The present invention provides non-toxic Gram-negative bacteria. In particular, the present invention provides viable Gram-negative bacteria (e.g., E. coli) substantially lacking lipopolysaccharide (LPS, endotoxin) within the outer membrane. The present invention further provides methods of generating viable non-toxic Gram-negative bacteria and uses thereof. The present invention also provides compositions and methods for inducing immune responses and for researching and developing therapeutic agents.