Food products are subject to various influencing factors possibly leading to a microbial contamination during production, treatment and entry into circulation. A possible ensuing germ multiplication within the food product involves a high risk of food poisoning or food perishing. Of great importance for germ multiplication - during which log phase generation time can be determined - are milieu conditions such as temperature and pH-value. An accumulation of bacteria and/or their toxins can be prevented by appropriate manipulation of milieu conditions. To achieve this, increased measurements, for example of the pH-value, temperature or temperature- variation, are carried out within the framework of quality assurance systems (HACCP). It is essential to have exact knowledge of the breeding and multiplication behaviour of microbes in order to arrange pH-value / temperature / temperature-variation parameters effectively and / or to allow predictions of multiplication or of stagnation in the breeding of microorganisms (and finally to allow predictions as to how long food can be safely kept [predictive microbiology]). The speed at which multiplication takes place is manifested by the generation time. The international literature on aspects of growth and multiplication of bacteria as well as on generation time and milieu conditions has been collected and analysed in the literature review of the present study. It is evident that the information on generation time of microorganisms so far been available is still insufficient because of methodical problems. Efforts to complete data material have only been made in the 1990s, and were facilitated by the use of rapid new methods amongst which impedance measurement has proved to be especially suitable. Impedance measurement offers the possibility of quickly and effectively determining generation times indirectly under various conditions and in greater scales. Previous culture methods involved determination of germ counts and were therefore costly as far as time and materials were concerned. Impedance measurement measures the change of conductivity (impedance) within 2 samples with differing germ numbers. Generation time is determined on the basis of the difference of the initial germ numbers in proportion to the difference of the impedimetrically determined detection time. Within the framework of this project, generation intervals of 30 different strains of food-hygienically relevant Enterobacteriaceae-species and -genera were determined. The model strains belonged to 10 genera and differed in their characteristics and in their origins. The importance of these microbes was explained in the literature review, in which special attention was given to the clinical and epizootiological factors of Enterobacteriaceae-infections and -intoxications in humans as well as the possibility of perishability caused by these germs. Various illnesses caused by these bacteria in humans and animals have also been listed. The aim of the experiments was the determination of generation times of Enterobacteriaceae in relation to different milieu conditions (with temperatures ranging from 12°C to 40°C and pH-values between 5.0 and 7.3). In view of the generation intervals of the examined genera it was essential to check to what extent generation intervals differed between the strains of one species or between the species of one genus, especially at low temperatures. Clear relations could be established between the length of generation time and temperature. Within the examined temperature sector it was ascertained that the mean of the generation times of the model strains (except the Yersinia spp. strain) was, as expected, shortest with 22 min at 35°C and 37°C respectively and grew steadily with the deviation of temperature from the optimum. Under favourable milieu conditions (temperatures from 30°C to 37°C, pH-value from 6.0 to 7.3) the generation times of the model strains were almost homogenous. Only temperatures, especially those under 25°C, and acid initial pH-values such as pH 5.0 and partly even pH 5.5 caused a slow in growth and multiplication processes. With the reduction of incubation temperatures, the generation times of the model strains grew disproportionally. Generation time at 20°C was triple of that at 37°C, increasing at 18°C to quadruple, at 15°C to six-fold and at 12°C to nine-fold. Below 12°C an impedimetric determination of generation time was only possible with a few of the strains and over a considerable period of time. Temperatures under 15°C or a pH-value of 5.0 (or considered together a temperature ? 20°C in interaction with a pH of ? 5.5) lead to a considerable slowing down of growth and multiplication processes. Upon evaluation of the results, it was conspicuous that the majority of the generation times of the examined Enterobacteriaceae-strains were almost identical, but that the Yersinia showed significally longer generation intervals and therefore had to be analysed separately. Under unfavourable milieu conditions, the mean variation of results increased, whereby for example, the genera Salmonella and Shigella were more restricted in their speed of growth and multiplication, their generation time increasing in contrast to the other genera. The acquired results complete and broaden the knowledge of multiplication speed of microbes, especially those from the Enterobacteriaceae-family. These results can contribute to a better assessment of possible hazards to the human health and to the minimization of food perishing.