If asked, most people would say without hesitating that the place in their home where most germs accumulate is the lavatory. However, they would be wrong. New high performance DNA sequencing techniques have meant that detailed information can be obtained about micro-organisms and, with regard to homes, the findings are surprising.
The origins of domestic micro-organisms differ, but most of them come from us. Every individual carries a 'cloud' of personal microbes - the latest forensic technology has successfully identified someone who rented an apartment through the 'bugs' he left behind - but they can be transferred, either through intimate contact (hugs, kisses, caresses) or social contact such as shaking hands. This means that we share germs all the time, at a rate of one million every hour. That is not a great loss: a group of Israeli scientists concluded last year that a human body has about 30 billion cells and 39 billion bacteria.
These organisms start to colonise us at birth, firstly through the birth canal - the vagina, like the skin and other mucous membranes, is a magnet for microbes - and then through food. Throughout our lives, we create our own microbiota, formed by thousands of different species, adapted to the specific place which they inhabit.
“There are between 60 and 90 different species in each person's mouth alone, and each one has different clones,” says Victor Jiménez Cid, who is also a professor of microbiology at the Complutense university.
The vast majority of these microbes do not cause illnesses; on the contrary, they protect us from them. In these stable colonies, “there is a law of survival of the strongest, especially when there are few resources. If an outsider arrives, it has very few possibilities to integrate. Pathogenic micro-organisms, before facing the immune system, have to compete with our microbiota, and it doesn't give way easily,” explains Victor.
Many researchers are trying to find out why probiotic foods don't succumb as soon as they reach the intestine and encounter powerful native flora. The mechanisms through which the bacteria fight between themselves to stay with our liquids and waste are similar to those produced by animals in nature. They even know the art of war: “There are some species which fire a type of poison arrow to destroy their competitors. It's amazing,” says the professor.
The most common bacteria inhomes are those which come from our skin -household dust is full of them - such as the gram-positives, which include staphylococci and micrococcus. In damp, dark places the gram-negatives can proliferate, and colonies of fungus and yeasts. These are what form the black marks which occur between the bathroom tiles or on the shower mat.
Food is a rich source of micro-organisms, as are ornamental plants and domestic pets. In many countries in Asia, northern Europe and Oceania it is customary to remove one's shoes when entering the house. That's a very sensible idea: different germs, parasite eggs and, of course, bits of excrement and sputum stick to the soles of shoes. You don't need to be an epidemiologist to understand that wearing your shoes from the street to the rug in the bedroom where you sleep is a risk factor,” says Victor.
The five-second rule
The belief that a floor does not represent a threat to health is the basis of the famous theory of five seconds, which says that if an item of food falls onto the floor and we pick it up within that time, the germs won't have had time to 'climb' up to our mouth. Various experiments have shown the opposite, but the most important, carried out by Paul Dawson, of Clemson university, found that it makes no difference if the food is on the floor for five seconds or half an hour, because the bacteria are transferred almost immediately. The quantity which does so depends on the type of floor - curiously, in the experiment there was more contamination on wooden and tiled surfaces than on carpet - and how dirty it is. For example, Escherichia coli can infect an organism at very low doses and cause diarrhea, and even death, in children and people who are immunosupressed. Conclusion: if you want to live dangerously, carry on believing the theory.
If we had special glasses to see the micro-organisms easily, we would be horrified to find that the kitchen, which is one of the areas we clean most, is full of them and that, curiously, their main vehicles are cleaning utensils.
Sponges and clothes are often in contact with remains of food, are porous and are nearly always damp, which makes them an ideal home for germs. If you wipe a cloth over the work surface and it leaves an unpleasant smell, it's because it is covered in them.
However, they are not the only ones. Chopping boards and knives are a frequent source of cross-contamination: if you cut meat with salmonella or campylobacter, when you cook the meat the germs disappear. If you then go on to use the same items to prepare foods which are eaten raw, such as salad, there is a risk of toxic infection.
The kitchen sink and drain are also hot spots: for example the remains of food we leave after eating, or when we rinse the dishes before putting them in the dishwasher are a nursery for microbes. They should be cleaned in a bleach solution at least once a day.
Other kitchen equipment such as the cooker, oven or microwave are also complicated because we usually touch them with dirty hands and - unlike surfaces such as the sink or worktop - they can't be washed because they are electrical appliances. Solutions with an alcohol base are the best to make them hygenic.
In the bathroom, which is the place most of us suspect of being unhygienic, the potentially most dirty items are not the ones we expect. In the 1970s, a researcher in Arizona discovered that, when we flush the cistern, a cloud of bacteria and viruses comes out of the lavatory and can float in the atmosphere for two hours. And another object to watch out for is the toothbrush which, as well, has its own ecosystem thanks to the damp and bacteria in dental plaque. Solution: close the lid of the lavatory before flushing, clean and dry the toothbrush after every use and change it regularly.
Researchers have found fecal material, staphylococci and fungus in bathtubs, and flu and catarrh virus on shower curtains. Razors, the bath mat, makeup brushes and contact lenses are other potential nests of microbes.
In the living room, the record for dirtiness is held by the TV remote control. It is passed from hand to hand, often used while eating, falls on the floor and people even cough over them. Researchers at the University of Virginia discovered cold virus in half of the remote controls they analysed.
The computer keyboard is similar - some hold so many remains of organic material that they should cause a sanitary alert - and so are mobile phones. One study found traces of excrement in one out of every six. They should be wiped with a cloth and disinfectant solution periodically.
In the bedroom, the bed is one of the places with the most biodiversity: it absorbs sweat, saliva and particles of dead skin every night. In the pillows and eiderdowns, the feather filling is a potential breeding ground for fungus and dust mites. The bedclothes, just like bathroom towels, should be washed at the highest possible temperature in the washing machine. If clean towels have a persistent damp smell it means they contain resistant fungus: it's time to get rid of them.
Door handles, house keys, handbags, water bottles and the laundry basket are other fantastic vehicles for germs to spread, and we rarely think to disinfect them.
In most cases the microbes are not dangerous, but domestic and personal hygiene is particularly essential when someone with a contagious infection is in the house.
Experts agree that the cheapest and most effective way to prevent infections is to wash your hands with soap and water, because hands are the principal way the micro-organisms (which as we have seen are omnipresent in homes) penetrate our body. This is because we continually touch our mouth, eyes and nose. According to the World Health Organisation, washing our hands five times a day reduces by half the possibilities of catching a virus such as flu or spreading food infections.
In his book 'Life of poo', British biologist Adam Hart says people tend to lie about their hygiene habits: in surveys, 95 per cent say they wash their hands after going to the lavatory, but you only have to spend any time in a public toilet to see that isn't true. One scientist did just that and came to alarming conclusions: only 61 per cent of women and 37 per cent of men washed t heir hands, and half of them didn't use soap.
That's why Hart decided on the subtitle to his book: “Why you should think twice before shaking hands (especially with a man).