Researchers examined the relationship between gut microbiomes and depression. To do so, they collected fecal samples from people with depression and also a control group of individuals with no signs of depression. They then performed a fecal microbiota transfer (FMT) by adding the samples to rats that had no gut microbiota and examined behaviors associated with depression and anxiety as well as species of bacteria that ended up growing in the rats’ guts. The results are presented below. Graph (a) shows the rats’ interest in a pleasurable experience (drinking sugar water). Graph (b) shows the amount of time rats spent out in the open versus along the edge of an area (a sign of anxiety). Graph (c) shows the number of species observed in rats after FMT.

Patients from the depression group had relatively high levels of bacteria in the genus Paraprevotella (a Gram-negative bacterium) relative to the control group of patients. Which of the following is a structural difference between Paraprevotella and methicillin-resistant Staphylococcus aureus (MRSA), a Gram-positive bacterium?

a. Paraprevotella contains a thick layer of peptidoglycan in its cell wall and MRSA lacks peptidoglycan.

b. The plasma membrane of Paraprevotella is surrounded by a cell wall that contains a relatively thin layer of peptidoglycan and an outer membrane and MRSA cells are surrounded by cell walls with a relatively thick layer of peptidoglycan,

c. Paraprevotella produces endospores but MRSA does not.

d. Paraprevotella lacks a plasma membrane but MRSA cells are surrounded by two plasma membranes.