Growth of microbes on (inner) surfaces of bioreactors is a well known phenomenon; it is generally termed “wall growth”. However, this phenomenon is often very undesired for several reasons. The organisms in the bio-film are more or less subject to diffusion control of both substrate supply and product removal (Atkinson and Fowler, 1974; Atkinson and Daoud, 1976). They, hence, may either be in a different physiological state (compared with those cells suspended in the liquid phase), or they suffer seriously from limitations resulting in death and/or lysis. These organisms can be less or even non-productive. In kinetic studies, these organisms represent a major problem; their amount is not exactly known, nor is their activity, nor their contribution to the biomass balance in open flow systems. They act as a constant inoculum of unknown quality and quantity. Considerable artifacts may result if wall growth is neglected (Topiwala and Hamer, 1971; Laforce, 1987). Leaching of heavy metal ions from stainless steel bioreactors has sometimes resulted in serious inhibition of microbial cultures (Sonnleitner et al., 1982). Growth of extremophilic microbes on a steel surface might result in even more extensive solubilization of heavy metal ions which, in turn, would cause inhibition of growth. The degree and rate of solubilization would depend on the accessible surface. The minimization of the accessible surface surrounding a given volume is a reasonable objective. Besides construction, the utilization of steel with extremely smooth surfaces, e.g. achieved by high quality polishing, is a practiced means to approach this objective.