Here's why:

* Thioglycollate broth is a rich medium designed to cultivate a variety of bacteria, including both aerobes and anaerobes. It contains ingredients like:

* Sodium thioglycollate: This chemical reduces oxygen levels in the broth, creating a gradient of oxygen concentration from the top to the bottom.

* Resazurin: This dye acts as an indicator, turning pink in the presence of oxygen.

* Obligate anaerobes are bacteria that cannot grow in the presence of oxygen. They are harmed or even killed by oxygen.

In thioglycollate broth:

* Obligate anaerobes would only grow in the bottom of the tube where oxygen levels are lowest. This is because the sodium thioglycollate creates a reducing environment, minimizing oxygen concentration at the bottom of the tube.

* However, even in the lower regions, there might be some residual oxygen.

* If you see growth near the top of the tube, where the broth is pink (indicating oxygen presence), this is not an obligate anaerobe.

To summarize: While thioglycollate broth is designed to support a variety of bacteria, obligate anaerobes would only grow in the very bottom of the tube, where oxygen levels are minimal.