Little bugs have littler bugs upon their backs to bite ‘em ….

My post about John Snow pointed out that he made the link between contaminated water and disease without actually knowing what we now know about germs.  In effect, Snow had made an inference based on the association between cases of cholera and the closest pump but correlation, as we tell our students, is not the same as causation. Elsewhere in London and beyond, others were desperately searching to identify the culprit itself.

Various theories had been put forward, dividing roughly into those suggesting a chemical origin and those suggesting a biological cause. One of the proponents of the latter was a doctor called Arthur Hill Hassall who looked down his microscope at samples he had collected from the reservoirs which supplied London’s drinking water and thought he had found the answer.   The drawings he published in The Lancet show water teeming with algae and if this sounds preposterous, remember that this was still 20 years before Pasteur and Koch discovered bacteria, a group of organisms far too small to be seen with the microscopes available to Hassall.

Hassall published the first authoritive guide to the freshwater algae of Britain and described  several new species including a diatom called Asterionella formosa which is very common in the plankton of lakes in the spring.   The Latin name translates as “beautiful little star” and finding it in a sample always brings a wry smile to my face, as I recall the walk-on part this and other algae played in the story of the struggle to unravel the causes of cholera.


Asterionella formosa collected from Dannemarche Reservoir in Jersey in June 2013 by Dave John.  The scale bar indicates 10 micrometres (1/100th of a millimetre).

The individuals photographed here come from a reservoir in Jersey.  I spend a day each year teaching on an algal identification course based in Durham. It relies on water samples brought along by the tutors and participants which means that there is always a rich assortment of material from all over the country to examine.  I was checking this sample before the class when I noticed the beaded appearance of the Asterionella.   Under higher magnification, these “beads” resolved into yet tinier organisms, unicellular fungi called “chytrids” which had infected the alga.


Just as cholera was able to spread rapidly through the densely-populated regions of London in the nineteenth century, so chytrids thrive when Asterionella is most abundant.  It is a reminder that diseases and infections are a natural feature of animal and plant populations, not just human scourges and, indeed, are an entirely natural way of regulating population numbers:

Little bugs have littler bugs
upon their backs to bite ‘em.
And littler ones have littler ones,
and so on, ad infinitum …

A drink of water with John Snow

I’m in a pub in Soho on a humid Friday afternoon with a glass of water on the table in front of me.  Water, rather than beer, seemed appropriate in light of the story that I am about to tell.   This is the John Snow pub, an old-fashioned London boozer with dark wood fittings, a Lino floor and just half a dozen or so customers quietly drinking.  These are not part of the tourist hordes which I had pushed through on my way here from Oxford Circus station and who were milling along Regents Street and Carnaby Street but (I’m guessing) office workers on their lunchtime breaks.


My glass of water in the John Snow pub in Broadwick Street, Soho, London.

The relevance of this location to a blog entitled Microscopes and Monsters lies partly with the pub’s namesake and partly with a pump on the opposite corner from the pub.  This looks, from afar, like many of the other reminders of bygone eras that are scattered around London’s streets save for one thing: this pump does not have a handle.

Back in the middle of the nineteenth century, this area of Soho was hit by one of many outbreaks of cholera that were ravaging London at the time.  John Snow was a doctor working in this part of London who was trying to combat the disease.  This was, remember, some 30 years before Pasteur and Koch finally identified bacteria as the causal agents – microscopic monsters, in other words – in diseases such as cholera.  John Snow had, as a result, little evidence to help him except for his observation that this particular cluster of incidents of cholera were clustered around one particular pump on Broad (now Broadwick) Street.  Maybe, he reasoned, this was no coincidence.  Maybe a simple measure such as removing the handle of the pump would stem the tide of the disease in this part of London.

Now we know that there is a link between contaminated water and disease this all sounds self-evidently rational, but Snow was taking a step into the unknown with no more than circumstantial evidence with which to justify himself.  The locals would have seen him less of a saviour and more of a nuisance, necessitating a longer walk to bring their water home.  Yet it was a hunch which proved right, with the number of cholera cases falling in the period immediately following the handle’s removal.  And, in due course, a crack in the wall of a nearby cess pit was identified as the source of the contamination in the well.


The cool, dark, quietness inside the John Snow pub created an almost sepulchral atmosphere.  Sipping from a glass of clean water here takes on an almost Eucharistic purpose, reminding me of the contribution that John Snow and others made in the middle of the nineteenth century and which we take for granted today.  I emptied my glass and picked up my bag. I have one more destination on this brief pilgrimage to London before the day is over …