Sanford School of Medicine, Division of Basic Biomedical Sciences, University of South Dakota, Vermillion, SD, USA.
J Med Entomol. 2023 Sep 12;60(5):875-883. doi: 10.1093/jme/tjad076.
The German cockroach (Blattella germanica) has been linked to transmission of Salmonella enterica serovar Typhimurium (S. Typhimurium), but infection dynamics within this vector are poorly characterized. Our recent work has focused on S. Typhimurium infection in the cockroach gut. However, microbial dissemination to the hemolymph is an essential aspect of many vector-borne pathogen transmission cycles and could potentially contribute to S. Typhimurium colonization of cockroaches. Therefore, the goal of this study was to examine the ability of S. Typhimurium to disseminate, survive, and proliferate in the hemolymph of cockroaches after oral infection. We detected only low numbers of bacteria in the hemolymph of a minority of insects (~26%) after oral infection. Further, S. Typhimurium was unable to survive overnight in cell-free hemolymph. Several hypotheses to explain the inability of S. Typhimurium to colonize hemolymph were tested. First, we investigated the ability of S. Typhimurium to metabolize trehalose, the primary sugar in hemolymph. S. Typhimurium grew efficiently in vitro using trehalose as a sole carbon source and mutant strains lacking trehalose metabolism genes exhibited no growth deficiencies in media mimicking the composition of hemolymph, suggesting that trehalose metabolism ability is not a factor involved in restricting survival in hemolymph. On the other hand, heat-inactivated cell-free hemolymph was permissive of S. Typhimurium growth, demonstrating that survival in hemolymph is limited specifically by heat-labile humoral factors. The involvement of cellular immune responses was also investigated and cockroach hemocytes in culture were observed to internalize S. Typhimurium within 1 h of exposure. Most hemocytes harbored few to no bacteria after 24 h, indicating that hemocyte responses are additionally involved in clearing infection from the hemolymph. However, dense intracellular clusters of S. Typhimurium were observed sporadically, suggesting a small subset of hemocytes may serve as reservoirs for bacterial replication. Together, our results reveal that a minute proportion of ingested S. Typhimurium is able to escape the cockroach gut and enter the hemolymph, but this systemic population is limited by both humoral effectors and hemocytes. Thus, we conclude that invasion of the hemolymph appears minimally important for colonization of the cockroach vector and that colonization of the gut is the main driver of vector-borne transmission. Our insight into the antimicrobial mechanisms of cockroach hemolymph also highlights the strong ability of these prevalent pests/vectors to cope with frequent infectious challenges in septic habitats.
德国蟑螂（Blattella germanica）已被证实可传播肠炎沙门氏菌血清型鼠伤寒（S. Typhimurium），但这种媒介体内的感染动态仍知之甚少。我们最近的工作重点是研究蟑螂肠道中的 S. Typhimurium 感染。然而，微生物向血淋巴的传播是许多载体传播病原体传播周期的一个重要方面，并且可能有助于 S. Typhimurium 在蟑螂体内的定植。因此，本研究的目的是研究 S. Typhimurium 在经口感染后在蟑螂血淋巴中传播、存活和增殖的能力。我们仅在少数昆虫（约 26%）的血淋巴中检测到低数量的细菌。此外，S. Typhimurium 无法在无细胞血淋巴中过夜存活。我们测试了几种解释 S. Typhimurium 无法定植血淋巴的假说。首先，我们研究了 S. Typhimurium 代谢血淋巴中主要糖——海藻糖的能力。S. Typhimurium 在体外有效地利用海藻糖作为唯一碳源生长，并且缺乏海藻糖代谢基因的突变株在模拟血淋巴组成的培养基中没有生长缺陷，这表明海藻糖代谢能力不是限制其在血淋巴中存活的因素。另一方面，热失活的无细胞血淋巴允许 S. Typhimurium 生长，表明其在血淋巴中的存活受到热不稳定的体液因子的限制。我们还研究了细胞免疫反应的参与情况，并观察到培养中的蟑螂血细胞在暴露于 S. Typhimurium 后 1 小时内内化了该细菌。24 小时后，大多数血细胞中仅有少量至无细菌，表明血细胞反应也参与了从血淋巴中清除感染。然而，偶尔会观察到 S. Typhimurium 的密集细胞内簇，这表明一小部分血细胞可能是细菌复制的储存库。总之，我们的结果表明，摄入的 S. Typhimurium 只有一小部分能够逃脱蟑螂肠道并进入血淋巴，但这种全身种群受到体液效应物和血细胞的限制。因此，我们得出结论，血淋巴的入侵对于蟑螂载体的定植作用很小，而肠道的定植是载体传播的主要驱动力。我们对蟑螂血淋巴抗菌机制的深入了解也突出了这些普遍存在的害虫/载体应对在感染环境中频繁面临的感染挑战的强大能力。
