Eggshells as hosts of bacterial communities: An experimental test of the antimicrobial egg coloration hypothesis

Abstract

Oviparous animals have evolved multiple defenses to prevent microbes from penetrating their eggs and causing embryo mortality. In birds, egg constituents such as lysozyme and antibodies defend against microbial infestation, but eggshell pigments might also impact survival of bacteria. If so, microbes could exert an important selective pressure on the evolution of eggshell coloration. In a previous lab experiment, eggshell protoporphyrin caused drastic mortality in cultures of Gram positive, but not Gram negative, bacteria when exposed to light. Here, we test this “photodynamic antimicrobial hypothesis” in a field experiment. In a paired experimental design, we placed sanitized brown, protoporphyrin‐rich chicken eggs alongside white eggs that lack protoporphyrin. We deployed eggs for 48 hr without incubation, as can occur between laying and incubation, when microbial infection risk is highest. Eggs were placed on the open ground exposed to sunlight and in dark underground storm‐petrel burrows. We predicted that the proportion of Gram‐positive bacteria on brown eggs should be lower when exposed to sunlight than when kept in the dark, but we expected no such difference for white eggs. Although our data revealed variation in bacterial community composition, the proportion of Gram‐positive bacteria on eggshells did not vary by egg color, and there was no interaction between egg color and location. Instead, Gram‐positive bacteria were proportionally more common on eggs on the ground than eggs in burrows. Overall, our experiment did not support the photodynamic antimicrobial hypothesis. The diverse range of avian egg colors is generated by just two pigments, but over 10 hypotheses have been proposed for the evolution of eggshell color. If our results are generalizable, eggshell protoporphyrin might not play a substantial role in defending eggs against microbes, which narrows the field of candidate hypotheses for the evolution of avian eggshell coloration.

A Preliminary Investigation of Salmonella Populations in Indigenous Portuguese Layer Hen Breeds

“…Also, Leleu et al [54] found a large variation in cuticle coverage and quality within groups of white and brown eggs from old hens, and Ishikawa et al [55] demonstrated that brown eggshells and their pigments were active against Gram-positive bacteria but not against Gram-negative bacteria including Salmonella Enteritidis. However, these results were not corroborated by Dearborn et al [56], where the proportion of Gram-positive bacteria on eggshells did not vary by egg color.…”

Eggshell spotting pattern is related to hatching asynchrony, hematocrit value and growth of nestling great titsParus major

“…For example, it is possible that eggshell pigments modify the amount and wavelength of light transmitted through the eggshell and thus affect embryonic development (Maurer et al 2015, Lahti andArdia 2016). Eggshell pigments may play a thermoregulatory role (Bakken et al 1978, Westmoreland et al 2007, and pigments may also decrease trans-shell contamination by microbes (Ishikawa et al 2010, but see Fargallo et al 2014, Dearborn et al 2017. It is also possible that eggshell pigments strengthen thin eggshells (Gosler et al 2005, García-Navas et al 2010, Hargitai et al 2013.…”

Mixed-Mode Bacterial Transmission via Eggshells in an Oviparous Reptile Without Parental Care

“…For example, establishing some bacteria (e.g., Enterobacteriaceae ) on the eggshells of lizard ( Sceloporus virgatus ) can improve egg hatching success by protecting the embryos from fungal infection ( Bunker et al, 2021 ). In these respects, we speculate that the eggshell acts as a transgenerational carrier for symbiotic microbes ( Dearborn et al, 2017 ; van Veelen et al, 2018 ). In oviparous birds, symbiotic microbes are transmitted from mother to eggshell via brood patches, nest material, and maternal skin, but not the maternal cloaca ( van Veelen et al, 2018 ).…”