Final answer:
The phospholipid barrier surrounding the peptidoglycan layer in gram-negative bacteria is called the outer membrane, which is an additional layer not found in gram-positive bacteria.
Step-by-step explanation:
The phospholipid barrier surrounding the peptidoglycan layer in gram-negative bacteria is called the outer membrane. In gram-negative bacteria, the peptidoglycan layer while being much thinner compared to that in gram-positive bacteria, is encapsulated by an additional outer membrane which is not present in gram-positive bacteria. This outer membrane is significant because it contains substances like lipopolysaccharides (LPS) which are endotoxins, as well as proteins like porins that allow particular molecules to pass through, and efflux pumps which help in expelling harmful substances such as antibiotics, potentially contributing to multi-drug resistance.