Issue #2: Microbes, viruses and face masks in the COVID-19 Petri dish

Setting it Straight - Issue #2

Written by Nobel Laureate Professor Peter Doherty

You’ve heard it on TV: we’re all in this Petri dish together! Australia: just one big COVID-19 Petri dish! What, or who, is a Petri? It’s capitalised, so does it refer to, say, Petra, the archaeological site in Southern Jordan? Were the first Petri dishes made of baked clay?

No, early Petri dishes were glass and they came from late 19th century Berlin. That’s where microbiologist, Julius Petri, worked with the great Robert Koch, who discovered the lethal bacterium Mycobacterium tuberculosis that killed millions, including poet John Keats and composer Frederic Chopin. Antibiotic resistant strains of M. tuberculosis are still lethal, especially in poor countries with limited access to the drugs that control HIV (human immunodeficiency virus), the cause of the acquired immune deficiency syndrome (AIDS).

So, pathogenic viruses and bacteria, or microbes as they’re sometimes referred to, can act together to cause disease and death, but why the different names? When pioneer microscopist, Antonie van Leeuwenhoek (1632-1723), first viewed wriggling bacteria through his tiny, single-lens optics, he called them “animalcules” implying, correctly, that these are life forms in their own right. What nobody saw until the 1931 invention of the mega-magnifying electron microscope (by Ernst Ruska and Max Knoll in Berlin) were viruses. So, that’s one difference – bacteria are mostly big and viruses are mostly small.

Let’s talk about size. A major white blood cell, the monocyte, is 15-18 microns (a millionth of a metre) across. Oxygen-carrying red blood cells are 6-8 microns, the rod-like M.tuberculosis bacterium is 2-4 x 0.2-0.5 microns, and the SARS-CoV-2 virus that causes COVID-19 is around 0.07 microns. The N95 (N means it’s not oil-resistant, you don’t need that for medical usage) masks we’re trying to conserve for our frontline healthcare workers trap 95% of particles down to about 0.3 microns – how does that protect against the much smaller SARS-CoV-2 virus?

Well, most (hopefully all) SARS-CoV-2 isn’t floating-free, like dust in the air. It’s sneezed or coughed-up in droplets of mucus, snot or whatever. So, the job of the mask is to trap the droplets, which are generally in the 1.0 to 6.0 micrometre range. And, while a sneezed 6.0 micrometre droplet may fly as far as 1.5 metres then fall to the ground, a 1.0 micrometre droplet will do that much closer to the source. That’s why a homemade mask – say a multi-folded, fine-weave, cloth bandana – may provide a measure of protection in a supermarket that’s observing the 1.5 metre distancing rule. The N95 (or better) masks need to be reserved for intensive care unit (ICU) doctors and nurses, who are exposed to small droplets as they insert a tube into the trachea of a very sick, coughing and spluttering, patient.

Back to Petri. We’ve said who he was, but the dish? You’ve all seen them on TV: a white-coated scientist holds a circular flat, 10 centimetre plastic plate, angled and held up to the light. It looks red, and the surface may have “creamy” streaks across it. The flat, semi-solid, surface is agar, a “jello” made from seaweed that’s also used to “stabilise” ice cream. The red is sheep or horse blood (5-10 per cent) dispersed in the agar. Our Petri dish, as the box label reads, is now a “blood agar plate”. And it could be called a “Hesse plate”, after Fanny Hesse, who worked alongside her husband at the Koch Institute. Fanny knew about agar from jam making, and introduced it into bacteriology.

Blood agar: that sounds incredibly primitive. The blood provides the nutrients that support bacterial growth. The “lumpy” (in close-up) creamy streaks are colonies of piled up, multiplying bacteria. Bacteria, and other microbes like the fungi and protozoa, are cells in their own right, with an outer membrane, a cytoplasm, a nucleus (the control centre), mitochondria (the energy factories) and so forth. Bacteria are everywhere, including our gastrointestinal tract (the microbiome) and, if they’re pathogens like M.tuberculosis, in our lungs.

Viruses like SARS-CoV-2 are, on the other hand, “obligate intracellular parasites”, subcellular particles that have to invade, and take over, some of the machinery of our living cells so that they can multiply and, when released from that initial “factory” cell, infect more cells. I’ll write much more about SARS-CoV-2 replication later, but what’s important for now is to understand that viruses have no independent life, and they’re always passengers that don’t move themselves around.

When virologists grow pathogens like SARS-CoV-2, they pipette virus-containing fluid onto “lawns” of mammalian cells that have attached themselves to the flat side of specialised, screw-top, plastic tissue culture flasks. The cells are bathed in red, liquid, tissue culture medium containing nutrients and the dye phenol red, which changes colour if the growing conditions become too acid or too alkaline. Especially with a dangerous virus like SARS-CoV-2, we would never use an open-topped Petri dish for such a purpose. And, unless the virus is a bacteriophage (the viruses that infect bacteria) they won’t grow on Fanny Hesse’s agar plates either.

So, that’s the science story of Petri and his dish. But symbols are important, imagery is important. If that mind picture of a bug-infected, blood-red Petri dish helps you to engage with the reality of living through this COVID-19 time, go for it!