Strain improvement in antibiotic‐producing microorganisms

Abstract

Abstract In the context of commercial strain development for improved antibiotic production it is important to stress the efficiency and practicability of the procedures used. Typical constraints are the novelty of the microorganism, time and manpower. Established and more novel techniques applied to strain improvement can be divided into the induction of beneficial alterations and the screen for and recognition of such altered mutants. Recent advances have contributed mainly to the former of these two divisions. The empiricism of mutagenesis can be removed as more is known about mutation induction with respect to repair phenomena. Mutagens must be assessed for efficiency with rapid test systems for forward and reverse mutation. Enhanced mutation frequencies can be achieved with repair‐deficient mutants or chemical inhibitors of repair processes. It is widely accepted that recombinational methods are an important part of strain development although published examples of its successful application are rare. Parasexuality in asexual fungi and protoplast fusion in both bacteria and fungi facilitate recombination and therefore the construction of improved strains. As a result of these advances it has become necessary to deviate from the classical, linear mutation and selection programmes. The application of recombinant DNA technology to strain development is undoubtedly promising. In the field of antibiotic research this approach may necessitate extensive basic research before its potential can be realised. Major constraints may be poor knowledge of both the biosynthesis of the antibiotic and the genetic systems in the producing microorganism. It will also be necessary to develop suitable vector systems. This approach clearly consumes time and resources which are important considerations in commercial terms. The ability to recognise and isolate improved mutants is just as important as their production and again time and efficiency is paramount. Shake‐flask screening has been central to most programmes although its format may be changing to accommodate multiline screens to capitalise on recombination techniques. Its major limitation as a primary screen is in the work involved in testing a comparatively small number of strains. To circumvent this problem many laboratories are miniaturising screens or moving towards ‘plate‐based’ screens in which colony productivity is assessed on agar. Plate screens also permit less empirical screening procedures such as reversion of mutants impaired in antibiotic production or the use of chemical inhibitors of antibiotic biosynthesis. This approach has the advantage of allowing large numbers to be screened as well as providing information of a biosynthetic nature if the mutants and inhibitors are subject to investigation in their own right.