The Selection Of Strains Resistant To Infection

Bacterial fermentations may be affected very seriously by phage infections, which may result in the lysis of the bacteria. A possible method for reducing the risk of failure due to phage contamination is to select bacterial strains which are resistant to the phages isolated in the fermentation plant (Hongo et al, 1972) It is important that the apparent resistant strains isolated are not lysogenic as the carrying of a population of phages in the fermentation is a source of potential lytic phage mutants. The use of phage-re-sistant mutants does not ensure immunity from phage infections because new host range phages may be introduced on to the plant or phage mutants may appear. Plant hygiene is essential to minimize the risk of contamination and it is also possible to utilize chemical agents in the fermentation which selectively inhibit phage replication (Hongo et al., 1972).

It may be possible to design the host organism of a recombinant fermentation such that it is more resistant to phage infection. Primrose (1990) suggested that the inclusion of one or more host-restriction and modification systems (HRM) may achieve this objective. H RM is a mechanism whereby foreign nucleic acids which enter a cell may be destroyed. Most HRM systems reduce the infectivity of bacteriophage DNA by a factor of 102 to 104. The genes for many HRM systems have been cloned and they can be introduced into host strains provided, of course, that they do not degrade the foreign DNA deliberately introduced into the production strain. Also, recombinant fermentations may be made phage-resistant by changing the host organism. Although this may seem a drastic step it would be feasible if the rationale were built into the development of the process.

Infection of some fermentations with 'wild' microorganisms may be made more easily controlled by selecting commercial strains which are resistant to various antibiotics. The antibiotics to which the commercial strain is resistant may then be used to control the level of contaminants. The danger inherent in this technique is that resistant contaminants will also tend to be selected, but reasonable protection should be given provided that stringent sterilization of a contaminated fermenter is carried out before the vessel is used again so that any antibiotic resistant contaminants are removed.

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