bacterial cell wall and peptidoglycan
/ bak-TEER-ee-ul SEL WAWL and PEP-tih-doh-GLY-kan /
A bacterium lives in water that is usually less salty than its own insides, so water constantly tries to rush in and swell the cell like an overfilled water balloon. Without something tough holding it together, the cell would burst. The bacterial cell wall is that rigid outer corset — a strong mesh wrapped around the cell membrane that gives the cell its shape and keeps it from popping.
The material that makes the bacterial wall special is peptidoglycan, found nowhere else in nature. Picture a chain-link fence: long chains of sugar molecules run side by side, and short chains of amino acids (peptides) cross-link them together into a single giant net-like molecule that surrounds the whole cell. This sugar-plus-peptide construction is what the name 'peptidoglycan' literally describes. The thickness and arrangement of this layer differs between bacteria, and that difference is exactly what the Gram stain detects.
Peptidoglycan is a prime target in medicine precisely because it is unique to bacteria — your cells have nothing like it, so a drug that attacks it can hit bacteria without harming you. Penicillin and related antibiotics work by blocking the construction of peptidoglycan; the wall can no longer be built or repaired, and the swelling cell ruptures. Two cautions: archaea also have walls but never made of peptidoglycan, and your own cells have no cell wall at all (that is why the plant-style 'cell wall' is a different topic).
Lysozyme, an enzyme in your tears and saliva, snips the sugar chains of peptidoglycan and pops bacteria open. It is part of why a small mouth wound rarely gets infected — your own spit is quietly dissolving bacterial walls.
The enzyme lysozyme attacks the same peptidoglycan target that penicillin does.
Peptidoglycan is found only in bacteria — not in archaea, plants, or animals. That uniqueness is the whole reason cell-wall antibiotics can be selectively toxic to bacteria.