You inoculate bacteria in a broth culture and bacteria only grew at the very bottom of the tube. How would you best describe the oxygen utilization of this bacterium?

as an anaerobic bacteria

Explanation:

These kind of bacteria do not need oxygen to survive, could it be that is a kind of facultative anaerobic bacteria, that means that sometimes they use oxygen for some processes and sometimes they do not need it.

Another possibility is that they are microaerophyls, that means, that they are going to use some oxygen to it metabolism but in a very small quantities.