When Small Meets Smaller: Immune Modulation and Virulence Strategies in Insect–Bacteria Interactions

Insects represent powerful experimental systems for investigating host–microorganism
interactions, providing valuable insights into bacterial pathogenicity, immune regulation,
symbiosis, and antimicrobial discovery. This review examines the complex relationships
between insects and bacteria, focusing on the mechanisms that control infection, immune
activation, and microbial adaptation. Particular attention is given to the routes of pathogen
entry and to the conserved innate immune pathways that coordinate host defenses, including
the Toll, Imd, Duox, and Jak/Stat signaling cascades. The review illustrates how
bacterial pathogens exploit toxins, immune evasion strategies, and metabolic adaptation
to overcome host defenses, while insects rely on tightly regulated cellular and humoral
responses, antimicrobial peptides, melanization, and microbiota-mediated homeostasis.
Interactions between pathogenic and commensal bacteria in the insect gut are discussed
in the context of immune tolerance, dysbiosis, and ecological adaptation. The dual role
of bacterial virulence factors in both pathogenesis and symbiosis is highlighted through
examples involving entomopathogenic bacteria such as Photorhabdus spp., Xenorhabdus
spp., and Bacillus thuringiensis. In addition, the review summarizes the use of insect models,
including Drosophila melanogaster, Galleria mellonella, Bombyx mori, and Apis mellifera, in
experimental infections aimed at studying virulence mechanisms, host immune responses,
and antimicrobial efficacy. Finally, multi-omic approaches, including transcriptomics,
metabolomics, epigenomics, and single-cell technologies are discussed as transformative
tools for dissecting host–microbe interactions at molecular and systems levels. Overall, insect–bacteria interactions emerge as dynamic and evolutionarily shaped systems in which
immunity, metabolism, microbiota composition, and environmental factors are closely interconnected,
offering important perspectives for both basic research and the development of sustainable biocontrol and antimicrobial strategies.