The invention relates to improved methods of producing and recovering sporulation-deficient B. anthracis mutant stains, and for producing and recovering recombinant B. anthracis protective antigen (PA), especially modified PA which is protease resistant, and to methods of using of these PAs or nucleic acids encoding these PAs for eliciting an immunogenic response in humans, including responses which provide protection against, or reduce the severity of, B. anthracis bacterial infections and which are useful to prevent and/or treat illnesses caused by B. anthracis, such as inhalation anthrax, cutaneous anthrax and gastrointestinal anthrax.