As we have previously shown that deletion of the lpp gene attenuated WT CO92 strain in mouse models of pneumonic and bubonic plague (up to 3 LD50; Sha et al., 2008, 2013a), we generated ΔlppΔypo0815, ΔlppΔypo2884, and ΔlppΔcyoABCDE double deletion mutants to determine whether additive or synergistic attenuation could be achieved in a pneumonic mouse model. The gene discussed is LPP; the disease is plague.