A recent post from IFLScience.com claims that microbiologists from London Metropolitan University conducted swabbing on touch screens in McDonald's restaurants in London and Birmingham, finding coliforms, Bacillus and Listeria. They also found Staphylococcus, Pseudomonas, Enterococcus faecalis and Klebsiella.
All of these bacteria are capably of causing infections, and you will find some discussion of these bacteria on this blog site - see labels.
But should we be surprised and should customers at McDonald's be concerned? Well, I am not at all suprised. If a microbiologist conducts swabbing in any public place, such as a supermarket, they will find all of these bacteria, perhaps on trolley handles, push plates on doors, in the toilets etc. As for the customers, if they are concerned, they could order their meal on the touch screen, then wash their hands, or use a hand sanitiser. Be honest - if you go to any fast food joint after you have been shopping in the mall, do you always wash your hands before you eat?
The original research was done for Metro (see https://metro.co.uk/2018/11/28/poo-found-on-every-mcdonalds-touchscreen-tested-8178486/). Not surprisingly, when this post was published on FaceBook, there were 1.2 thousand comments and nearly a thousand shares. My personal opinion is that this article was written and published for shock value and to boost circulation. Interestingly, a video clip posted on this same site showed that the majority of customers were not surprised by the findings and would still go and purchase food there.
Whatever you think of McDonald's or any other fast food outlet, in my opinion, the researchers are making a mountain out of a molehill, and IFLScience is making it worse. Don't pick on just one fast food chain, and think about how you protect yourself and your family from infection.
And while you are at it, think about a similar scenario in your own home.
Food for thought: How do you recognise the microbiologists around you? They are the people who use their little fingers or elbows to open doors, use a paper towel to turn the taps off in a public washbasin, use a knuckle on the touch screen and who often use hand sanitiser before eating in public.
Showing posts with label coliform. Show all posts
Showing posts with label coliform. Show all posts
Thursday, December 27, 2018
Tuesday, August 7, 2018
FDA issues an update statement on Yuma E coli outbreak
Earlier this year, a serious multi-state outbreak of Escherichia coli O157:H7 infection was associated with consumption of Romaine lettuce from Yuma, Arizona.
The US FDA has now issued an update statement, though the investigation is not completed. Tests of canal water close to the lettuce growing farms has tested positive for the outbreak strain of E. coli O157:H7, which is capable of producing Shigatoxin. Infection with this microorganism can result in serious disease - 210 people in 36 states have become ill, with 96 hospitalizations and five deaths.
No single farm has been determined as the source of the outbreak, nor has the distribution chain been implicated so far.
The lettuce growing farms and canal are close to a Concentrated Animal Feeding Operation (CAFO), which can hold in excess of 100,000 head of cattle at any one time. Since cattle can harbour the STEC E. coli involved in the outbreak, there is the possibility that the presence of the bacteria in the canal water originated from the CAFO. Further testing of the CAFO and the animals would be necessary to establish a link.
The means by which the lettuces became contaminated must also be investigated. Was the canal water used directly for irrigation, or were the bacteria somehow transferred from the canal water to the leaves?
The US FDA has now issued an update statement, though the investigation is not completed. Tests of canal water close to the lettuce growing farms has tested positive for the outbreak strain of E. coli O157:H7, which is capable of producing Shigatoxin. Infection with this microorganism can result in serious disease - 210 people in 36 states have become ill, with 96 hospitalizations and five deaths.
No single farm has been determined as the source of the outbreak, nor has the distribution chain been implicated so far.
The lettuce growing farms and canal are close to a Concentrated Animal Feeding Operation (CAFO), which can hold in excess of 100,000 head of cattle at any one time. Since cattle can harbour the STEC E. coli involved in the outbreak, there is the possibility that the presence of the bacteria in the canal water originated from the CAFO. Further testing of the CAFO and the animals would be necessary to establish a link.
The means by which the lettuces became contaminated must also be investigated. Was the canal water used directly for irrigation, or were the bacteria somehow transferred from the canal water to the leaves?
Friday, December 2, 2011
Your all-time favourites
I have just reviewed the most common search terms resulting in visits to this blog.
By far the most common has been "Coliforms in food". This has been a regular search since I started writing Safe Food in 2006, but has perhaps appeared more frequently since the Escherichia coli O104:H4 outbreak in Europe.
Coliforms are used as "indicator organisms" to show whether the food has been processed under hygienic conditions. Presence of these bacteria mean that the food has potentially been contaminated with faecal material and hence faecal organisms, but to confirm this, we look for Escherichia coli presence in the food.
Surprisingly, the next most common search has been "Safety of Probiotics". Probiotic bacteria are those that confer some benefit on the host. Thus many foods, such as yoghurt, contain live bacteria that may colonise the gut of the consumer and confer some benefit. These bacteria are often lactic acid bacteria. Many claims have been made for the benefits of consumption, though there are some indications that administration to critically ill patients with acute pancreatitis may have deleterious effects. There is also some evidence that administration to infants under 6 months of age may render them more prone to development of sensitivity to allergens. For the majority of consumers, however, probiotics will not be hazardous and may be beneficial.
Do not confuse probiotics with "prebiotics". These were so named by Marcel Roberfroid in 1995, who later wrote the definition: "A prebiotic is a selectively fermented ingredient that allows specific changes, both in the composition and/or activity in the gastrointestinal microflora that confers benefits upon host well-being and health." A major class of prebiotic is soluble fibre.
Much lower down the list are "Listeria in vegetables" and "Safety of chicken". There has been a small spike in Listeria searches, probably as a result of recent recalls and the cantaloupe outbreak in USA that has killed approximately 29 people. The incidence of listeriosis is very low and victims are usually neonates, the elderly and those people with compromised immune systems, but this outbreak seems to have been particularly dangerous. Chicken has made regular appearances in the news, either being a vehicle for Salmonella or Campylobacter. The incidence of Campylobacter infections in New Zealand has dropped significantly since the introduction of more stringent controls and biosecurity systems.
One new search now appearing is a bit disturbing: "Will eating spoiled food make you sick?". I have previously written on this after receiving a direct question from a reader. Is the increased interest a result of hardship in the community, or merely a thirst for knowledge? I hope it is the latter.
By far the most common has been "Coliforms in food". This has been a regular search since I started writing Safe Food in 2006, but has perhaps appeared more frequently since the Escherichia coli O104:H4 outbreak in Europe.
Coliforms are used as "indicator organisms" to show whether the food has been processed under hygienic conditions. Presence of these bacteria mean that the food has potentially been contaminated with faecal material and hence faecal organisms, but to confirm this, we look for Escherichia coli presence in the food.
Surprisingly, the next most common search has been "Safety of Probiotics". Probiotic bacteria are those that confer some benefit on the host. Thus many foods, such as yoghurt, contain live bacteria that may colonise the gut of the consumer and confer some benefit. These bacteria are often lactic acid bacteria. Many claims have been made for the benefits of consumption, though there are some indications that administration to critically ill patients with acute pancreatitis may have deleterious effects. There is also some evidence that administration to infants under 6 months of age may render them more prone to development of sensitivity to allergens. For the majority of consumers, however, probiotics will not be hazardous and may be beneficial.
Do not confuse probiotics with "prebiotics". These were so named by Marcel Roberfroid in 1995, who later wrote the definition: "A prebiotic is a selectively fermented ingredient that allows specific changes, both in the composition and/or activity in the gastrointestinal microflora that confers benefits upon host well-being and health." A major class of prebiotic is soluble fibre.
Much lower down the list are "Listeria in vegetables" and "Safety of chicken". There has been a small spike in Listeria searches, probably as a result of recent recalls and the cantaloupe outbreak in USA that has killed approximately 29 people. The incidence of listeriosis is very low and victims are usually neonates, the elderly and those people with compromised immune systems, but this outbreak seems to have been particularly dangerous. Chicken has made regular appearances in the news, either being a vehicle for Salmonella or Campylobacter. The incidence of Campylobacter infections in New Zealand has dropped significantly since the introduction of more stringent controls and biosecurity systems.
One new search now appearing is a bit disturbing: "Will eating spoiled food make you sick?". I have previously written on this after receiving a direct question from a reader. Is the increased interest a result of hardship in the community, or merely a thirst for knowledge? I hope it is the latter.
Saturday, March 28, 2009
The temperature's rising (but why?)
I was recently asked to explain some of my comments on detection of faecal coliforms. <See Coliforms and Faecal Contamination Wednesday, July 18, 2007>
I have extracted the essentials of the questions below:
“Since the coliforms when tested… are grown at optimum growth temperature of 37C, why would you need to raise the faecal coliform test temperature to 44.5C to show thermotolerant E. coli are present? Why would food safety people be looking for thermotolerant bacteria in food rather than bacteria that grow at normal temperatures?”
In my article, I provided some background information on "faecal indicator microorganisms", which we use to show that a food or water sample may have been contaminated with faeces. These tests originated in the early days of public health services and safety of public water supply. The coliforms are easier to detect and enumerate than are Salmonella or Shigella or faecal viruses. (Salmonella and Shigella are not coliforms). The reason for incubating at 44.5C to demonstrate the presence of Escherichia coli dates from 1904 when Eijkman suggested it as a means of separating the "B. coli" originating in the faeces of warm blooded animals from the strains characteristic of cold blooded animals and thus providing us with a means to detect faecal contamination of water supplies by warm blooded animals, including humans.
So the answer to the first question is “we are not particularly interested in thermotolerant coliforms; rather we want to show that the water supply may have been contaminated by human wastes and hence potentially contains faecal pathogens”.
Having said that, of course there are other areas of food microbiology where we are very interested in the presence of thermotolerant or thermophilic bacteria. If we pasteurize milk with plate heat exchangers (the standard method) we may find that the cooling stages become colonized by thermotolerant streptococci, which may cause spoilage. In the case of canned foods, we find that some sporeforming bacteria can survive even a very rigorous thermal process. These bacteria are of no public health significance, but they may cause spoilage if the cans are held at high temperatures (greater than about 40C) as might be found in storage facilities in very hot countries or in a restaurant kitchen.
“Since the coliforms when tested… are grown at optimum growth temperature of 37C, why would you need to raise the faecal coliform test temperature to 44.5C to show thermotolerant E. coli are present? Why would food safety people be looking for thermotolerant bacteria in food rather than bacteria that grow at normal temperatures?”
In my article, I provided some background information on "faecal indicator microorganisms", which we use to show that a food or water sample may have been contaminated with faeces. These tests originated in the early days of public health services and safety of public water supply. The coliforms are easier to detect and enumerate than are Salmonella or Shigella or faecal viruses. (Salmonella and Shigella are not coliforms). The reason for incubating at 44.5C to demonstrate the presence of Escherichia coli dates from 1904 when Eijkman suggested it as a means of separating the "B. coli" originating in the faeces of warm blooded animals from the strains characteristic of cold blooded animals and thus providing us with a means to detect faecal contamination of water supplies by warm blooded animals, including humans.
So the answer to the first question is “we are not particularly interested in thermotolerant coliforms; rather we want to show that the water supply may have been contaminated by human wastes and hence potentially contains faecal pathogens”.
Having said that, of course there are other areas of food microbiology where we are very interested in the presence of thermotolerant or thermophilic bacteria. If we pasteurize milk with plate heat exchangers (the standard method) we may find that the cooling stages become colonized by thermotolerant streptococci, which may cause spoilage. In the case of canned foods, we find that some sporeforming bacteria can survive even a very rigorous thermal process. These bacteria are of no public health significance, but they may cause spoilage if the cans are held at high temperatures (greater than about 40C) as might be found in storage facilities in very hot countries or in a restaurant kitchen.
Saturday, May 3, 2008
Is hamburger a greater risk for E. coli?
The subject of coliforms in food and specifically the question about hamburger comes up often in the referrals to this blog.
Coliforms are used in the food industry as indicators of general hygiene. This sounds simple, but it is not quite straightforward; we need to understand what coliforms are and how they are measured before we can draw conclusions.
The term “coliform” does not refer to a unique species. In fact, coliforms are a group of bacteria defined by the test used to detect them. Thus, we can carry out a simple culture test that will count or show the presence or absence of this group. Each country uses slightly different tests. One protocol defines coliforms as:
“Gram negative rods that produce acid and gas from lactose in the presence of bile salts at 37 C in 24 to 48 hours”
Notice that there is no mention of genus or species or connection with faecal contamination. In fact, the group includes several genera that are capable of growing in the environment and have no faecal connotation, so finding coliforms on lettuces should not be cause for undue concern. What we can say is that all of these bacteria should be killed by pasteurization temperatures, so if they are found in pasteurized foods, the hygiene of the process has been poor.
If we want to show that faecal contamination of food has occurred, we do a test for Escherichia coli, which is found at high concentration in faeces. This test for “faecal coliforms” was originally used to test water, where the organism cannot grow. The situation in food is a little more complicated because the organisms may grow in the food, but the finding of E. coli in food is good evidence of faecal contamination. Properly pasteurized food should not contain any faecal coliforms. Raw foods, such as leafy vegetables, can become contaminated by faecal coliforms if birds or animals have access to the crops. This was the probable cause of E. coli O157:H7 contamination of raw spinach last year.
Meat animals all have E. coli in their intestines and there is a possibility of contamination of the carcase by spillage of gut contents during slaughtering. This contamination will initially be on the meat surface, but will be distributed throughout the meat if it is minced or ground. This is why steak can be eaten “rare”, since the interior of meat from healthy animals is sterile – all that is required is for the exterior to be cooked to a temperature of about 70 – 75 C. However, hamburger patties are made from ground meat and must therefore be cooked right through to ensure destruction of faecal bacteria that may have been mixed in. Americans tend to eat hamburger cooked less thoroughly than in other countries. The USDA suggests 71.1 C (160 F) for beef burgers. So the real issue is not that hamburgers are at greater risk of contamination, but that undercooking them may allow E. coli to survive.
See Guaranteed safe At what cost (?) October 17, 2007
http://foodsafetywithjaybee.blogspot.com/2007/10/
guaranteed-safe-at-what-cost.html
Coliforms are used in the food industry as indicators of general hygiene. This sounds simple, but it is not quite straightforward; we need to understand what coliforms are and how they are measured before we can draw conclusions.
The term “coliform” does not refer to a unique species. In fact, coliforms are a group of bacteria defined by the test used to detect them. Thus, we can carry out a simple culture test that will count or show the presence or absence of this group. Each country uses slightly different tests. One protocol defines coliforms as:
“Gram negative rods that produce acid and gas from lactose in the presence of bile salts at 37 C in 24 to 48 hours”
Notice that there is no mention of genus or species or connection with faecal contamination. In fact, the group includes several genera that are capable of growing in the environment and have no faecal connotation, so finding coliforms on lettuces should not be cause for undue concern. What we can say is that all of these bacteria should be killed by pasteurization temperatures, so if they are found in pasteurized foods, the hygiene of the process has been poor.
If we want to show that faecal contamination of food has occurred, we do a test for Escherichia coli, which is found at high concentration in faeces. This test for “faecal coliforms” was originally used to test water, where the organism cannot grow. The situation in food is a little more complicated because the organisms may grow in the food, but the finding of E. coli in food is good evidence of faecal contamination. Properly pasteurized food should not contain any faecal coliforms. Raw foods, such as leafy vegetables, can become contaminated by faecal coliforms if birds or animals have access to the crops. This was the probable cause of E. coli O157:H7 contamination of raw spinach last year.
Meat animals all have E. coli in their intestines and there is a possibility of contamination of the carcase by spillage of gut contents during slaughtering. This contamination will initially be on the meat surface, but will be distributed throughout the meat if it is minced or ground. This is why steak can be eaten “rare”, since the interior of meat from healthy animals is sterile – all that is required is for the exterior to be cooked to a temperature of about 70 – 75 C. However, hamburger patties are made from ground meat and must therefore be cooked right through to ensure destruction of faecal bacteria that may have been mixed in. Americans tend to eat hamburger cooked less thoroughly than in other countries. The USDA suggests 71.1 C (160 F) for beef burgers. So the real issue is not that hamburgers are at greater risk of contamination, but that undercooking them may allow E. coli to survive.
See Guaranteed safe At what cost (?) October 17, 2007
http://foodsafetywithjaybee.blogspot.com/2007/10/
guaranteed-safe-at-what-cost.html
Wednesday, October 17, 2007
Guaranteed safe (?) At what cost?
You may have read of the shocking toll taken by Escherichia coli O157:H7 in the U.S. and the apparent increasing incidence of the disease. E. coli O157:H7 infection often causes severe bloody diarrhoea and abdominal cramps that may last for 5 to 10 days. In some people, particularly young children and the elderly, the infection can also cause haemolytic uremic syndrome (HUS), in which the red blood cells are destroyed and the debris causes kidneys failure. About 8% of patients may develop HUS. Sometimes the patients recover after a long illness, but a significant number of children have required kidney transplants.
Topps Meat Company, a US company specializing in manufacture of frozen ground beef, initially recalled 330,000 pounds of beef hamburger patties because of E. coli contamination. There had been a number of cases of O157:H7 infection tied to its products. On the 29th September, the company extended the recall to 21.7 million pounds of ground beef, bringing the total recalls in the U.S from April this year to over 30 million pounds of red meat, mostly hamburger. A week later, the company announced that it was going out of business, citing inability “to overcome the reality of a recall this large”.
Clearly, 21.7 million pounds is more than a single day’s production, so there is some sort of systemic failure in the plant. An inability to identify unique batches of product probably contributed to the size of the recall. The USDA also cited the company for “inadequate process controls in the non-ground meat production line”.
The recall highlights the problem of regulations based on product testing. In the US, E. coli O157:H7 is regarded as an adulterant; there is zero tolerance for its presence, so if it is found, the whole product lot must be withdrawn. However, it is extremely difficult to demonstrate total absence of a micro-organism from a product – even with large numbers of samples, it is highly likely that a low-level non-homogeneous contamination would be missed. I am told that the product in question had actually passed two levels of testing and that the recall began only when consumers became ill.
The problem is compounded by the predilection of Americans for consumption of hamburgers cooked rare. Doug Powell at Kansas State University has said many times (and again today) that colour is a poor indicator of sufficient cooking and that consumers should use a meat thermometer. Nobody should be made ill by the food they consume and no company has a right to claim that their small size should exempt them from running properly controlled safe production processes, but consumers must take some responsibility for their own safety. At least one of the consumers who contracted the O157:H7 infection admitted that she had cooked the burgers until they were pink inside.
Raw ground meat is inherently hazardous – it is manufactured from animals that may carry bacteria capable of causing human disease. It is almost impossible to guarantee that some of these bacteria will not be found on the carcase. When the meat is ground, the exterior surfaces and interior tissues are all mixed together, spreading the contamination throughout the meat. (That’s why you can cook a steak rare and produce a beautiful, safe meal, but cooking ground meat rare can leave the pathogens in the centre undamaged and capable of causing illness).
The whole thing is getting out of hand in the US. Companies are recalling huge amounts of food, the authorities are closing processing plants, lawyers are suing for massive damages and consumer advocates are calling for guarantees that food is safe. The result can be only a loss of confidence in the food supply and an increase in costs to the consumer. I see some parallels developing in the case of Campylobacter in poultry in New Zealand
From the industry point of view, the answer lies in greater process control based on a thorough risk analysis, not on discredited end product testing.
Topps Meat Company, a US company specializing in manufacture of frozen ground beef, initially recalled 330,000 pounds of beef hamburger patties because of E. coli contamination. There had been a number of cases of O157:H7 infection tied to its products. On the 29th September, the company extended the recall to 21.7 million pounds of ground beef, bringing the total recalls in the U.S from April this year to over 30 million pounds of red meat, mostly hamburger. A week later, the company announced that it was going out of business, citing inability “to overcome the reality of a recall this large”.
Clearly, 21.7 million pounds is more than a single day’s production, so there is some sort of systemic failure in the plant. An inability to identify unique batches of product probably contributed to the size of the recall. The USDA also cited the company for “inadequate process controls in the non-ground meat production line”.
The recall highlights the problem of regulations based on product testing. In the US, E. coli O157:H7 is regarded as an adulterant; there is zero tolerance for its presence, so if it is found, the whole product lot must be withdrawn. However, it is extremely difficult to demonstrate total absence of a micro-organism from a product – even with large numbers of samples, it is highly likely that a low-level non-homogeneous contamination would be missed. I am told that the product in question had actually passed two levels of testing and that the recall began only when consumers became ill.
The problem is compounded by the predilection of Americans for consumption of hamburgers cooked rare. Doug Powell at Kansas State University has said many times (and again today) that colour is a poor indicator of sufficient cooking and that consumers should use a meat thermometer. Nobody should be made ill by the food they consume and no company has a right to claim that their small size should exempt them from running properly controlled safe production processes, but consumers must take some responsibility for their own safety. At least one of the consumers who contracted the O157:H7 infection admitted that she had cooked the burgers until they were pink inside.
Raw ground meat is inherently hazardous – it is manufactured from animals that may carry bacteria capable of causing human disease. It is almost impossible to guarantee that some of these bacteria will not be found on the carcase. When the meat is ground, the exterior surfaces and interior tissues are all mixed together, spreading the contamination throughout the meat. (That’s why you can cook a steak rare and produce a beautiful, safe meal, but cooking ground meat rare can leave the pathogens in the centre undamaged and capable of causing illness).
The whole thing is getting out of hand in the US. Companies are recalling huge amounts of food, the authorities are closing processing plants, lawyers are suing for massive damages and consumer advocates are calling for guarantees that food is safe. The result can be only a loss of confidence in the food supply and an increase in costs to the consumer. I see some parallels developing in the case of Campylobacter in poultry in New Zealand
From the industry point of view, the answer lies in greater process control based on a thorough risk analysis, not on discredited end product testing.
Wednesday, July 18, 2007
Coliforms and Faecal Contamination
I recently read a newspaper article about the potential contamination of ground water from failing septic tanks. Sanitarians had found septic tank systems close to a restaurant that might be feeding human faecal matter to the ground water. However, wells in the surrounding area, both up and downstream of the potentially contaminated site tested negative for Escherichia coli. The significance of this finding is that the sanitarians could not demonstrate that the wells were in fact contaminated by human faeces or the outflow from the septic tanks.
Escherichia coli is a bacterium found in the intestines of man and animals, but it doesn’t grow in water and doesn’t grow well in the environment. So if it is found in water, the implication is that the water has been contaminated with faecal material. Some strains of E. coli, such as O157:H7, cause serious disease, though most do not. The micro-organism is used as a Faecal Indicator. The importance of finding any E. coli in water or food is that other enteric pathogens, such as Salmonella or Norovirus, might be present.
The article went on to say that 28% of the wells tested did contain “coliforms” and this indicated that the water system was at risk of more serious contamination. This is probably correct, because a properly constructed well should draw water from deep in the ground; such water has often been in the ground for hundreds of years and has been filtered through the soil and should have a low bacterial count. Though the article was correct, it may have caused some confusion in the mind of the reader.
The finding of coliforms in water is not necessarily indicative of danger. The coliform group is defined by the tests* that we use, i.e. “coliform” is not a species, it simply says that these organisms give us a positive result in the tests we apply. This means that some bacteria are included in the group that have no faecal connotation at all. These bacteria may be found in plant material that has never come in contact with faeces of man or animals and the bacteria may even be transmitted in the seeds of the plants. So if we find coliforms in the water or in food, it is not a cause for immediate panic. Obviously, if we find large numbers present, we should look to find out why. The well may be affected by inward leakage of surface water; fresh salad vegetables may have these micro-organisms present at low numbers on their tissues, but high numbers suggest poor kitchen hygiene or temperature abuse.
If we wish to demonstrate that contamination by faeces has occurred, we need to do further tests on the bacteria to see if they are actually E. coli. The tests involve different culture media, different incubation temperatures and biochemical tests.
*The tests that we use to detect coliforms vary somewhat between countries. The tests are based on the ability of coliforms to grow in the gut, so they must be able to grow in the presence of bile salts. Bile salts are secreted into the intestine to aid in digestion of fats; they are natural detergents. So some media formulations include bile salts, while others employ synthetic detergents. The media also contain lactose as a carbon and energy source for the bacteria and the tests are incubated at the optimum temperature for the growth of the target organisms. So the coliform group is defined by their ability to produce a positive result:
Acid and gas production in 24 to 48 hours from lactose at 37C in the presence of bile salts.
Escherichia coli is a bacterium found in the intestines of man and animals, but it doesn’t grow in water and doesn’t grow well in the environment. So if it is found in water, the implication is that the water has been contaminated with faecal material. Some strains of E. coli, such as O157:H7, cause serious disease, though most do not. The micro-organism is used as a Faecal Indicator. The importance of finding any E. coli in water or food is that other enteric pathogens, such as Salmonella or Norovirus, might be present.
The article went on to say that 28% of the wells tested did contain “coliforms” and this indicated that the water system was at risk of more serious contamination. This is probably correct, because a properly constructed well should draw water from deep in the ground; such water has often been in the ground for hundreds of years and has been filtered through the soil and should have a low bacterial count. Though the article was correct, it may have caused some confusion in the mind of the reader.
The finding of coliforms in water is not necessarily indicative of danger. The coliform group is defined by the tests* that we use, i.e. “coliform” is not a species, it simply says that these organisms give us a positive result in the tests we apply. This means that some bacteria are included in the group that have no faecal connotation at all. These bacteria may be found in plant material that has never come in contact with faeces of man or animals and the bacteria may even be transmitted in the seeds of the plants. So if we find coliforms in the water or in food, it is not a cause for immediate panic. Obviously, if we find large numbers present, we should look to find out why. The well may be affected by inward leakage of surface water; fresh salad vegetables may have these micro-organisms present at low numbers on their tissues, but high numbers suggest poor kitchen hygiene or temperature abuse.
If we wish to demonstrate that contamination by faeces has occurred, we need to do further tests on the bacteria to see if they are actually E. coli. The tests involve different culture media, different incubation temperatures and biochemical tests.
*The tests that we use to detect coliforms vary somewhat between countries. The tests are based on the ability of coliforms to grow in the gut, so they must be able to grow in the presence of bile salts. Bile salts are secreted into the intestine to aid in digestion of fats; they are natural detergents. So some media formulations include bile salts, while others employ synthetic detergents. The media also contain lactose as a carbon and energy source for the bacteria and the tests are incubated at the optimum temperature for the growth of the target organisms. So the coliform group is defined by their ability to produce a positive result:
Acid and gas production in 24 to 48 hours from lactose at 37C in the presence of bile salts.
If we wish to demonstrate that Escherichia coli is present, then we may subculture into a similar medium containing selective agents (which prevent other similar micro-organisms from growing) and raise the temperature to 44.5C and again look for gas production. At the same time we may look for the ability of the micro-organisms to convert tryptophan to indole; produce sufficient acid to change the colour of methyl red pH indicator; produce acetoin from glucose and grow on citrate as the sole carbon and energy source. This series of tests is known as the IMViC tests. E. coli normally gives a "++--" profile. Finally, we may grow the culture on Eosin Methylene Blue agar and look for the formation of a metallic green sheen. If all these tests give the correct result, we can be pretty sure that E. coli is present and faecal contamination has occurred.
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