Salivary gland microbiome of Anopheles gambiae: a mini-review of acquisition, composition, and functional significance

The salivary gland (SG) is the final barrier for Plasmodium transmission to humans but remains comparatively understudied relative to the midgut microbiome. This review synthesizes current knowledge on SG microbiome acquisition routes, composition, and functional significance. Acquisition may occur via larval filter feeding, vertical (egg smearing), transstadial, or horizontal transmission during blood feeding, though their relative contributions are unknown. Compositional studies show Gram-negative genera Serratia , Elizabethkingia , Acinetobacter , Pseudomonas , and Asaia predominate; Plasmodium infection correlates with increased Serratia and decreased Elizabethkingia abundance. While immune-related genes (e.g., cecropins, defensin, GNBP, SRPN6) expressed in the SG may be modulated by resident bacteria, direct evidence of their effect on sporozoite invasion remains lacking. Gram-negative bacteria trigger Toll, Imd, and JAK-STAT pathways, but emerging evidence suggests the SG may mount a distinct, locally independent immune response compared to the systemic pathway. Paratransgenesis using Asaia shows promise, yet SG-targeted effector delivery remains untested. Ecological pressures common in West Africa, including agricultural pesticides, insecticide resistance, and larval water contamination, may influence mosquito-associated bacteria, but no studies explicitly link these to the SG microbiome. Significant knowledge gaps persist, notably the absence of field studies in high-burden regions like Nigeria and the lack of experimental manipulation to establish causality. Addressing these priorities is critical to determine whether the SG microbiome can be exploited as a transmission-blocking target.