Chagas disease, also known as American trypanosomiasis, is a vector-borne disease that can cause death. It is estimated around 8 million people are infected worldwide, mostly in Latin America where the disease represents a public health problem [1].

The disease is caused by the bite of kissing bugs such as Rhodnius. prolixus, Triatoma. dimidiata, Triatoma. maculata and Triatoma. venosa [2] belonging to the Triatominae subfamily known to transmit the protozoan parasite, Trypanosoma. cruzi [1].

Recently, different studies showed that R. prolixus prefers bite the human face and feet [3], [4] and this specific attraction would be due to different factors such as CO2 [4], [5], heat, humidity and chemical compounds released during sweating [6] as well as the presence of volatile organic compounds (VOCs) produced by different bacteria [4], [7].

Indeed, human skin is colonized by a wide variety of beneficial microorganisms belonging to the phyla Proteobacteria (Pseudomonas, Janthinobacterium…), Actinobacteria (Corynebacterium, Kocuria, Microbacterium, Propionibacterium, Micrococcus), Firmicutes (Staphylococcus, Clostridium), Bacteroidetes, Cyanobacteria and Acidobacteria [8].

They form the skin microbiota and allow to inhibit the growth of pathogens and to promote the processing of proteins and free fatty acids on the skin [8], [9].

Moreover, the skin microbiota plays an important role in the generation of human odors [9]–[11] involving in the human-vector interactions via the releasing of more than 350 identified compounds [12], [13] whose 150 VOCs are released with the help of these bacteria [14] and are playing a role in attracting bloodsucking insects [10], [15] such as Anopheles gambiae mosquito [16], a phenomenon which would be dependent on the growth phase of the bacteria [16], [17].
Indeed, behavioural experiments showed that the combination of VOCs produced during the stationary growth phase of Corynebacterium minutissimum, Staphylococcus epidermidis, Brevibacterium epidermidis and Bacillus subtilis was attractive to An. gambiae [17].

Based on results obtained with An. gambiae and the preferences of R. prolixus to bite human face or feet, researchers from the University of Los Andes (Colombia) suggested that the attraction of R. prolixus to the human face could be a result of the VOCs produced by the bacterial microbiota present on facial skin.

Researchers studied the effect of VOCs produced in vitro by 8 bacterial species isolated from human skin on the attraction of R. prolixus and identified some of these VOCs produced by these bacterial strains tested behaviourally with R. prolixus.

Their results are published in PLOS Neglected Tropical Diseases [18].

From human facial skin of 10 volunteers, researchers identified 7 bacteria species by sequencing whose Staphylococcus. epidermidis which was the only species common to all volunteers
Among them, 6 bacterial strains were selected and 2 other ones were added (the last two in the list below) to make a growth analysis via  a culture in a standard-liquid medium:

  • Staphylococcus epidermidis 1,
  • Staphylococcus caprae 7P,
  • Staphylococcus capitis 11C
  • Citrobacter koseri 6P,
  • Micrococcus luteus 23
  • Dermacoccus nishinomiyaensis 9C.
  • Staphylococcus warneri
  • Brevibacterium epidermidis

All species showed similar growth curves and reached the stationary phase at 12-14h.
A first step consisted to evaluate the behavioural response of 68 kissing bugs to the VOCs released in vitro by the bacteria, using a dual-choice-T-olfactometer composed by 2 vials:

- one delivering a standard liquid medium without bacteria (control)
- the other delivering standard liquid medium with each species of bacteria at one or the   other growth phase (exponential or stationary phase).

Results showed that R. prolixus was not attracted by the control but was really attracted by the standard liquid medium containing 3 of 4 Staphylococcus species evaluated: 
S. capitis 11C and S.warneri in the exponential phase and S. epidermidis 1 in the stationary phase (Table 1).

However, R. prolixus showed no attraction for Citrobacter koseri 6P, Microccocus luteus 23 and Brevibacterium epidermidis in one or both growth phases (Table 1) and a total indifference to the VOCs produced by S. epidermidis 1, M. luteus 23 during exponential phase as well as by S. capitis 11C, S. warneri and B. epidermidis during the stationary phase but also by S. caprae 7P and D. nishinomiyaensis during the 2 growth phases (Table 1)

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Table1: Summary table showing behavioural response of R. prolixus to the VOCs released by the different bacteria (based on results of Tabares et al. 2018)

The second part aimed to identify different VOCs produced in vitro by these previous tested bacterial strains using gas chromatography coupled to mass spectrometry.
Researchers identified a total of 34 produced VOCs by these bacteria and C. koseri 6P and M. luteus 23 species were the ones showing the highest number of VOCs while S. caprae 7P had the lowest number.

Among all VOCs, CO2 and indole were the only two ones shared by all 8 species and presented differences in the average percentage area between species in exponential and stationary phases.

CO2 plays an important role to attract hematophagous insect [4], [5], [19], while indole is known to be a volatile compound released by the  microbiota in the oviposition sites in Anopheles, and it is also a major component of sweat and breath in humans [20]–[22].

In addition, the 3 attractive bacteria shared a common VOC: the benzene-acetaldehyde.
However, this VOC is also shared by other Staphylococcus species which didn’t show any attractiveness of kissing bugs.

Reciprocally, no-attractive bacteria shared 2 supplementary VOCs: the dimethyl disulfide and the phenol but they are also part of the chemical profile of other bacteria-phase which in the behavioural analysis didn’t show any, or even attraction.

Finally, statistical analysis showed a differentiation in profiles of the VOCs released in vitro by bacterial species-growth phase. Moreover, they indicated that behavioural responses of R. prolixus were highly dependent on the complex mixture of VOCs released by bacteria and not on single compounds, and this reminds similar results obtained with Anopheles [14].

This study is the first reporting the effect of VOCs released by in vitro bacteria isolated from human facial skin, on the R. prolixus behaviour.

It supports the tripartite interaction R. prolixus-bacteria-humans previously observed by other studies [4], [7], showing that these bacteria produce VOCs which could play an important role in the interaction between kissing bugs and human.

This study shows that Staphylococcus species tested here were attractive for R. prolixus,while non-Staphylococcus species were non-attractive. It contrasts with other results obtained with other vectors such as An. gambiae showing a clear preference for other species of bacteria different from Staphylococcus [17] suggesting a difference of attraction between these 2 hematophagous species. Indeed, while B. epidermidis in stationary phase was very attractive for An. gambiae [17], no behavioural response was observed from R. prolixus to this bacteria species. Moreover, the attractiveness of R. prolixus to this same bacteria in exponential phase was negative.

Thus, the behavioural response of vectors to bacteria seems to be very complex and seems to be dependent on the simultaneous interaction of several synergistic VOCs acting with antagonist or agonist manner [23].

Even if other analysis are necessary to evaluate the ecological functions of VOCs in R. prolixus, these findings highlight the potential of bacterial VOCs for biotechnological use and the identified mixtures can be used as baits or repellents to avoid R. prolixus-humans contact and reduce the risk of vectorial transmission of Chagas disease mediated by this species of triatomine.

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Bibliography

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