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# Facultative anaerobes can grow with or without free oxygen in their environment e g gut dwelling bacteria obligatory anaerobes require absence of

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bacteria). Obligatory anaerobes require absence of oxygen. You are researching how a facultative anaerobe transitions from anaerobic to aerobic respiration along the length of the human intestine (small and large intestine), where oxygen levels – and with them respiration - vary from atmospheric levels of oxygen (aerobic respiration) to no oxygen (anaerobic respiration). You grow cultures of a facultative anaerobic bacteria in flasks with different levels of oxygen that mimic the conditions in the 7.5m long human intestines and collect data on the growth rates of the bacteria (number of bacterial colonies) under the different conditions. Time food spends in the small intestine, 2.5m long, is 4 hrs. The time taken for food to pass through the entire length of the intestine is between 24 and 72 hrs. To determine oxygen levels in your flasks, you use the ratio of oxygen to nitrogen, which you vary between 0 (no oxygen) and 0.25 (atmospheric levels of oxygen). You assume that for conditions where r<0.01 your bacteria will be obligatorily anaerobic, and aerobic respiration will be observed from r=0.05. If the oxygen ratio goes from 0.25 to 0 over 7.5 m, and rO2 = 0.25 – 0.03x (x=distance along the intestine) describes the relationship between oxygen ration and the length of path travelled by the bacteria in the intestine, how long will it take bacteria to pass from the small into the large intestine? Explain how you arrived at your estimate.
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