A facultative aerobe is an organism that can survive in the presence of oxygen as well as in environments where there is no oxygen. The process for generating energy in the two cases is different.
When energy is generated using glucose in the presence of oxygen, there is a production of 38 molecules of ATP as the sugar is completely broken down into carbon dioxide and water. Anaerobic production of energy yields a much smaller number of ATP molecules.
Therefore the bacterium will ideally use processes that utilize oxygen to break down food. But the absence of oxygen does not lead to its death; instead it then uses anaerobic processes to break down food.