Antibiotic classification (peptide vs non-peptide): Which of the following is NOT an amino-acid/peptide antibiotic?

Introduction / Context:
Antibiotics can be grouped by their chemical nature as well as their target. Many exam questions ask you to distinguish peptide (amino-acid–based) antibiotics from non-peptide classes such as beta-lactams, macrolides, and quinolones. This classification helps predict spectrum, mechanism, pharmacology, and resistance patterns.

Given Data / Assumptions:

• The options include well-known agents from different structural families.
• “Amino acid and peptide antibiotic” means the active molecule is a peptide or polypeptide chain (often cyclic) built from amino acids.
• We must choose the single option that is not a peptide antibiotic.

Concept / Approach:
Peptide antibiotics are built from amino acids (e.g., bacitracin, actinomycin, polymyxins). By contrast, penicillins are beta-lactam antibiotics derived from a bicyclic beta-lactam–thiazolidine core; they are not peptide antibiotics. Tetranactin is a macrotetrolide (polyether) produced by Streptomyces; although non-peptidic, it is less commonly tested than penicillin for this contrast. Standard exam framing expects penicillin as the non-peptide prototype.

Step-by-Step Solution:
Identify peptide antibiotics among the options: bacitracin (polypeptide), actinomycin (peptide chromopeptide), polymyxin B (cyclic lipopeptide) → peptide class. Recognize penicillin's core: beta-lactam ring + thiazolidine ring → non-peptide. Select the agent that is clearly non-peptidic in standard classifications: penicillin. Confirm that exam convention typically contrasts penicillin (non-peptide) with peptide agents such as bacitracin.

Verification / Alternative check:
Pharmacology texts classify penicillins as beta-lactams (cell-wall inhibitors) distinct from polypeptides. Bacitracin, polymyxins, and actinomycin are routinely labeled peptide or polypeptide antibiotics.

Why Other Options Are Wrong:
Actinomycin: chromopeptide; clearly peptide-based. Bacitracin: classic polypeptide antibiotic. Tetranactin: macrotetrolide; although non-peptide, it is not the expected answer in conventional MCQs contrasting penicillin vs peptide antibiotics. Polymyxin B: cyclic lipopeptide.

Common Pitfalls:
Assuming “contains nitrogen” implies peptide; structural class, not nitrogen presence, defines peptide antibiotics. Also, do not confuse mechanism (cell-wall action) with chemical class.

Final Answer:
Penicillin.