Here's a rule French learners come to love: after ne … pas, the partitive collapses to plain de — d' before a vowel.

Affirmative	Negative
Je mange du pain.	Je ne mange pas de pain. — I don't eat bread.
*Elle boit **de l'*eau.	*Elle ne boit **pas d'*eau. — She doesn't drink water.
On a des fruits.	On n'a pas de fruits. — We don't have any fruit.

The same happens to un, une: J'ai un chien. → Je n'ai pas de chien. — I don't have a dog.

But le, la, les don't change: J'aime le fromage. → Je n'aime pas le fromage. — I don't like cheese.