Mosquitoes use a variety of cues to orient toward and land on potential blood meal hosts. These host attraction mechanisms involve physical cues such as heat and moisture, visual cues for day-feeding mosquitoes, and chemical cues. Carbon dioxide (CO2) is widely acknowledged as an important general host attraction cue, but it is less important for certain human-feeding specialist mosquitoes that are tasked with distinguishing between human hosts and other mammalian organisms. Such anthropophilic mosquitoes still utilize CO2 for long distance orientation toward hosts, but it can function as a deterrent at close range unless human skin odors are also present. For close range attraction and landing, anthropophilic mosquitoes rely primarily on olfactory cues like body odor, which are detected using specialized semiochemical receptors on their antennae.
Skin bacteria may have helped this mosquito locate her blood meal.
Human body odor is produced when secretions of the skin glands are metabolized into volatile fatty acids by bacteria that inhabit the skin. These bacteria-mediated volatiles play a critical role in the attraction of certain mosquitoes. The first evidence that the primary source of attraction was not the human skin substrate itself, but rather its microbiota, came when Knols and DeJong demonstrated that the anthropophilic malaria vector Anopheles gambiae was attracted to Limburger cheese. For maturation, this particular type of cheese depends on a Coryneform bacteria that is closely related to a bacteria found on human skin and involved with the production of similar fatty acids on both substrates. Eliminating skin microorganisms, such as by washing with bactericidal soap, removes important olfactory cues and significantly alters the feeding site selection of A. gambiae. Even in the absence of other host cues like heat and CO2, volatiles produced by human skin bacteria are attractive to mosquitoes.
There are certain volatiles that are consistently found across human hosts and known to be attractive to anthropophilic mosquitoes, such as a blend of ammonia, lactic acid, and carboxylic acids. However, there also exists differential attractiveness between humans due to their body odor emanations. These differences remain relatively stable over time and are influenced by the variation in composition and abundance of bacteria between individuals. Verhulst and colleagues demonstrated that as microbial diversity increases, relative attractiveness to anthropophilic mosquitoes decreases. This could be due to attenuation by other less attractive or even repellant compounds, as not all microorganisms and their subsequent volatiles contribute equally to host attraction.
Scanning electron micrograph of S. epidermidis, a strain of bacteria being used in host attraction research.
A recent article in Smithsonian Magazine discusses how a team of researches at Texas A&M University are exploiting bacteria’s role in host attraction as a way to potentially reduce human biting rates. Tomberlin and his colleagues are using a mutant form of the common skin bacteria Staphylococcus epidermidis to alter bacterial communication, making it more difficult for mosquitoes, particularly the yellow fever mosquito Aedes aegypti, to receive signals from the bacterial semiochemicals. When performing blood-feeding assays comparing the normal S. epidermidis to the silenced mutant type, twice as many mosquitoes were attracted to the wild type. This type of research could lead to the formulation of non-pesticide mosquito repellant alternatives.
The Reiskind lab is also investigating the role of bacterial communities and volatile chemical production in host attraction. We will share more about this project soon!