Fresh medium- Flow-rate from reservoir regulator Sterile air or→ other gas Gaseous Culture 路 Culture 8 e Effluent containing microbial cells FIGURE 5.21 Aerobic, anaerobic, facultative, mi- croaerophilic, and aerotolerant anaerobe growth, as re- vealed by the position of microbial colonies(depicted here as black dots) within tubes of a culture medium. A small FIGURE 5.9 Schematic for a continuous culture device amount of agar has been added to keep the liquid from be.(chemostat). In such a device, the population density is con- stu wnIc trolled by the concentration of limiting nutrient in the reser- as a redox indicator (a)Oxygen penetrates only a short dis- voir, and the growth rate is controlled by the flow rate(see tance into the tube, so obligate aerobes grow only at the sur- Figure 5. 11). Both parameters can be set by the experimenter. face.(b) Anaerobes, being sensitive to oxygen, gro away from the surface. (c) Facultative aerobes are able to grow in either the presence or the absence of oxygen and thus grow throughout the tube. (d) Microaerophiles grow away from the most oxic zone. (e)Aerotolerant anaerobes grow throughout the tube