National Engineering Laboratory for Animal Breeding and MOA Key Laboratory of Animal Genetics and Breeding, College of Animal Science and Technology, China Agricultural University, Beijing, China.
PLoS One. 2019 Jul 24;14(7):e0220054. doi: 10.1371/journal.pone.0220054. eCollection 2019.
In this study, we compared the antibacterial effectivity of the eggs of six precocial and four altricial bird species using Escherichia coli, based on their eggshell traits. The ultrastructure of eggshell was observed using a scanning electron microscope (SEM). According to SEM results, eggs from precocial birds (chicken, turkey, quail, duck, ostrich, and goose) had cuticle on the eggshells, while eggs from altricial birds (pigeon, budgerigar, munia, and canary) did not. The environment/selection pressure may induce the divergent evolution process in eggs of precocial and altricial birds. The E. coli experiment results showed that chicken, turkey, quail, duck, and goose eggs, with a high cuticle opacity, exhibited a much lower E. coli penetration rate. In contrast, the eggs with poor (ostrich) or without (pigeon, budgerigar, munia, and canary) cuticle exhibited a higher penetration rate. It is suggested that cuticle is a main barrier against bacterial penetration in precocial birds' eggs. Turkey and quail eggs showed the lowest E. coli contamination rate (3.33% and 2.22%, respectively), probably because of the tightly connected nanosphere structure on their cuticle. As for altricial birds' eggs, the eggs of budgerigar, munia, and canary with small pore diameter (0.57 to 1.22 μm) had a lower E. coli penetration rate than pigeon eggs (45.56%, 66.67%, 50%, and 97.78%, respectively, P < 0.05), indicating that pore diameter played a significant role in defending against bacterial trans-shell invasion. We found that eggshell thickness and pore area decreased with egg size. The cuticle quality had no relationship with egg size, but was closely related to the bird species. The E. coli penetration rate of altricial birds' eggs was significantly higher than that of precocial birds' eggs, mainly because the pores are exposed on the eggshell surface and cuticle protection is absent. This study provides detailed information on the eggshell cuticle, which gives insight into the cuticle evolution process that occurred in precocial and altricial bird species. Moreover, the results of E. coli penetration may help understanding the antibacterial behavior in birds.
在这项研究中，我们基于蛋壳特征比较了六种早成鸟和四种晚成鸟的卵的抗菌效果，所用细菌为大肠杆菌。使用扫描电子显微镜（SEM）观察蛋壳的超微结构。根据 SEM 结果，早成鸟（鸡、火鸡、鹌鹑、鸭、鸵鸟和鹅）的卵壳有壳层，而晚成鸟（鸽子、虎皮鹦鹉、金丝雀和雀）的卵壳没有。环境/选择压力可能会导致早成鸟和晚成鸟的卵在进化过程中出现差异。大肠杆菌实验结果表明，壳层不透明性高的鸡、火鸡、鹌鹑、鸭和鹅的卵，大肠杆菌穿透率较低。相比之下，壳层差（鸵鸟）或无（鸽子、虎皮鹦鹉、金丝雀和雀）的卵，大肠杆菌穿透率较高。这表明壳层是早成鸟卵抵御细菌穿透的主要屏障。火鸡和鹌鹑的卵大肠杆菌污染率最低（分别为 3.33%和 2.22%），这可能是由于其壳层上紧密连接的纳米球结构。对于晚成鸟的卵，虎皮鹦鹉、金丝雀和雀的卵由于其较小的孔径（0.57 至 1.22μm），大肠杆菌穿透率较低（分别为 45.56%、66.67%、50%和 97.78%，P<0.05），表明孔径在抵御细菌穿透蛋壳方面起着重要作用。我们发现蛋壳厚度和孔面积随卵的大小而减小。壳层质量与卵的大小无关，但与鸟类密切相关。晚成鸟的卵大肠杆菌穿透率显著高于早成鸟的卵，主要是因为卵壳表面暴露的孔和缺乏壳层保护。本研究提供了有关蛋壳壳层的详细信息，深入了解了早成鸟和晚成鸟种中发生的壳层进化过程。此外，大肠杆菌穿透率的结果可能有助于了解鸟类的抗菌行为。
