Secondary symbionts of insects: acetic acid bacteria

The insect digestive system is emerging as a new niche from which acetic acid bacteria (AAB) have been recovered and characterized. An environment permeable to oxygen and rich in different kinds of sugars, according to the diet followed by the insect, and the acidic conditions, typically associated to specific parts of the digestive system, are the main characteristics of this AAB isolation source. AAB are indeed well known for the capability to grow and flourish in acidic conditions and to produce several acids and alcohols metabolizing/consuming sugars. To date, symbiotic AAB have been identified from different insect orders, among which Diptera, such as the fruit fly Drosophila melanogaster, the olive fruit fly Bactrocera oleae, and the mosquitoes of the genus Anopheles and Aedes, Hymenoptera, such as the honey bee Apis mellifera, and Hemiptera, such as the leafhopper Scaphoideus titanus and the mealybug Saccharicoccus sacchari. Interestingly these insects rely on sugar-based diets, represented by sugars from fruits, nectars or phloematic sap, thus giving a clue to a possible bacterial contribution to the insect nutritional function. Recently, AAB have been described as involved in the insect innate immune response participating in the maintenance of the insect gut homeostasis. As documented for Asaia symbionts, these bacteria are vertically and horizontally transmitted and are able to cross-colonizing insects that belong to phylogenetically distant orders and genera. AAB possess interesting characteristics that could be exploited in the development of symbiotic control strategies.