Microbe of the Month: The cleaner element of dirty water: A 'natural vaccine' that prevents dysentery may be found in contaminated sources, Bernard Dixon says

Whether in the Third World, on a polluted Mediterranean beach, or in a makeshift camp formed by refugees from one of today's many wars, polluted water is a potentially horrendous source of infection. By the same token, pure water supplies are both a safeguard and a symbol of health.

But there may be an important exception. In certain circumstances, contaminated water may prevent, rather than cause, the severe intestinal disease known as dysentery. That is the implication of a paper published recently in the Lancet by Dr David Sack and colleagues at John Hopkins University and the University of Maryland, Baltimore.

The link between impure water and intestinal infections is so pervasive that it has rarely been out of the news since August 1854 when the removal of the handle from the Broad Street pump in Soho, London, at the request of Dr John Snow, brought an end to a raging cholera epidemic.

Only last year, doctors at St Thomas's Hospital, London, discovered that a young girl who became severely ill after falling into the Thames had been infected with not only Salmonella but also three other disease-causing bacteria. Last month, microbiologists in London and Leicester, after finding food-

poisoning viruses in British bathing waters, called for an end to the discharge of raw sewage into the sea. And waterborne transmission of the bacterium Vibrio cholerae has played a major role in the cholera epidemic now affecting several South American countries.

Like cholera, bacterial dysentery is a potentially fatal disease, especially in the young, the old and the malnourished of all ages. The bacteria penetrate the lining of the large intestine, causing victims to pass, frequently and painfully, stools containing blood and mucus. High fever, chills, and abdominal cramps are among other symptoms. Contamination of water supplies from patients' faeces is a highly efficient mechanism through which this foul disease is spread.

Dr Sack and his collaborators have been especially interested in the contrast between two of the species of the bacterium Shigella responsible for the malady. S. sonnei is more common in industrialised societies, whereas S. flexneri occurs more often in developing countries. The disparity is surprising, given that they are transmitted in the same way. However, it also squares with evidence that as countries advance economically there is also an increase in the proportion of dysentery cases caused by S. sonnei.

There are various possible reasons for this pattern, none totally consistent with the evidence. The explanation which begins to seem most plausible is that in developing countries exposure to another bacterium, one that partially resembles S. sonnei, triggers the formation of antibodies that make a large proportion of the population immune to S. sonnei as well. In other words, such a microbe would comprise a naturally occurring vaccine. It would have to be relatively common and ingested by large numbers of people in the community - perhaps through polluted surface water. If this did occur, it would account for the contrast between the prevalence of S. sonnei and S. flexneri in developed and developing regions. It would also explain the comparative rise in the occurrence of S. sonnei as increasing prosperity facilitates improvements in water supplies.

When Dr Sack and his co-workers studied possible candidates as this 'natural vaccine' they settled on Plesiomonas shigelloides. A waterborne bacterium, P. shigelloides has been isolated from surface water samples in many developing countries. A typical resident in such a country, drinking about a litre of surface water a day, would consume about 105 cells of P. shigelloides. Only because special methods are required to grow it in the laboratory has the significance of the organism been overlooked in the past.

The candidature of P. shigelloides was greatly strengthened by the discovery that part of the cell wall of one particular strain, a molecule known as lipopolysaccharide (LPS), was identical with the LPS found in S. sonnei. Infection with this bacterium could, therefore, induce the formation of antibodies that would also deal with S. sonnei. The part of the dysentery bacterium that is most important in eliciting immunity is indeed its LPS. When the Baltimore researchers injected rabbits with P. shigelloides they found that it protected them against S. sonnei infection. Studies in volunteers will be necessary to determine whether the same thing occurs in humans.

Oddly, there is uncertainty as to whether P. shigelloides itself causes diarrhoea. Some studies have suggested that it does. Others have shown no such association. Inoculation of P. shigelloides, even in large quantities, into human volunteers has not led to any illness. Like artificial vaccines against conditions such as poliomyelitis and diphtheria, P. shigelloides would, of course, not need to produce symptoms in order to induce immunity. It may well be that the bacterium does cause mild inflammation of the intestinal wall, at the same time inducing antibodies protective against S. sonnei.

Dr Sack and his colleagues emphasise that their hypothesis, despite the strong and varied evidence in its favour, does not amount to an argument against the desirability of improving water supplies. The benefits of sewage treatment and water purification in excluding a whole host of microbial killers remain paramount. Nevertheless, P. shigelloides appears to be the exception that proves the rule.

(Photograph omitted)

Start your day with The Independent, sign up for daily news emails
  • Get to the point
Latest stories from i100
Have you tried new the Independent Digital Edition apps?
Independent Dating

By clicking 'Search' you
are agreeing to our
Terms of Use.

iJobs Job Widget
iJobs General

Ashdown Group: Front-End UI Application Developer

£30000 - £40000 per annum + Benefits: Ashdown Group: Front-End UI Application ...

Recruitment Genius: Digital Account Executive

£18000 - £26000 per annum: Recruitment Genius: They work with major vehicle ma...

Recruitment Genius: Service Engineers - Doncaster / Hull

£27000 per annum: Recruitment Genius: Domestic Service Only Engineers are requ...

Recruitment Genius: Employability / Recruitment Adviser

£23600 - £27500 per annum: Recruitment Genius: The Employability Service withi...

Day In a Page

Where the spooks get their coffee fix: The busiest Starbucks in the US is also the most secretive

The secret CIA Starbucks

The coffee shop is deep inside the agency's forested Virginia compound
Revealed: How the Establishment closed ranks over fallout from Loch Ness Monster 'sighting'

How the Establishment closed ranks over fallout from Nessie 'sighting'

The Natural History Museum's chief scientist was dismissed for declaring he had found the monster
One million Britons using food banks, according to Trussell Trust

One million Britons using food banks

Huge surge in number of families dependent on emergency food aid
Excavation at Italian cafe to fix rising damp unearths 2,500 years of history in 3,000 amazing objects

2,500 years of history in 3,000 amazing objects

Excavation at Italian cafe to fix rising damp unearths trove
The Hubble Space Telescope's amazing journey, 25 years on

The Hubble Space Telescope's amazing journey 25 years on

The space telescope was seen as a costly flop on its first release
Did Conservative peer Lord Ashcroft quit the House of Lords to become a non-dom?

Did Lord Ashcroft quit the House of Lords to become a non-dom?

A document seen by The Independent shows that a week after he resigned from the Lords he sold 350,000 shares in an American company - netting him $11.2m
Apple's ethnic emojis are being used to make racist comments on social media

Ethnic emojis used in racist comments

They were intended to promote harmony, but have achieved the opposite
Sir Kenneth Branagh interview: 'My bones are in the theatre'

Sir Kenneth Branagh: 'My bones are in the theatre'

The actor-turned-director’s new company will stage five plays from October – including works by Shakespeare and John Osborne
The sloth is now the face (and furry body) of three big advertising campaigns

The sloth is the face of three ad campaigns

Priya Elan discovers why slow and sleepy wins the race for brands in need of a new image
How to run a restaurant: As two newbies discovered, there's more to it than good food

How to run a restaurant

As two newbies discovered, there's more to it than good food
Record Store Day: Remembering an era when buying and selling discs were labours of love

Record Store Day: The vinyl countdown

For Lois Pryce, working in a record shop was a dream job - until the bean counters ruined it
Usher, Mary J Blige and Will.i.am to give free concert as part of the Global Poverty Project

Mary J Blige and Will.i.am to give free concert

The concert in Washington is part of the Global Citizen project, which aims to encourage young people to donate to charity
10 best tote bags

Accessorise with a stylish shopper this spring: 10 best tote bags

We find carriers with room for all your essentials (and a bit more)
Paul Scholes column: I hear Manchester City are closing on Pep Guardiola for next summer – but I'd also love to see Jürgen Klopp managing in England

Paul Scholes column

I hear Manchester City are closing on Pep Guardiola for next summer – but I'd also love to see Jürgen Klopp managing in England
Jessica Ennis-Hill: 'I just want to give it my best shot'

Jessica Ennis-Hill: 'I just want to give it my best shot'

The heptathlete has gone from the toast of the nation to being a sleep-deprived mum - but she’s ready to compete again. She just doesn't know how well she'll do...