Polypeptide antibiotics are a chemically diverse class of anti-infective and antitumor antibiotics containing non-protein polypeptide chains. Examples of this class include actinomycin, bacitracin, colistin, and polymyxin B.
Source Wikipedia

They feature 'polypeptide' rings, i.e. complex amino-acid polymers in a ring-like structure.

The first examples were 'polymyxins' discovered in 1947 from bacterial cultures. Some are now produced from fungi. It has since been discovered that polypeptide antibiotics appear to be produced, to some degree, by all living organisms; and are assumed to function as natural host defence.

When used as medical antibiotics, some have toxic side effects and so tend to be used as last-resort treatments, or for very specific uses where they have been found to out-perform other types.

The mechanism of action of this entire group of antibiotics is currently unknown.

Despite thousands of articles about hundreds of these polypeptide antibiotics over several decades, basic questions about their mechanism of action remain unanswered. Most investigators have concluded that they act solely by permeabilizing the bacterial cell membrane. Although strong evidence for another mechanism of action has not yet emerged, this conclusion may not represent the whole story because polypeptide antibiotics clearly have other significant effects on bacteria

Source Biophysical Journal 94(5): 1549–1550.