Because the Russian sentence is negative? I think 'Isn't there any bread here' is OK, but 'Is there any bread here?' seems too far-fetched.
I agree, but "Is there no bread here?" or even "Isn't there any bread here?" are sort of uncommon expressions in English (the latter is more common, but usually indicating that you actually did expect that there was bread and are unable to find it, and/or potentially sarcastic) - what I'm wondering about is the intent behind the Russian sentence; while it's negative, is there an expectation that there is actually bread, or not? If not, it seems like an artifact of Russian sentence construction rather than the same meaning that the negative sentence conveys in English.
EDIT: One of my Russian-speaking friends confirms that it's the same sense as English (confirming that your expectation of bread has not been satisfied), so that answers my question.
-ы is added in feminine decension if the stem ends is hard consonant (ма́ма — stem мам- ends in м — ма́мы).
The word хлеб belongs to masculine declension (stem is the same, хлеб-), so we add -а (if the stem ends in hard consonant, г/к/х/ш/ж/ч) or -я (if the stem ends in soft consonant). So, хлеб — genitive is хле́ба, учи́тель — genitive is учи́теля.