In July 2022, Cheng et al. reported that acetophenone released from the skin microbiota of flavivirus-infected hosts (mice and patients with dengue fever) acts as a potent attractant for
Aedes mosquitoes, which are vectors for dengue (DENV2), chikungunya (CHIKV), and Zika (ZIKV) virus transmission, thereby increasing flavivirus transmission to aggravate dengue, chikungunya, and Zika diseases
[11][5].
Aedes mosquitoes prefer to feed on mice infected by dengue and Zika viruses due to the released acetophenone compared to uninfected mice in control groups, in which acetophenone is absent. Female
Aedes aegypti (
Ae. aegypti) and
Aedes albopictus (
Ae.
albopictus) mosquitoes were tested in these experiments, which showed that flaviviruses can stimulate the proliferation of acetophenone-producing skin commensal bacteria (especially the
Bacillus genus) by suppressing the expression of the essential antimicrobial resistin-like molecule-α (RELMα) protein that protects against pathogenic bacterial skin infections, thereby killing the microbes. In particular, viruses can change host odors to attract mosquitoes.
It is worth noting that the use of isotretinoin, a naturally occurring retinoic acid and a vitamin A derivative, reduces attractiveness to mosquitoes through the activation of the expression of the RELMα protein in the skin of flavivirus-infected animals
[11][5]. This interesting and important finding caught the attention of scientists who made brief but substantial and picturesque comments on Cheng’s work in prestigious journals such as
Science [12][6],
Nature Reviews Microbiology [13][7], and
Cell [14][8]. Regardless,
wresearche
rs wanted to examine this simple ketone from the perspective of synthetic and applied organic chemistry and chemical ecology fields.
It should be noted that the connection between the skin microbial population and attractiveness to mosquitoes and other blood-sucking insects has been studied, for example, in the African malaria mosquito
Anopheles gambiae (
An. gambiae) (infected with
Plasmodium falciparum sporozoites), which plays an important role in malaria transmission
[15,16[9][10][11][12],
17,18], and the triatomine bug
Rhodnius prolixus, which is the main vector transmitting the parasite
Trypanosoma cruzi, the causative agent of Chagas disease
[19,20][13][14]. Their host-seeking behavior is influenced by host scents, i.e., appropriate concentrations of volatile odors (a mixture of molecules, so-called molecular stimuli, such as ammonia, isobutylamine, acetone, lactic acid, isobutyric acid, and CO
2)
[19][13] present in human (or animal) skin microflora that can activate, attract, and even repel these blood-sucking insects.
Blood-sucking insects have excellent sensory abilities for detecting and following the physical and chemical signals (semiochemicals) emitted by their hosts;
Aedes mosquitoes are not an exception. Cheng’s work
[11][5] showed that the volatile acetophenone in the host skin microbiota produced a considerable electrophysiological response detectable by the antenna of
Ae. aegypti, and thus, high levels of acetophenone attracted more mosquitoes than uninfected mice and healthy individuals in control groups. Nevertheless, close derivatives of acetophenone (
1), 4-ethylacetophenone (
2), and 4-ethoxyacetophenone (
3) (
Figure 1) had repellent effects on the malaria mosquito
An. gambiae [18,21][12][15].
Moreover, acetophenone (
1) and its
para-ethyl analog
2 were found to exhibit significant repellency for the male Asian tiger mosquito,
Ae. albopictus, but only at a high concentration (10
−2)
[22][16]. The latter metabolite has been described as a male-swarming aggregation pheromone for
Ae. aegypti mosquitoes
[23][17], which increases female attraction and mating success. However, none of the three acetophenone derivatives
1–
3 were identified among specific semiochemicals, i.e., acetoin (3-hydroxy-2-butanone), sulcatone (6-methyl-5-hepten-2-one), octanal, nonanal, and decanal, released from the males of
Anopheline species—
Anopheles arabiensis and
An. gambiae [24][18]. Such pheromones can remarkably manipulate these highly dangerous insects.
On the other hand, acetophenone (
1) was found to be an effective repellent in field experiments
[25][19] for tsetse flies (
Glossina spp.) infected by
Glossina hytrosavirus, the primary vector of African trypanosomes, which cause human African trypanosomosis (HAT or sleeping sickness) and African animal trypanosomosis (AAT or nagana)
[26][20]. Recently, it was reported that acetophenone-containing Zebra skin odor repels the savannah tsetse fly
[27[21][22],
28], confirming previous results from field experiments
[25][19]. Therefore, the diametrically opposite bioeffects of acetophenone (
1) mentioned above could be used in the control and prevention of heralded diseases. This volatile ketone could be a suitable biomarker for detecting pathogens that actively manipulate host odors. However, the development of these volatile-based diagnostics, which use the “signal” of disease and the background “noise” of genetic and environmental variations, is still in its infancy
[29,30][23][24]. On the other hand, odor-bait technology as a surveillance and control tool for insect vectors, such as tsetse flies or
Aedes mosquitoes, has been promoted as a new and viable component of the integrated vector management program
[31,32,33][25][26][27]. In this context, some extracts and essential oils from aromatic plants could be more advantageous in the biological control of pest insects. Nonetheless, there is some skepticism over this aspect
[34][28].
Another analog of acetophenone, i.e., 2-aminoacetophenone, secreted by
Pseudomonas aeruginosa, which is a ubiquitous opportunistic human pathogen, can facilitate attraction to food for several fly species, including
Musca domestica,
Ceratitis capitata, and
Drosophila melanogaster [35][29].
Investigations on the involvement of plant volatiles in the feeding attraction behavior of mosquitoes were conducted before Chang’s work
[11][5]. Mosquitoes visit flowers for nectar and may, in turn, act as pollinators for plants
[36][30]. Like animal skin microflora, the composition of the inflorescence odor of plants can be an attractant or a repellent for mosquitoes
[37][31].
Gonzalez-Audino et al. demonstrated that freshly cut inflorescences of the perennial herb “alyssum”
Lobularia maritima (Cruciferae) stimulated a positive flight response in both sexes of
Ae. Aegypti [38][32]. Moreover, acetophenone and four other compounds (1-octanol, 2-phenylethanol, benzyl cyanide, and benzyl isothiocyanate) were identified from the headspace of
Lobularia maritima (
L. maritima) by direct comparison with analytical standards. By testing the flight preference toward single synthetic compounds identified in the headspace of
L. maritima, the
resea
uthorchers showed that acetophenone (
1) was the first plant-derived compound that elicited positive flight behavior in
Ae. aegypti (
Figure 2).