All natural waters contain a variety of living micro-organisms. Micro-organisms are responsible for several serious waterborne diseases and so the presence of pathogenic bacteria in drinking water is something to be avoided. On the other hand most bacteria are harmless to man and although drinking water is not normally completely sterile it has been disinfected in an attempt to remove the pathogenic species, the causative agents of disease.
Incidentally the term micro-organisms covers such species as bacteria, viruses, fungi, algae, and crustaceans but only bacteria will be considered here.
There are two very broad categories of bacteria in drinking water of interest to the analyst. The most obvious being the total bacteria and the other E. coli.
1. Total bacteria is a measure of the total number of colony forming units (cfu) of bacteria in water. It is an indication of the number of live bacteria in the water. It is determined by incubating the bacteria on an agar medium which contains a dye. Any bacterial colonies which flourish take in the dye and appear as coloured spots on the agar. Counting the number of colonies gives and indication of the level of total bacteria in the water. This in turn gives an idea of the level of disinfection, pollution of water supply and possible health risk. It does not however give any indication of whether the bacteria present are dangerous or not.
It should be mentioned here that bacteria which take in the dye on agar are called “gram-positive”. However there are some bacteria which do not take a dye and so do not show up on the agar, these are called “gram-negative”.
One very important gram-negative bacterium is legionella pneumophila, the causative bacteria of Legionnaires Disease, which is not detected by the total bacteria test.
Legionella can be found in the home in storage tanks, on washers and in shower heads. It only becomes a threat to health if it is breathed into the lungs. The bacteria is carried to the lungs by small water droplets, therefore showers and spray taps are potential sources of the bacteria.
To ensure that legionella is controlled, shower heads should be cleaned and disinfected every six months. All washers used should be of an approved type which does not support the bacteria. It is a good idea to flush the system regularly to ensure that a build-up of the bacteria does no occur.
2. E.Coli is a shorthand term for Escherichia Coliform bacteria. Polluted drinking water carries a considerable danger of spreading waterborne disease by transporting pathogenic bacteria. The presence of such organisms can be demonstrated by using the indicator organism E.Coli. This bacteria is usually present in the gut of humans and warm blooded animals and is excreted in large numbers. It would therefore be present in sewage and polluted water in much higher numbers than pathogenic bacteria.
Pathogenic bacteria are also more likely to die more rapidly in the unfavourable environment outside the body. E.Coli should not be detected in 100ml of tap water; at this level it is highly unlikely that any pathogenic bacteria will be present. This is a very safe level because more than 1 bacteria needs to be ingested to cause infection. In fact 20 volunteers were each fed 1000 typhoid bacteria in a recent survey – none developed typhoid. The absence of E.Coli is therefore good evidence of a bacteriologically uncontaminated water supply.
Outbreaks of diseases due to waterborne bacteria are rare, but one organism, cryptosporidium is currently causing a great deal of concern. It is widely found in nature and was responsible for an outbreak of gastro enteritis among water consumers in Oxfordshire and Wiltshire. The cause of the outbreak was traced to the drinking water which contained the organism. The bacteria is not eliminated by current water treatment methods. Symptoms of the disease can include diarrhoea, stomach cramps, vomiting and fever.
Standard tests carried out by the Water Authorities do not detect the organism. Infants and older people are especially at risk, together with those whose immune systems are not up to full strength, e.g. cancer patients. In the United States of America cryptosporidiosis is the third most common cause of death in Aids sufferers. Scientists are currently examining ways of detecting the organism and treating it.
WHO limit = 0 per 100ml (higher levels are allowed in occasional samples)
EC MAC = 0 per 100ml
WHO limit = ----
EC guide level= 10 per ml at 37 deg C
= 100 per ml at 22 deg C
EC MAC = ----