[Changes in murine skin odors following Plasmodium infections and their impact on mosquito attraction].

OBJECTIVE: To investigate the alterations in skin volatile odors in mice following Plasmodium infections and their effect on mosquito attraction, and to analyze the changes in murine skin microbiota, so as to provide the scientific evidence for unraveling pathogen-host-vector interactions and management of vector-borne diseases.

METHODS: Twenty 6-week-old female mice of the C57BL/6 strain were randomly divided into the infection and control groups, of 10 mice in each group. Mice in the infection group were each injected with 1 × 106 Plasmodium yoelii via the tail vein, and mice in the control group received an equiv-alent volume of phosphate-buffered saline (PBS). Blood samples were collected from mouse tail vein daily on days 1 to 6 post-infection for preparation of blood smears for microscopic observation to dynamically monitor changes in parasitaemias. A triplecage olfactometer was deployed to compare the numbers of Anopheles stephensi attraction to mice between the two groups. Mouse cutaneous volatile odors were collected with adsorbents and analyzed by gas-chromatography-mass-spectrometry (GC-MS) to identify odorous molecules, and the amounts of odorous molecules on mouse skin were compared between groups. In addition, mouse skin microbiota was collected with cottonswabs for 16S rRNA gene amplicon sequencing to compare the relative abundance of bacteria in mouse skin microbiota between the two groups.

RESULTS: The parasitaemias were 0, (2.30 ± 0.87)%, (8.00 ± 4.34)%, (31.30 ± 3.51)%, (42.00 ± 2.65)% and (51.00 ± 3.61)% in mice in the infection group on days 1 to 6 post-infection with Plasmodium (F = 165.60, P < 0.001), and the gametocytaemias were 0, (0.14 ± 0.06)%, (0.39 ± 0.10)%, (0.63 ± 0.15)%, (1.10 ± 0.10)% and (1.53 ± 0.31)%, respectively (F = 44.58, P < 0.001). Pairwise comparisons showed the highest parasitaemias and gametocy taemias in mice 6 days post-infection (both P values < 0.05), and linear regression analysis revealed that both the parasitaemias (b = 11.36, t = 14.43, P < 0.001) and gametocytaemias (b = 0.31, t = 12.80, P < 0.001) appeared a tendency towards a rise over days. The proportions of mosquito attraction to mice were 50.45% (106/210), 49.55% (119/240), 49.18% (112/227), 55.87% (132/236), 66.84% (159/237), 61.32% (138/226) and 54.65% (126/230) in the infection group on the day of infection and on days 1 to 6 post-infection, which appeared a tendency towards a rise over days (χ2 = 9.54, P < 0.05). A total of 24 odors were identified in mouse skin surface, and Plasmodium-infected mice exhibited significantly higher enrichment of p-cresol (134 954.86 ± 40 485.75 vs. 34 700.13 ± 4 774.68; t = 4.260, P = 0.013), ethylbenzene (1 214 980.59 ± 111 546.49 vs. 355 445.01 ± 53 369.70; t = 12.04, P = 0.00) and nonanal (62 215.11 ± 11 348.82 vs 24 040.15 ± 8 557.10; t = 4.35, P = 0.02), and lower contents of toluene (61 833.23 ± 2 755.23 vs. 152 906.21 ± 10 199.69; t = 14.93, P = 0.00), benzaldehyde (583 921.81 ± 39 764.63 vs. 1 071 368.84 ±254 069.28; t = 3.28, P = 0.00) and indole (10 991.89 ± 582.76 vs. 27 275.57 ± 3 995.59; t = 6.99, P = 0.00) relative to controls. In addition, higher relative abundance of Streptococcus (0.29 ± 0.12 vs. 0.12 ± 0.09; t = 2.54, P = 0.03) and Rothia (0.16 ± 0.05 vs. 0.04 ± 0.06; t = 3.52, P = 0.01) and lower abundance of Lactococcus (0.02 ± 0.04 vs. 0.27 ± 0.20; t = 2.73, P = 0.03) were found on mouse skin surface in the infection group than in the control group.

CONCLUSIONS: Upon infection with Plasmodium, the volatile odor profile emitted from the skin of mice undergoes alterations, resulting in increased attractiveness to mosquitoes. This phenomenon may be attributed to parasite-induced changes in the skin microbiota.