Humans contain fewer bacterial cells than expected

By REGINA PALATINI | February 11, 2016

B9_candy1-678x1024 Consuming sugar can worsen the impact of the mouth’s microbiome.

The expression “we are not alone” is truly an understatement when considering the number of microbes that live in and on the human body.

For the last several decades, researchers and the scientific community at large believed that bacterial cells outnumbered our own cells by a ratio of approximately 10 to one. However, this estimate was recently updated by a study published in the journal Cell, which revealed that the average adult is actually made up of around 30 trillion human cells and about 40 trillion bacterial cells.

Although the ratio of bacterial to human cells is now a lot closer to 1:1, we are still essentially more bacteria than self.

So, are these foreign invaders good or bad, and do they help or impede our survival? Do they serve a purpose in our bodies, or are they just along for the ride?

Some of the microbes on and inside the human body, collectively referred to as the human microbiome, can cause diseases. However, a growing body of research suggests that the many of them are beneficial and vital to our health and well-being. Certain bacteria can be so beneficial that a growing number of scientists are referring to our personal biomes as our “second genome.”

According to Elizabeth Grice, co-author of a recent paper published in The Annual Review of Genomics and Human Genetics, the microbes within humans can increase genetic diversity, provide humans with increased immunity and facilitate human digestion, among other benefits.

The human genome has been referred to as the blueprint of human biology. In recent years, the development of genomic technology has allowed researchers to take a closer look at the human microbiome and to consider it as a contributor to the human genome.

Our genome consists of about 20,000 genes. We may have a few thousand different species of bacteria residing in our bodies, and each one of those species has a few thousand genes, so it can be beneficial to gain a better understanding of how these non-human genes are expressed.

Take our mouth, for example. It is considered to have its own “microbiome” due to the variable environmental conditions that it presents, which provide ideal conditions for bacterial growth.

“Scientists have estimated that up to 25,000 different types of bacteria can live in the mouth under certain chemically diverse circumstances. Over 1,000 of these bacterial species live in the dental plaque ecosystem as microorganisms that are extremely adaptable to changing mouth environments,” Harold Katz, who works at the University of California, Los Angeles School of Dentistry, told TheraBreath.

The human tooth is the only part of the body that does not contain some type of regulating system that periodically sheds its surface. Because of this, various colonies of microorganisms readily adhere to and remain on each tooth’s surface.

Dental plaque film is thought to be a part of an oral defense mechanism that is intended to prevent seriously pathogenic bacteria from rapidly destroying tooth enamel. However, this plaque can cause dental and gum disease when these microbes are presented with sugar, such as from a sugary beverage or candy bar. Being presented with sugar can cause the plaque to overgrow and to secrete acid that can damage the teeth.

A recent study conducted by Mashkoor Choundhry and published in PLOS ONE found that in patients who had suffered severe burns there was a substantial increase in Enterobacteriaceae, a strain of potentially harmful bacteria. There was a corresponding decrease in the beneficial bacteria that normally keep these harmful bacteria in check.

According to George Weinstock, the associate director of The Genome Institute and leader of work on the Human Microbiome Project at Washington University in St. Louis, the project is aiming to understand the microbiome in addition to human genomes.

“The project is going to sequence the second human genome, which are the genomes of all these organisms,” he said. “We’re going to try to understand what’s going on in your microbiome What does it do that makes it healthy? What is going wrong when it causes problems for you? How can we manipulate it and control it and make it better?”

In the continuing effort to understand how the human body functions, it is becoming more and more important to understand the trillions of foreign organisms that reside within us.

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