What aspect of mollicutes primarily differentiates them from traditional bacteria?

Mollicutes are a unique group of bacteria that are characterized primarily by their size and lack of a cell wall. Unlike traditional bacteria, which typically possess a rigid cell wall made of peptidoglycan, mollicutes have a much simpler structure. This absence of a cell wall not only influences their shape—making them often pleomorphic (able to take on various forms)—but also their sensitivity to certain antibiotics that target cell wall synthesis, such as penicillin.

Their small size allows mollicutes to pass through filters that retain most other bacteria, which complicates their study and contributes to their classification as a distinct group. This unique combination of features, particularly the lack of a cell wall, sets mollicutes apart from more common bacterial forms, which have a more typical structural organization.

The other choices do not accurately reflect the defining characteristics of mollicutes. They do not possess flagella, they do not form endospores, nor do they exist as multicellular organisms. These distinctions further affirm the uniqueness of mollicutes among bacterial taxa.