While they are single-celled organisms like bacteria, they are classified as fungi because of their similar characteristics, such as:

* Cell wall structure: Yeast have cell walls made of chitin, a substance also found in the cell walls of fungi.

* Mode of nutrition: Both yeast and fungi are heterotrophs, meaning they obtain nutrients from other organisms.

* Reproduction: Yeast reproduce through budding, a process similar to the reproduction of some fungi.

So, even though yeast is a single-celled organism, it's more closely related to multicellular fungi than to bacteria.