In Drosophila, the Toll pathway was discovered to be contributed to fungal and bacterial infections and activation by Spaetzles and PGRPs. For regulation of the innate immune system in Drosophila, PGRP-SA cooperates with PGRP-SD in the hemolymph and is required for the activation of Toll pathway by binding to Lys-type PGN (peptidoglycan), and PGRP-LC and PGRP-LE recognize the DAP-type PGN of gram-negative bacteria to activate the Imd pathway (Zhang et al., 2019). This evidence concerns the gene TLR4 and bacterial infectious disease.