In gram-positive cells, how do NAM-NAG backbones link together?

In gram-positive bacteria, the peptidoglycan layer is crucial for maintaining the cell wall's structural integrity. The backbone of this layer is composed of alternating units of N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG).

NAM and NAG units are linked together by glycosidic bonds, specifically beta-1,4 glycosidic linkages, which are not the options listed. The units form a long chain of polysaccharides that provide a rigid structure to the cell wall.

However, the cross-linking that strengthens and stabilizes the peptidoglycan layer is facilitated by peptide interbridges between the amino acids of adjacent NAM units. This feature is characteristic of gram-positive bacteria, where the peptide cross-linking significantly contributes to the overall strength and stability of the cell wall.

Thus, the correct answer points to the nature of how these structural elements interact through peptide interbridges rather than the formation of peptide bonds within proteins or through hydrogen or covalent bonds that don't describe the specific connection in this context. These peptide interbridges allow for the complex three-dimensional structures necessary for the functionality of the bacterial cell wall.