Questions: Thioglycolate medium is used to determine the oxygen requirements of a microorganism. You inoculate a tube of this medium with a bacterium and observe dense growth at the bottom of the tube. From this you can conclude that Multiple Choice - the organism is fastidious. - the organism can be cultured on the desk top. - the organism is a facultative anaerobe. - the organism is an obligate aerobe. - the organism is an obligate anaerobe.

The answer is the last one: the organism is an obligate anaerobe.

Explanation for each option:

1. The organism is fastidious.

   • Incorrect. Fastidious organisms have complex nutritional requirements and this characteristic is not determined by oxygen requirements or growth patterns in thioglycolate medium.

2. The organism can be cultured on the desk top.

   • Incorrect. This statement is too vague and does not relate to the oxygen requirements of the organism. Growth at the bottom of the tube indicates oxygen sensitivity, not the ability to be cultured in a specific location.

3. The organism is a facultative anaerobe.

   • Incorrect. Facultative anaerobes can grow in both the presence and absence of oxygen. They typically show growth throughout the tube, with more growth at the top where oxygen is present.

4. The organism is an obligate aerobe.

   • Incorrect. Obligate aerobes require oxygen for growth and would show growth at the top of the tube where oxygen concentration is highest.

5. The organism is an obligate anaerobe.

   • Correct. Obligate anaerobes cannot tolerate oxygen and will grow only at the bottom of the tube where the oxygen concentration is lowest.

In summary, dense growth at the bottom of a thioglycolate medium tube indicates that the organism is an obligate anaerobe, as it cannot survive in the presence of oxygen.