First, try to think if the bacteria is aerobic or anaerobic.
Fermentation is a metabolic process, which means some kind of reactions are taking place inside the cell.
Just think what kind of metabolic reactions will take place inside each bacterial cell.
As you are aware, bacteria do not have all cell organelles like other eukaryotic cells, just think how a metabolic reaction will take place inside a bacterial cell and where does it take place.
When we say it is metabolic process, it means some reactants are needed.You will have to think what kind of reactants or substrate are broken down during the reaction.
For example, substrate can be glucose, lactose, etc.
If these substrates or reactants are broken down, then think about what products are formed, and then you need to explain why these carbohydrates are broken down.
What you have to explain is how fermentation is an important process in bacteria and without the fermentation process, bacteria cannot survive.
You can also try to explain how fermentation in bacteria is useful in food industry.
For example: Yogurt is one of the fermented products. Just think how bacteria convert milk into yogurt.