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In natural and man-made environments microorganisms often grow in the presence of a diversity of functionally similar substrates. The pattern of utilization of these mixed substrates is generally dependent upon their concentration. When substrates are present in high (not growth-limiting) concentrations, sequential utilization and diauxic growth is often observed and the substrate that supports the highest growth rate is utilized preferentially from the mixture. When the substrate concentrations are growth-limiting, simultaneous utilization of the various compounds present in the mixture appears to be the general response. Recent studies on mixed substrate utilization in both batch and continuous cultures have thrown light on the strategies of the control mechanisms that, in microbes, govern the utilization of the various substrates. But perhaps more importantly these studies have indicated the possible significance of mixed substrate utilization in microbial competition in nutrient-limited natural ecosystems.

Microbes, through their specialized metabolic processes and ecologies, play a fundamental part in man's economy, and their economic roles are widely recognized as important. The sulphur bacteria have had in past geological eras, and are having today, numerous and diverse effects, often deleterious, scattered throughout the economy. Many of their economic roles are not widely known. Their economic and environmental effects arise, according to the type or combination of species involved, principally from (1) the production of acid, (2) the formation or removal of hydrogen sulphide, and (3) the removal of oxygen. In this paper their various effects on industry and the environment are surveyed.