# Bacterial Semichemicals and Transkingdom Interactions with Insets and Plants
## 1 Introduction
### Bacteria and Eusocial Insect Communication  
In social nest-living insects, BVOCs regulate social behaviors. For example, in _Atta sexdens_ (Hymenoptera, Formicidae), 2,5-dimethylpyrazine and 3-ethyl-2,5-dimethylpyrazine were identified in poison gland secretion and work as powerful trail pheromones (Figure 1). In _A. sexdens_ var rubropilosa (Hymenoptera, Formicidae), _Serratia marcences_, a common endosymbiotic strain in insects, is involved in pyrazines biosynthesis starting from L-threonine as a precursor [37]. In addition to bacteria, yeast species also contribute to semiochemical communication in Hypernoptera. In North American yellowjacket wasps (Hymenoptera, Vespidae), two species of fungi produce VOCs that attract conspecific individuals. _Hanseniuspora uvarum_ and _Lachancea thermotolerans_ were attractive to three species of yellowjacket wasps (_V. pensylvanica_, _V. germanica_, and _V. alascensis_) when grown on grape juice-infused yeast peptome dextrose (YPD) agar [38]. Gas chromatography-mass spectrometry analysis revealed that ethyl acetate (only _H. uvarum_), phenylethyl acetate, isoamyl acetate, 2-phenylethyl alcohol, and 2-methyl-butanol were the most abundant headspace volatiles produced by these yeasts. This information was instrumental to formulate a synthetic blend that was proven to be effective in attracting _V. pensylvanica_, but not the other yellowjacket species [38]. Another important type of communication in social insects is based on chemical recognition of CHCs. In termite species, the aggressive response is common when a parasite or non-nestmate insect penetrates the nest. In _Reticulitermes speratus_ (Isoptera, Rhinotermitiidae), antibiotic feeding increases the mortality and aggression of test individuals [39]. Similar effects were obtained with the addition of extracts from gut bacteria of non-nestmate individuals [39]. The death or reduction of microbial symbionts also affects the fitness of individuals, showing a reduction in body size and in mechanical resistance of the exoskeleton formed by a thinner cuticle [40]. Direct evidence of CHC metabolism by gut bacteria was obtained in _Zootermopsis nevadensis_ (Isoptera, Termopsidae) using radiolabeled succinate, a precursor of methyl-branched hydrocarbons, which was transformed by bacteria into propionate and incorporated in the cuticle as 5-methyl-heneicosane or 5,17-dimethylheneicosane by insect cells [41]. A specific analysis of the effects of gut microbiota on cuticular hydrocarbon metabolism in ants was performed by [PERSON] and colleagues [41] based on three point-of-view approaches. These experiments investigated the relationship between variation in bacterial community, differences in CHC profiling, and nestmate recognition. The authors manipulated the gut microbiota of _Acromyrmex echinator_ leaf-cutting ants (Hymenoptera, Formicidae) using tetracycline. Before partially restoring the microbiota through fecal droplet administration, they analyzed the relationship between microbiota structure and insect survival and found that survival was enhanced by endosymbionts. Moreover, they proved that antibiotic treatment modified the normal microbiota structure and affected the CHC chemical profile. Additionally, results showed that modified CHC chemical profile was significantly correlated with enhanced aggression and misrecognition. Variations in CHC levels were significant for 4-oxo-octanoic acid, 4-oxo-decanoic acid and in long-chain linear alkanes (n-C36 and n-C40), which decreased in antibiotic-treated insects compared to untreated controls [42]. Finally, in eusocial insects, the gut microbiota assemblage reflects the exposure to social interactions: Oral trophallaxis enhanced the development a healthy \"core microbiota\" in the bee. Gram negative bacteria, such as _Gilliamella apicola_, _Frischella perrara_, and _Snodgrassella alvi_, are acquired with either social interaction between young individuals, nurse/workers individuals, or those that have contact on hive surfaces.