Questions: Antimicrobial Peptides and Natural Host Defenses

Question 1

Defensins are small, cationic (positively charged) peptides that form pores in bacterial membranes and kill bacteria. Why don't they also lyse the human cells they contact?

• A. Defensins are too large to insert into the tightly packed lipid bilayer of mammalian cell membranes
• B. Mammalian cell membranes present a neutral outer leaflet (stabilized by cholesterol) while bacterial membranes expose negatively charged phospholipids, so defensins are selectively attracted to bacterial surfaces
• C. Human cells produce receptor proteins that sequester and neutralize defensins before they reach the membrane
• D. Defensins require peptidoglycan to trigger insertion, and mammalian cells lack this structure

Answer: (B)

The selectivity of cationic AMPs depends on charge complementarity: bacterial membranes are rich in negatively charged phospholipids (like phosphatidylglycerol) exposed on the outer leaflet, while mammalian outer leaflets are predominantly neutral, with cholesterol dampening any surface charge. Defensins are electrostatically attracted to the negatively charged bacterial surface, insert into it, and form pores. On mammalian cells, the electrostatic attraction is minimal, so defensins do not insert efficiently. This charge-based discrimination is why AMPs kill bacteria at concentrations that leave host cells unharmed.

Question 2

Gram-negative bacteria are generally less susceptible to lysozyme than Gram-positive bacteria. What structural feature explains this difference?

• A. Gram-negative bacteria produce a lysozyme-degrading protease in their periplasmic space
• B. Gram-negative bacteria have a thicker peptidoglycan layer that is harder for lysozyme to penetrate than the thin layer of Gram-positives
• C. Gram-negative bacteria have an outer membrane that shields their thin peptidoglycan layer from lysozyme access
• D. Gram-negative bacteria lack N-acetylmuramic acid, so the lysozyme cleavage target is absent from their cell walls

Answer: (C)

Gram-positive bacteria have a thick, exposed peptidoglycan layer directly accessible on the cell surface — lysozyme cleaves its glycosidic bonds, destroying wall integrity. Gram-negative bacteria have a thin peptidoglycan layer buried between an inner membrane and an outer membrane; the outer membrane is a lipopolysaccharide-containing barrier that prevents lysozyme from reaching its substrate. This structural difference is why Gram-positive organisms are especially vulnerable to lysozyme in tears and saliva.

Question 3

Antimicrobial peptides and conventional antibiotics work by the same basic mechanism — both disrupt specific enzymatic reactions or protein synthesis steps within bacteria.

• T. True
• F. False

Answer: False

Conventional antibiotics typically target specific bacterial proteins — cell wall synthesis enzymes (penicillin targets transpeptidases), ribosomes (tetracyclines, aminoglycosides), or DNA gyrase (fluoroquinolones). Most AMPs work by physically disrupting the membrane through electrostatic insertion and pore formation, exploiting the charge differential between bacterial and host cells. This physical mechanism, rather than protein-target specificity, is part of why bacteria have greater difficulty evolving resistance to AMPs compared to conventional antibiotics.

Question 4

Bacterial pathogens that successfully colonize human tissues have often evolved mechanisms to modify their membrane surface charge, reducing the electrostatic attraction that drives AMP binding.

• T. True
• F. False

Answer: True

Modifying surface charge is a key virulence mechanism in many bacterial pathogens. For example, some bacteria add positively charged amino acids to lipid A in their LPS, reducing the net negative charge of the outer surface and decreasing defensin binding. Others produce proteases that degrade AMPs, or form biofilms whose polysaccharide matrix physically excludes AMPs. These adaptations directly reflect evolutionary pressure from host AMP production and help explain why certain organisms are pathogenic while related species remain harmless commensals.

Question 5

Why does the charge difference between bacterial and mammalian membranes allow antimicrobial peptides to selectively kill bacteria without destroying host tissues?

Model answer: Bacterial membranes expose negatively charged phospholipids on their outer surface, providing an electrostatic target for cationic (positively charged) AMPs. Mammalian cell outer leaflets are predominantly neutral — phosphatidylcholine and sphingomyelin dominate, with cholesterol dampening surface charge — so there is no electrostatic attraction to drive AMP insertion. AMPs are thus selectively concentrated at bacterial membranes, where they insert and form pores, while mammalian membranes are largely ignored.

This selectivity is the evolutionary and practical key to AMPs as innate defenses: they kill bacteria at concentrations found in tears, saliva, and neutrophil granules without causing tissue damage. It also explains why loss of AMP production — through burns, genetic defects, or chronic disease — dramatically increases susceptibility to surface infections. The host uses membrane composition as the discrimination criterion between self and microbial non-self.