Nearly (?) sterile avian egg in a passerine bird

Abstract

During early ontogeny, microbiome affects development of the gastrointestinal tract, immunity and survival in vertebrates. Bird egg has been suggested to be either (1) initially sterile (Sterile egg hypothesis) and (2) colonized through horizontal trans-shell migration after egg laying, or (3) initially seeded with bacteria through vertical transfer from mother oviduct. Little empirical data illuminate so far the contribution of these mechanisms to gut microbiota formation in avian embryos. We evaluated microbiome of the egg content (day 0; E0-egg), embryonic gut at day 13 (E13) and female faeces in a free-living passerine, the great tit (Parus major), using a methodologically advanced procedure combining 16S rRNA gene sequencing and microbe-specific qPCR assays. Our metabarcoding revealed that avian egg is (nearly) sterile, barely distinguishable in microbial composition from negative controls. Of the three potentially pathogenic bacteria targeted by qPCR, only Dietzia was found in E0-egg (yet also in controls), E13 gut and female samples, which might indicate its possible vertical transfer. Unlike in poultry, we have shown in passerines that major bacterial colonisation of the gut does not occur before chick hatching. We stress that protocols carefully checking for environmental contamination are critically important in studies using samples with low bacterial biomass.

Impacts of coprophagic foraging behaviour on the avian gut microbiome

“…Like mammals, where colonization of the gut microbiome begins during and after birth by maternally acquired bacteria (de Goffau et al ., 2019; Kuperman et al ., 2020), initial acquisition of microbiota in birds occurs after hatching, as the egg generally remains sterile (Grond et al ., 2017; Zhou et al ., 2020 a ). Although bacteria from the oviduct and/or cloaca of the female parent may penetrate the egg and inoculate the growing foetus (Ballou et al ., 2016; Ilina et al ., 2016; Ding et al ., 2017; Broom & Kogut, 2018; Dietz et al ., 2019; Lee et al ., 2019 b ; Těšický et al ., 2023) this may only occur when the bacteria are pathogenic, rather than commensal (Ilina et al ., 2016). The vast majority of colonization and recruitment therefore involves post‐hatch exposure to microbes acquired through the environment (Grond et al ., 2019) and foraging (Grond et al ., 2017; Kobayashi et al ., 2019).…”

The strength of gut microbiota transfer along social networks and genealogical lineages in the house mouse

“…Vertebrate embryos generally develop in a sterile or near-sterile environment, and the colonization of their bodies by symbiotic bacteria begins as early as hatching or birth (Perez-Muñoz et al 2017 , Kennedy et al 2023 , Těšický et al 2024 ). Parental transmission of bacteria is an important pathway that facilitates the establishment of a normal microbiota in newborns.…”