Bacteria, invisible passengers travelling in the Metro


Every day, 5.5 million users travel through the 12 lines of the Mexico City Metro, carrying and bringing with them an unimaginable number of invisible passengers that are exchanged by breathing, talking, coughing, sneezing, and physical contact.

Pandemia: influenza Mexico metro. Photo: Conacyt Information Agency
Pandemia: influenza Mexico metro. Photo: Conacyt Information Agency

Public transportation, and in particular the Metro, has attracted the attention of researchers because of the high concentration of people coexisting there. In Mexico City, the microbiota of this transportation system is studied because of the large influx it has, an amount almost similar to that of the New York Subway, with the difference that this network consists of 436 kilometers and 468 stations, while in the Mexican capital there are 226 kilometers and 195 stations. This speaks to the overcrowding it presents.

A sea of people crowds the entrance of the Indios Verdes station of the Mexico City Subway. It is six o'clock in the morning and the usual hustle and bustle lead all these people to the access turnstiles, each one carrying an approximate load of 100 trillion cells of microorganisms such as bacteria, viruses, and fungi.

These microscopic agents are present in the air, on seats, walls, railings, and on any surface; and although it is frightening, we live all the time in similar scenarios that, when exposed to them, are beneficial to our health.

They can be public spaces where a great density of people is concentrated, such as stadiums, theatres or public transport; or closed places whose characteristic is human contact and little or no ventilation, such as offices, shops and the home itself.

In both scenarios, the microbiota -the microorganisms that live in a specific environment- is composed of bacteria inherent to humans, such as those associated with the digestive tract, the respiratory tract, the skin and mouth, the sexual organs, and so on.

Scientists around the world are studying the diversity of pathogens to which we are exposed in different environments in order to understand the risks of some diseases, their relationship with epidemiological aspects, the antibiotic resistance of bacteria or even the discovery of others that are so far unknown and that could be beneficial to humans.

City microorganisms

In 2013, Christopher Mason, a researcher at Cornell University, conducted a study to analyze the microbiological diversity of the New York Subway, identify potential biological threats and provide data that could be used for the design of a "smart city" for urban planning, management, and human health.

Through the metagenomic analysis of the samples taken with swabs from various surfaces, he found that almost half of the biological universe he obtained (48 percent) is unknown; the one identified corresponds to bacteria, viruses, fungi and animals, of which only 12 percent could be associated with some disease.

From this work arose the concern to understand the microbiota of the Metro in the largest cities in the world, forming the MetaSUB consortium. Research groups from 61 cities in the five continents participate in this consortium.

In Mexico, doctors Celia Alpuche Aranda and Jesús Martínez Barnetche, researchers from the National Institute of Public Health (INSP), are collaborating. In 2016 they carried out a pilot project in five stations of the Mexico City subway, collecting samples that were analyzed by massive sequencing in laboratories in New York.

This preliminary study detected bacteria present in the human microbiome, such as Pseudomonas, bacilli, staphylococcus, and streptococcus, among others, which do not represent a risk for people in good health, although not for those who are immunosuppressed.

"Most of what has been found in the Metro is what is found in the human microbiome; there has been no description of anything that might be risky. Pseudomonas were found, which are hospital-acquired pathogens that infect people who are immunosuppressed or who have a persistent medical condition. These agents were found there, just as they are found in the supermarket or in our own home," says Jesús Martínez Barnetche.

The preliminary study supports the view that each city has a particular antimicrobial resistance profile which, in the case of Mexico City, would indicate that environmental contamination is changing the microbiological profile. Hence, the implementation of this project is relevant, since the incipient results allowed the identification of genetic markers of resistance different from those of bacteria present in other cities of the world.

"With the data we obtained, we detected indications that suggest that certain metabolic pathways involved in the metabolism of chemical, toxic, and other compounds -products of pollution and the use of hydrocarbons- were overrepresented. My impression is that this could be due to or linked to the presence of certain components in the atmosphere that favors the growth of some microorganisms over others," says Martínez Barnetche, who works at INSP's Center for Research on Infectious Diseases (CISEI).

Knowledge to act

Motivated by the interest in revealing the microbiome of the great "orange worm", scientists from the Universidad Autónoma Metropolitana (UAM), the Universidad Nacional Autónoma de México (UNAM) and the Universidad de la Ciudad de México (UACM) are studying the bacteria present in this transport medium, through molecular analysis by deoxyribonucleic acid (DNA) sequencing.

This work is aimed to know the risk of being in contact with this great diversity of bacteria, but also the benefits that they can give us because many of these microscopic travelers can contribute to good digestion, to have a better metabolism, or to regulate hormonal processes.

Dr. Mariana Peimbert Torres, a researcher at UAM, Cuajimalpa unit, and who participates in this project called "Microbioma del Metro de la Ciudad de México", says that they have seen that the bacteria present are related to people.

Through the analysis of surface samples and the air breathed in by the Metro at 48 points on all the lines of the network, they are identifying existing microorganisms to find out their diversity in time and space.

"The network is made up of underground stations and others that go above the surface; we want to know if the microbiome is different in each of the areas of the city where the lines are located. In addition, we want to see how its presence changes over time", he explains.

The information that is being obtained with this work can contribute to decision making in public health matters, for example in cases of health contingencies, or in the operation of the Metro Collective Transport System (SCT).

According to the also head of the Division of Natural Sciences and Engineering of that university, the work opens the opportunity to explore other lines of research, such as studying the bacterial communities in the stations associated with hospitals, to know how much the diversity of microorganisms of the people who transit there influence; or those that connect with the bus terminals and the airport, because they are points of convergence of people who come from other places and whose bacterial load is different.

Bacteria, here and there

The exposure we have to the bacterial universe occurs anywhere, although it is greater in spaces with high human density. In the Metropolitan Zone of the Valley of Mexico, one breathes about 120 different types of live bacteria in the lower atmosphere.

For work, education, and other reasons, there are areas that bring together a large number of these people who move along the various transport routes and with them, a large cloud of microorganisms.

In order to find out what makes up this urban microbiome, scientists at the Centre for Research and Advanced Studies (Cinvestav) of the National Polytechnic Institute (IPN) analyzed bacteria in the lower atmosphere along public transport routes.

In Mexico City and its metropolitan area, the population density is 20.8 million people, making it the fourth most populated city in the world.

Dr. Jaime García Mena, a researcher from the Department of Genetics and Molecular Biology of Cinvestav and leader of the project, explains that they captured live airborne bacteria at a height of one meter in 67 sampling sites to make them grow and characterize them by massive sequencing.

The study identified about 120 different types of bacteria with which citizens could be in contact or breathing; although the number varies according to the season, as we found that the months of June and July are when there are more airborne bacteria.

This work also shows that human activities carried out in different places change the variety of microorganisms, even over distances of less than two kilometers, because "where there is a large concentration of people there is a great diversity of bacteria, but if we add to this that there are economic activities, such as transport of goods, then it increases more," says the researcher.

Are you afraid of bacteria?

For some people, being exposed to a huge amount of pathogenic microorganisms can trigger a phobia or obsessive-compulsive disorder. The first is an immense and irrational fear of some environmental situation, which leads the sufferer to become irritable, aggressive, and even to a point where it seems that the person freezes; these behaviors have a specific correlation to a stimulus.

According to Dr. Guillermo Peñaloza Solano, a psychiatrist assigned to the Psychiatric Care Services of the Ministry of Health, those who present obsessive-compulsive disorder do not control the images or ideas they are presented with.

Obsessive-compulsive disorder is generated from intrusive thoughts that produce apprehension, restlessness, worry, or repetitive behaviors, focused on reducing associated anxiety.

In the case of the fear of catching germs on public transport or in another specific place, they have answers such as avoiding being there or washing their hands many times a day, behaviors that they cannot stop doing.

In both scenarios, the cause of the mental illness is multifactorial, that is, it depends on each person's temperament, degree of neuroticism, or whether they witnessed a related traumatic event; but another factor is genetic inheritance, because if there have been cases of phobia or anxiety disorder in the family, it is likely that they can be inherited.

The specialist indicates that both phobias and obsessive-compulsive disorders can be treated with antidepressants and anxiolytics, but "the main and most important thing is that they must have cognitive-behavioral therapy, which helps them to better understand their thoughts and try to identify the emotions within that disorder," he says.

Immune system, the great ally

Undoubtedly, not only in the subway but anywhere we are exposed to infectious agents that can affect our health, but in front of it, we have a great ally: the immune system.

Our first line of defense is the skin, a very efficient physical barrier, which is complemented by the mucous membranes present in the body's compartments that are open to the outside world - such as the nose, the oral cavity or the vagina - and which may have antibacterial compounds.

As if it were an army of soldiers defining territory, the main strength of the immune system is the ability to recognize millions of bacteria, fungi, viruses, and parasites, to produce soluble molecules and specific cells against them, says Dr. Yvonne Rosenstein, a researcher at the UNAM Institute of Biotechnology.

"It is an organized set of cells that monitors attacks from the outside, a surveillance system that acts like a patrol that passes through the body and travels several times a day in search of pathogens. When it detects them, it produces molecules with antimicrobial activity; this is called innate immunity," she says.

Adaptive immunity is formed from the first response to pathogens, creating an immune memory, and generating cells (lymphocytes) that recognize it and specific antibodies.

According to the specialist, we don't get sick anymore in the subway or in crowded environments because of the germs that can be present there, not all of them are harmful. Many of the bacteria are microorganisms that are a natural part of the environment and our bodies.

"Being in crowded places can be beneficial because it allows us to enrich our microbiome, and we need it to be healthy," she adds.

In that sense, considers Dr. Jaime Garcia Mena, the airborne microbiota keeps our immune system alert so that, through the exercise of the production of the immune response (innate and adaptive), we can enjoy good health.

That is why Dr. Rosenstein recommends "not to demonize" the Metro, which, although it circulates many germs, much of them contribute to our complex ecosystem.

By Ana Luisa Guerrero. This work whose author is Agencia Informativa Conacyt is under a Creative Commons Attribution 4.0 International license.