Evaluation of Providencia rettgeri pathogenicity against laboratory Mediterranean fruit fly strain (Ceratitis capitata)

The Mediterranean fruit fly (medfly) Ceratitis capitata (Wiedemann) (Diptera: Tephritidae), is often referred to as the most severe agricultural pest. Its biological control is mainly through the Sterile Insect Technique (SIT). Colonization, mass-rearing conditions and the irradiation process impact the competitiveness of sterile males and disrupt symbiotic associations by favoring some bacterial species and suppressing others. Levels of Providencia species have been shown to fluctuate considerably in the gut of the medfly laboratory strain Vienna 8 under irradiation, increasing by up to 22%. This study aimed to determine the pathogenicity of Providencia rettgeri isolated from the gut of laboratory Vienna 8 medfly strains by examining the effects of 1) two different treatment doses on egg-hatching and development and 2) two infection methodologies (ingestion and injection) of male and female adults according to their mating status. Treatment of eggs with P. rettgeri (2%) significantly decreased the mean egg to pupae recovery rate. Our data showed significant high mortality in flies with both injection and ingestion after 24 hours without any effect of sex. Microbial counts demonstrated that the bacteria could proliferate and replicate in adult flies. There was a significant sex-dependent effect after infection, with mortality decreasing significantly for males more than females. Providencia rettgeri can be considered as a potential pathogen of C. capitata. Mating protected males and females against infection by P. rettgeri by triggering an immune response leading to double the levels of Cecropin being secreted compared to infected virgin adults, thus reducing the virulence of the bacteria.

地中海实蝇（Mediterranean fruit fly, medfly）*Ceratitis capitata*（Wiedemann）隶属于双翅目（Diptera）实蝇科（Tephritidae），被公认为危害最为严重的农业害虫之一。其生物防治主要依赖昆虫不育技术（Sterile Insect Technique, SIT）。种群定殖、大规模饲养条件以及辐照过程会影响不育雄虫的交配竞争力，并通过偏好部分细菌类群、抑制其他类群的方式破坏共生菌群的稳态平衡。研究表明，在辐照处理下，地中海实蝇实验室品系维也纳8号（Vienna 8）肠道内普罗威登斯菌属（Providencia）的丰度会发生显著波动，最高可增长22%。本研究旨在明确从维也纳8号实验室品系地中海实蝇肠道中分离得到的雷氏普罗威登斯菌（Providencia rettgeri）的致病性，具体通过两项实验开展：1）两种不同处理剂量对卵孵化率及幼虫发育的影响；2）基于成虫交配状态，探究两种侵染方式（饲喂侵染与注射侵染）对雌雄成虫的作用。使用2%浓度的雷氏普罗威登斯菌处理卵块，可显著降低卵到蛹的平均回收率。实验数据显示，无论是注射侵染还是饲喂侵染，成虫在侵染后24小时内均出现显著的高死亡率，且该死亡率不受性别影响。微生物计数结果证实，该细菌可在成虫体内增殖繁殖。但侵染后存在显著的性别依赖效应：雄虫的死亡率较雌虫出现了更为显著的下降。综上，雷氏普罗威登斯菌可被视为地中海实蝇的潜在致病菌。交配可通过激活免疫应答使雌雄成虫抵御雷氏普罗威登斯菌侵染：与未交配的侵染成虫相比，交配个体的天蚕素（Cecropin）分泌水平提升了一倍，从而降低了该细菌的毒力。