Foodborne diseases / Horizontal transmission

Horizontal transmission

Horizontal transmission of Salmonellae into eggs

Horizontal transmission means that Salmonellae are invading the egg after laying (this results in an infected egg for consumption). Salmonellae can penetrate whole shell eggs very rapidly. Research showed that Salmonellae can be present in the contents of whole shell eggs three days after contamination of the outer eggshell when stored at 25 ˚C.

The structure of an egg1. Shell and Cuticle
2. Shell Membranes (inner and outer)
3. Air Cell
4. Outer Thin Albumen (White)
5. Firm or Inner Thick Albumen (White)
6. Chalazae
7. Vitelline (Yolk) Membrane
8. Yolk

The chicken egg consists of different structures as shown in the figure. Structures 1, 2, 4, 5, 7 are contributing to the antimicrobial defence of the chicken yolk. In the following part, the role of these structures in the antimicrobial defence of the chicken egg is explained. Besides the antimicrobial or physical barriers of the egg, other factors, like temperature differential, moisture and the presence of contamination can influence the penetration of bacteria into the egg.

Vertical transmission of Salmonellae into eggs

Vertical transmission means that eggs become contaminated with Salmonellae through the infected hen. When the upper part of the oviduct is contaminated contamination of the forming eggs will occur and if the lower part of the oviduct is contaminated, penetration will occur. Different species of Salmonella enterica have the ability to colonise the reproductive tissue of laying hens. When the reproductive tissue is colonised with Salmonellae, the egg contents can become contaminated with Salmonellae. S. enteritidis and S. typhimurium can be present in the contents of eggs of infected hens. For S. enteritidis, research showed that it has the ability to contaminate the yolk. Several studies showed a different pattern in incidence of yolk contamination and incidence of albumen contamination with S. enteritidis.


When bacteria want to penetrate the egg the first barrier on their route is the cuticle. The cuticle is a mucilaginous layer and is deposited on the eggshell just before it is laid. The cuticle seems to play an important role in the prevention of the entry of bacteria for the first 96 hours after the laying of the egg. After this period of time the cuticle dries and cracks, resulting in the loss of the protective function of this layer. The bacteriological contamination within eggs is significantly higher in eggs with a poor cuticle compared with eggs with a medium or good quality. Although the cuticle seems to play a role in preventing antimicrobial growth the material of the cuticle is not inert.
The thickness of the cuticle is influenced by flock age. The older the flock, the thinner the cuticle. This observation may be a factor in the explanation why eggs from older flocks of laying hens are more sensitive to bacterial contamination.


The eggshell gives strength and shape to the egg and is the second barrier on the route of bacteria to the contents of the egg. The eggshell contains pores and some eggshell proteins have an antimicrobial function.
The only route for bacteria to penetrate intact whole shell eggs are the pores present in the eggshell. Pores are holes through the calcified layer of the eggshell and their number per egg varies from 7,000 to 17,000. The diameter of the pores varies from 15 to 65μm on the outside and from 6 to 23 μm on the inside of the egg. Domestic hen eggs are capped with organic spheres to prevent bacteria entering the egg through an unhindered pathway. The ability of S. enteritidis to survive on eggs seems to increase with decreased shell quality.

Shell membranes

The outer shell membrane has a porous structure and is not a barrier to bacterial entry. The inner shell membrane does have a fine structure and delays the entry of bacteria up to a few days. The different ways for bacteria to penetrate the shell membrane are uncertain but enzymic digestion can play a role.

Outer thin and inner thick albumen

The structure of the albumen is very important. Physical damage is enhancing the growth of bacteria in the albumen. The albumen of eggs does contain certain antimicrobial and enzyme-inhibitory systems to keep the embryo free of infection.

Antimicrobial and Enzyme-Inhibitory Systems in the Avian Egg

Antimicrobial/Anti-enzyme system

Mechanism of action


Chelates (strongly binds) the vitamin biotin, making it unavailable as a growth factor for microbial growth


Inhibits cysteine proteases

High pH (up to about 9.5)

Inhibitory to some micro-organisms and enhances the iron-binding effect of ovotransferrin


Hydrolyses β(1-4)-glycoside bonds in the peptidoglycan of bacterial cell walls, causing lysis


Chelates riboflavin, making it unavailable to micro-organisms


Inhibits serine proteases


Inhibits serine proteases

Ovotransferrin (conalbumin)

Chelates ions of copper, iron and zinc, making it unavailable for microbial growth

The pH of a fresh egg immediately after laying is between 7.6 and 7.9. When eggs are ageing, pH of eggs can rise up to 9.5 because of loss of CO2. This high pH has an inhibiting effect on the growth of bacteria. The proteins avidin and ovoflavoprotein make vitamins unavailable for micro organisms that require them as growth factors thus inhibiting the growth of bacteria.
Ovotransferrin is binding ferric iron very tightly. It can prevent microbial growth in media low in free iron such as egg white, because ferric iron is the limiting factor. About 12 percent of the dry weight of egg white consists of ovotransferrin, which has two binding sites for Fe3+. Some micro organisms, however, can produce their own iron chelators such as Salmonella typhymurium (S. typhimurium), which can produce enterochelin, making it possible to use iron from the egg white.
Lysozyme splits the bond between N-acetyl neuramic acid and N-acetylglucosamine in polysaccharides. This bond is a part of the walls of gram positive bacteria. Gram-negative bacteria, like Salmonellae are not sensitive to lysozyme.

Vitelline membrane

The vitelline membrane surrounds the yolk of the egg and plays a role in the separation of the yolk from the albumen. The egg yolk is a much better medium for the growth of bacteria than the albumen, so it is important to prevent bacteria from entering the yolk. The vitelline membrane is also a barrier for micro organisms on the route to enter the yolk. Salmonellae are able to penetrate the vitelline membrane after 48 hours at 15 ˚C when placed on the membrane.

Temperature differential

The temperature differential can be present when the warm egg cools just after laying. When the warm egg cools a negative pressure is created which can result in drawing of contaminated material down through the pores. The highest penetration rate occurs where a temperature differential (35°C to 4°C) is present.


When eggs are moved from the refrigerator into room temperature water droplets on the surface of eggs can be formed. This process is named sweating and is believed to cause bacterial penetration. There is also a positive correlation between the amount of water vapour present at the time of laying and the incidence of contaminated eggs.

Presence of contamination

Eggs are most susceptible to penetration at the point of lay. Because of that the contamination of eggs is related to the environment in which the egg is laid. A higher level of hygiene is thought to reduce the incidence of contaminated eggs. With the increase of flock age, eggs are likely to become more contaminated due to a general decline in the hygienic status during the life of a flock.