Egg contamination as an indicator of environmental health

Contamination of egg contents is not only a problem for human health, but can also be an indicator of environmental pollution. Contamination by bacteria (including multiply antibiotic resistant bacteria, or MARB), fungi, and heavy metals has been observed, and may occur either by direct transmission from the ovary and uterus (vertical infection), by horizontal infection from contaminated feces, or by transfer from the surroundings after the eggs have been laid. The avian egg is porous with a thick, hard shell and soft outer coat. Its structure varies widely among species. The shell consists of protein matrix and 95-97% calcium carbonate crystals, and below it is the fibrous shell membrane surrounding the albumen. Turtle eggshell consists of three layers: outer calcareous, middle multistrata and inner membrane. The calcareous layer has loose nodular units varying in shape and size without interlocking attachments, resulting in numerous spaces and openings. Each nodular unit has spicules arranged in folded stacks. The eggshell membrane is the major supplier of calcium, as well as traces of other elements, during embryogenesis. Penetration of bacteria into eggs is facilitated by the porous structure of the eggshell and egg membrane, and is primarily a function of time and the number of bacteria present. Microbial penetration of chicken and turtle eggshells takes place within a few minutes, depending on the type of bacteria. Pseudomonas aeruginosa was found to be the most concentrated in egg components followed by Salmonella typhimurium, Staphylococcus aureus and Escherichia coli, respectively. In addition, multiply antibiotic resistant bacteria (MARBs), containing R-plasmids with tetracycline efflux genes tet(A) and tet(B), have been isolated from commercial chickens and eggs. On their migratory routes, sea turtles are exposed to various pollutants. Microorganisms from polluted feeding areas can enter the digestive tract and subsequently infect the ovary and uterus. Examination of turtle eggs and oviductal fluid reveals that MARBs also infect turtle eggs. Since antibiotics are highly diluted in the marine environment, MARBs have been used as bioindicators of contaminated effluents. Pseudomonas is the most frequent isolate, and studies on the bacteria in tertiary treated sewage effluent (TTSE) reveal that the majority are MARBs. When TTSE enters the sea, these MARBs may infect fish and turtles. Hence, consumption of contaminated fish and turtle eggs could lead to serious health problems. Some fungi can grow on eggshells, infecting embryos and preventing gas exchange. Asperigillus flavus was found to produce aflatoxins which may interfere with embryonic development. In addition, bioaccumulation of heavy metals is common. Analysis of turtle eggs has revealed the presence of 12 heavy metals, of which Zn was the most concentrated.