in Chapter 6 and the aseptic operation and containment of fermentation vessels which are discussed in Chapters 6 and 7.

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