Unit of measurement
number/ 100ml
World Health Organisation Standards (1993)
Not mentioned
Standards for private water supplies in England (2016)
Standards for private water supplies in Scotland (2006)
Standards for private water supplies in Wales (2017)
European Union Drinking Water Directive (1998)


Clostridium is a variety of usually gram-positive, rod-shaped bacteria which is found in soil, water and the intestinal tracts of humans and other warm-blooded animals. Most species of Clostridium are anaerobic, meaning that they only grow in the complete absence of oxygen.

Although there are approximately 100 strains of clostridium, several specific strains are well-documented as being responsible for causing disease in humans. These are:

  • Clostridium difficile (also known as C.diff) - can flourish as a result of an individual taking antibiotics. The antibiotics kill much needed microorganisms found in the human gut which gives C.diff the chance to thrive. This strain can cause diarrhoea, nausea, stomach pain, loss of appetite and fever.
  • Clostridium botulinum - produces the toxin botulinum in foods and wounds which can cause the potentially fatal illness botulism. This disease is usually characterised by the initial symptoms of weakness, blurred vision and trouble speaking followed diarrhoea, vomiting and swelling of the abdomen. Interestingly, this toxin is also used in Botox procedures and is what causes the partial paralysis of the facial muscles.
  • Clostridium perfringens - this strain can cause a variety of unwanted ailments including food poisoning, fasciitis, cellulitis and gas gangrene.
  • Clostridium tetani - can cause tetanus by entering the body through a break in the skin, such as a cut or puncture, and releasing toxins which cause muscle spasms.

The presence of clostridium (Particularly clostridium perfringens as this is the strain tested for under the Private Water Supply Regulations) in drinking water acts as an indicator that the supply has faecal contamination. Clostridium bacteria are spore-forming and do not multiply in water but their spores are incredibly resistant to harsh conditions including those brought upon them by disinfection chemicals and UV treatment. This means that they can persist in a water supply for extended periods, making them a good indicator for historical faecal contamination.

Whilst clostridium perfringens spores are unlikely to present a problem when consumed directly from contaminated water in low levels, they can cause an issue when this contaminated water is used in the incorrect preparation, storing or cooking of foods.

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