It began, as many thought exercises do, with a discussion in the kitchen. My wife and I were debating whether a cooked egg was still safe to eat after spending the night on the kitchen counter. I am certainly not the type to throw away food lightly at the lightest hint of an issue. Confidently, I told her the egg would be fine. After all, eggshells are made of calcium carbonate, solid, sealed, and, in my mind, impenetrable to any contaminants.
It turned out to be one of those “smart dumb” moments. By the end of this journey, I would learn how spectacularly wrong I was, and how remarkable life is at engineering security.
A humble egg that is a marvel of security engineering.
My background is in sterile large molecule manufacturing within biotech, the kind of work where even a single microscopic breach within thousands of gaskets and kilometres of stainless steel can trigger an investigation lasting months or even years. I have spent much of my career designing systems to keep contaminants out and tracing them when they find a way in.
Yet for all the effort, precision, and technology we pour into security, after this curiosity-driven journey, I came to the conclusion that nothing I’ve seen in biotech compares to what nature achieves effortlessly inside an egg. To keep things simple, I focused on one scenario: an unfertilised, intact chicken egg.
As I went down the rabbit hole of Eggsecurity, I discovered that the egg is an astonishingly well-designed fortress. Before diving into the details, it is worth noting just how nutrient rich an egg truly is. It is so abundant in life-sustaining material that even today, many biopharmaceutical products are made using eggs. To a bacterium, reaching the egg yolk must be like winning the lottery. The yolk’s purpose, after all, is to nourish life itself.
My first question was simple: are eggshells porous? It turns out they are not only porous, but contain thousands of microscopic openings. The average pore size is in the single-digit μm range. That may sound tiny, but it is still large enough for a small bacterium to pass through. For context, sterile filtration uses filters with pore sizes of 0.2 μm. I even know some aseptic subject matter experts who would argue for 0.1 μm to ensure nothing microbial gets through. The benchmark for this comes from the smallest known bacterium used to challenge filter integrity, Brevundimonas diminuta, with a rod-like shape and diameter of 0.5 to 0.8 μm. This organism is introduced at high concentrations (107 Colony Forming Units/cm2) to challenge the system.
My next question was what is meant to pass through these pores? The answer was so obvious. How could I have missed this? Biology 101: a good exchange of oxygen and carbon dioxide is required for an embryo to grow. So the egg must allow gas molecules to pass while keeping everything else out. How does the simple egg achieve this?
The first protective barrier is the antimicrobial cuticle, a thin outer coating that acts like a natural sealant. Whether it remains intact or is partially washed away, it is the egg’s first line of defence. In countries where eggs are washed, this layer does not vanish instantly, so for a certain duration it will still provide some antimicrobial protection.
Beneath it lies the eggshell with thousands of microscopic pores. These tunnels are not straight shafts but winding labyrinths, long and twisted. The environment inside is dry, which makes bacterial movement difficult. The dry eggshell environment also does not strike me as nutrient rich, meaning it will be difficult for the contaminants to grow through the tunnel, which is another means of transport for microorganisms. Microbes thrive in warm, moist and nutrient rich environments. In a dry maze without airflow or nutrients, they would have to perform the microbial equivalent of cave exploration whilst immobilised and starving in search of a hidden treasure.
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