Wednesday 31 May 2017

Julius Petri

I must have mentioned Petri dishes aka boites de Petri or in their original language Petrischalen dozens of times in my ramblings about microbiology, several times shamefully without capitalising P for Petri. I should know better because some part of my brain had registered that they were named after their inventor Julius Richard Petri [R with beard]. Petri was born >!today!< 31st May 1852 in Wuppertal in the Eastern Rhineland and qualified as a Militärarzt - army doctor - who was seconded to Berlin to work with Robert Koch. Koch [prev on TB etc.] was biggest of big cheeses in the world of 19thC microbiology, second only to Louis Pasteur in Paris, so this was clearly a prestigious and stimulating place to work. Koch's laboratory was responsible for identifying the causative agents of TB Mycobacterium tuberculosis, cholera Vibrio cholerae and anthrax Bacillus anthracis. But his lasting claim to fame was the development of Koch's postulates for definitively tying down a disease state to an identifiable cause.

To establish a microbial cause, they had to be able to identify and grow that bacteria to the exclusion of all others in the local environment. It was really hard because contamination was rife. A huge break-through came when Fanny Hesse, working in Koch's lab as unpaid technical help to her husband Walter, brought an idea from her kitchen to use a seaweed extract called agar-agar as the vehicle for the various growth media. Resolution: write subsequent piece on this key woman of science. That had numerous advantages over the existing options [slices of potato, extract of calf's foot] and paved the way for Petri's contribution which was a pair of circular glass dishes the lid slightly larger than the base which held the media. It was the close-fitting lid that really gave Petri's system utility. It's neat because the lid's overlapping sides deter air-borne microbes from drifting onto the plate to contaminate the sample. And each plate was separate experiment that could be stacked and moved easily from bench-top to incubator to cold-room.
The dishes could be sterilised in an autoclave filled with appropriate media, inoculated with bacteria and then re-used. When I learned microbiology 100 years later in the 1970s, we were still using the same methodology. You can still buy glass Petri dishes at $188/dozen. For that price you get [see L] 'cover' and 'bottom' etched on the side and a life time's careful use. Nowadays, the plates are made in lightweight transparent polystyrene which is used once, heat-sterilised into a blob and sent to landfill. They cost about $1/dozen - you do the math. Plastic opens other functionality: you can buy [$6/dozen] plates split into three or four sections with little injection-moulded vertical walls. Or, if time is money, you can buy ready-to-go plates filled with the medium of your choice.

We were taught to pour the hot agar into plates and build them one at a time into a stack; that way the hot medium would warm the lid of the dish immediately below and so deter condensation on the lid. We were also requested-and-required to incubate the plates agar-side-up, so that no residual lid condensation would be able to drip down on the agar surface and mix the bacteria over the surface. Every year I have to re-teach at least some of students the elements of correct practice w.r.t. Petri dishes and agar.   There is a certain amount of irony in this because I am definitely not the safest pair of hands in a laboratory.

Particularly since Petri dishes featured as a Google doodle on 31st May 2013, their inventor has been sidelined as a one-trick pony. No fair! Petri went on to a stellar career carrying out research on TB and hygiene, progressing through various posts and institutions until he finished up in 1889 as Director of the Kaiserliches Gesundheitsamt in Berlin where he had invented the dish nearly 20 years earlier. His 1893 work Versuche über die Verbreitung ansteckender Krankheiten, insbesondere der Tuberkulose, durch den Eisenbahnverkehr und über die dagegen zu ergreifenden Maßnahmen on the spread of tuberculosis by the railways could, with advantage, be read by 21stC epidemiologists worrying about the spread of SARS, bird-flu and Ebola by today's higher-speed mass transport. Later still Julius Petri became known a circus act.

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