The worms/larva are probably mosquito larva.
How they got into your water bottle depends on any number of scenarios.
If you leave your bottle on the side and have left the screw top off for any length of time, maybe overnight, then that is a probable cause. Mosquitos (and ants) much prefer clean water than puddles.
Maybe the shop you buy your water from aren't being honest in their supply and are refilling bottles themselves from an unchecked water source. Always make sure your bottles come with the wrap around seal on the neck.
You should have an Or Yor number on the bottle, this is an identification number, as supplied by the FDA and Health Department. The number sits inside a white box and looks like this:-
The numbers inside the box correspond to the company whose water is inside the bottle (not the numbers on my example). You can always complain to the Tessabahn about the quality of the water and give them the Or Yor number, then they can then check that the company is doing everything correctly.
As I said, I'm not sure where your problem originated from, but always, always, replace the cap on the large bottled water containers after every use. Mosquitos are prone to find any open water source, especially during the nighttime.
Pete 
