Secondary mitotic human skin fibroblast populations in vitro underwent 53 ± 6 cumulative population doublings (CPD) in 302 ± 27 days. When the growth capacity of the mitotic fibroblasts is exhausted, and if appropriate methods are applied, the fibroblasts differentiate spontaneously into postmitotic fibroblast populations, which were kept in stationary culture for up to 305 ± 41 additional days. Mitotic and postmitotic fibroblast populations are heterogeneous populations with reproducible changes in the proportions of mitotic

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Microbial growth on water-insoluble carbon sources such as hydrocarbons is accompanied by metabolic and structural alterations of the cell. The appearance of surface-active compounds (biosurfactants) in the culture medium or attached to the cell boundaries is often regarded as a prerequisite for initial interactions of hydrocarbons with the microbial cell. Under this point of view, biosurfactants produced by hydrocarbon-utilizing microorganisms, their structures and physico-chemical properties are reviewed. The production of such compounds is mostly connected with growth limitation in the late logarithmic and the stationary growth phase, in which specific enzymes are induced or derepressed. Addition of purified biosurfactants to microbial cultures resulted in inhibitory as well as in stimulatory effects on growth. Therefore, a more differentiated view of microbial production of surface-active compounds is proposed. Biosurfactants should not only be regarded as prerequisites of hydrocarbon uptake, but also as secondary metabolic products.

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