genus of microbacteria that causes tuberculosis.
In the domain of health, more and more importance is being given to what’s known as the microbiota, or the set of all the microorganisms each person carries inside (and on the surface). It’s believed that, depending on which microbes inhabit this microbiota, we can be more or less prone to certain illnesses or conditions.
One example of this would be that the type of bacteria found in intestines could determine whether we gain weight or not, as was first suggested in a study from 2006. After isolating intestinal bacteria from mice of normal weight and from others that were obese, scientists found that bacteria from the latter contributed towards weight gain in the former, even when they continued with the same diet. Humans have between 500 and 1,000 different species of bacteria in the digestive tract and it might well be that these also have an influence on a person’s susceptibility to gaining weight. More recent studies support this theory. For example, in 2009 it was found that obese women have a high presence in their saliva of bacteria called Selenomonas noxia. By contrast, thin women show a very different set of bacteria.
Secondary effects
It’s been known for some time now that antibiotics can disrupt the balance of ‘good’ bacteria. Medicines eliminate infections but are unable to distinguish between aggressive and innocuous microbes. Depending on the treatment, even weeks can go by in some cases before the bacterial composition of the intestine, for example, completely recovers. This can then give rise to diarrhoea or new infections caused by other harmful bacteria, especially in people already weakened by illness.
This shows that it is not only our intestines that are full of microorganisms. Our mouths, too, normally have between six and thirty different types of bacteria. And skin is another organ that is home to thousands more. It was once believed that most of them were of the genus Staphylococcus, because when samples taken from human skin were cultivated in the laboratory, they were the most visible. But this doesn’t mean that there aren’t many more. There are others that don’t divide so quickly. Indeed, with the new tools of genetic analysis, it’s been possible to see that the set of denizens of human skin is much more complex than was previously thought, with up to 1,000 different species, which is to say, a number that’s comparable with that for the intestines. The skin behind the ear is the zone with the least diversity of bacteria, with only fifteen kinds, while the forearm has as many as forty-four. This varying distribution might explain whysome skin diseases appear in certain zones and not in others. As in the intestines, bacteria on the skin have important functions, so, for example, oilier zones have some bacteria that produce a moisturizing substance to stop the skin from cracking.
In recent years, several studies have set about the task of identifying all the microorganisms that are to be found in different organs, generally using modern techniques to read their genes in order to relate them to obesity or illness. These studies give us a general idea of the microbes we carry around with us, although each person’s flora is, in fact, unique. Almost like our DNA. It depends more on the zone in which we live than on our genes, and personal habits have a considerable influence as well. An article published in January 2009 demonstrated that sets of intestinal bacteria vary even between twins. Nevertheless, the members of a family living under the same roof have similar flora. The article also indicated that obesity reduces the diversity of flora, as well as altering the genes and metabolism of microorganisms. It’s speculated that this might have consequences for our health, but we still aren’t sure what they might be.
This knowledge we are acquiring about the microbes that coexist with us has led to questions about whether they can be used for therapeutic purposes. There are now studies looking into ways of changing the composition of a person’s microbiota as a way of curing illnesses and even regulating the metabolism with the aim of weight loss. The easiest way is to take microbes from a healthy person’s faeces and transfer them to the patient. Informally known as a stool transplant, this isn’t such a simple process as it may appear, because it requires, first of all, filtering out the bad microbes and other contaminants.
It’s still not known whether this procedure might have any real benefit, but what is undeniable is that the microbiota plays an important role in our health, both positively and negatively. This could be more far-reaching than initially imagined. Some studies have even shown that the microbes inhabiting our intestines could affect the brain and somehow influence behaviour.
The dark side
It’s well known that not all microbes are as beneficial as the ones I’ve just described. A group called pathogens, amounting to only 1,415 of all those that exist, have been found to cause infectious diseases in humans. Although they are clearly a minority, their impact on society has been, and is, immense.
Infections occur when one of these pathogens manages to enter our organism and overcome its defence systems. Problems arise when the microbe starts drawing on the resources of the organisms it has invaded to multiply nonstop. If this isn’t checked fast enough, it will end up interfering with the normal functioning of the body, presenting the symptoms characteristic of each infection, depending on the organs the invader prefers. Some of these symptoms are shared by many infectious diseases, for example, fever, shivers or feeling unwell in general.
It’s commonly believed that, thanks to the discovery of antibiotics, pathogenic microbes have ceased to be the terrible threat they were until just a few decades ago. To some extent, this is true. Nevertheless, we are a long way from being able to feel relaxed about this. On the one hand, bacteria that are resistant to the most commonly used antibiotics are constantly appearing. On the other, some serious illnesses still exist for which there are no vaccines or treatment. And there are still others that have both but, even so, we can’t stop them. Moreover, it should be recalled that antibiotics are only useful against bacteria, but they don’t work with viruses. It’s true that we have antivirals to fight these microbes, but they aren’t so effective, and we still haven’t produced such a wide range either. This, then, is a neverending struggle.
A bit of terminology
With regard to infections, there’s a series of terms that are frequently used to define their reach. For example, an outbreak is an infection localized among a relatively small group of people, for example, a family, a school or even a village. A typical case would be food poisoning, which tends to affect only those who have eaten food containing pathogenic microbes.
The next level is an epidemic, which is defined rather arbitrarily as an accumulation of infected people that’s bigger than ‘normal’. For example, if a disease is very rare, a mere handful of cases could be regarded as an epidemic. When an epidemic has spread through more than a continent or even the whole planet, we call it a pandemic. Technically speaking, the WHO officially declares a pandemic only when a disease goes beyond six phases, ranging from detection of the microbe in animals (phase 1) through to the continuing presence of the disease in more than one of the regions defined by the organization (phase 6).
An infectious disease that’s constantly present in a region without any significant fall or rise in the number of cases is said to be endemic. For example, malaria is endemic to many parts of Africa. Whether or not an outbreak turns into an epidemic, a pandemic or becomes endemic depends on many factors, among them the speed at which it spreads and the virulence of the disease it causes.
Which disease spreads fastest?
It has been estimated that the 1918 flu pandemic had an R0 of around 4 (meaning that each affected person infected four more). Seasonal flu (the annual one) normally has an R0 of between 1.3 and 3. The A(H1N1) flu pandemic of spring 2009 had an R0 of only 1.4, while that
