Nitric oxide synthase (NOS), the enzyme responsible for the production of endogenous nitric oxide from arginine, has been recently discovered in a number of Gram-positive bacteria. While bacterial NOS has been implicated in mediating nitrosative stress, much remains unknown about the functional role of endogenous nitric oxide in bacteria. Using the known NOS inhibitor aminoguanidine, we examined changes in the protein expression profile using two-dimensional gel electrophoresis. Treatment with aminoguanidine induced several changes in protein expression in Bacillus subtilis. In particular, mreB-like protein (Mbl) was fully down-regulated in the aminoguanidine-treated samples. The expression of Mbl was also examined by reverse transcriptase-polymerase chain reaction and Mbl was found to be fully down-regulated at the transcriptional level as well. Given the role that Mbl plays in the maintenance of cytoskeletal structure, it appears that bacterial NOS may participate in specific biosynthetic pathways with ramifications toward the regulation of antibiotic resistance.