The interactions between herbivorous insects and their symbiotic micro-organisms can be influenced by the plant species on which the insects are reared, but the underlying mechanisms are not understood. Here, we identify plant nutrients, specifically amino acids, as a candidate factor affecting the impact of symbiotic bacteria on the performance of the phloem-feeding aphid Aphis fabae. Aphis fabae grew more slowly on the labiate plant Lamium purpureum than on an alternative host plant Vicia faba, and the negative effect of L. purpureum on aphid growth was consistently exacerbated by the bacterial secondary symbionts Regiella insecticola and Hamiltonella defensa, which attained high densities in L. purpureum-reared aphids. The amino acid content of the phloem sap of L. purpureum was very low; and A. fabae on chemically defined diets of low amino acid content also grew slowly and had elevated secondary symbiont densities. It is suggested that the phloem nutrient profile of L. purpureum promotes deleterious traits in the secondary symbionts and disturbs insect controls over bacterial abundance.

The cells and tissues of many aphids contain bacteria known as "secondary symbionts," which under specific environmental circumstances may be beneficial to the host insect. Such symbiotic bacteria are traditionally described as intractable to cultivation in vitro. Here we show that two types of aphid secondary symbionts, known informally as T type and U type, can be cultured and maintained in three insect cell lines. The identities of the cultured bacteria were confirmed by PCR with sequencing of 16S rRNA gene fragments and fluorescence in situ hybridization. In cell lines infected with bacteria derived from aphids harboring both T type and U type, the U type persisted, while the T type was lost. We suggest that the two bacteria persist in aphids because competition between them is limited by differences in tropism for insect tissues or cell types. The culture of these bacteria in insect cell lines provides a new and unique research opportunity, offering a source of unibacterial material for genomic studies and a model system to investigate the interactions between animal cells and bacteria. We propose the provisional taxon names "Candidatus Consessoris aphidicola" for T type and "Candidatus Adiaceo aphidicola" for U type.