The billions of microbes living on you are called your skin microbiome. These microorganisms (sometimes called skin flora) are harmless or even beneficial—playing a vital role in your immune system and skin appearance. Evolved over thousands of years, the human microbiome consists of many distinct types of colonies, depending on the location and condition of the microenvironment.

The microbiota survives off of the salt, water, and oil (sebum) your skin releases to keep itself cool and lubricated. And several factors determine the habitat of the various microbiota, like:

• body temperature
• skin thickness
• amount and size of folds
• skin pH
• the density of hair follicles and glands

In other words—and not all too surprisingly—the microbiota on your face looks different from the microbiota on your armpits. Areas with higher density of oil glands, like your face, back, and chest, thrive off of the lipids (fats) in your sebum. Warm, humid areas, such as the groin and between the toes, host microorganisms that love a danker environment. Meanwhile, dry, cool patches—like your arms and legs—have far fewer micro-colonies than the rest of your body. In all, the average person carries around two pounds of microbes on their body at all times.

The sheer amount and diversity of skin flora may sound scary. But it’s actually a good and healthy thing. Having a bountiful, well-balanced microbiome plays an important role in your overall health and the appearance of your skin. The microorganisms help produce vitamins, hormones, and chemicals that affect everything from your mood to metabolism to the immune system.

Most people know the skin is the body’s first line of defense against injury or potential pathogens. But it’s not actually your skin’s cells that act as the front lines of the cavalry. It’s the skin’s microbiome.

Your skin’s inherent environment is rather unfriendly to bad bacteria. It’s cool and dry. The pH is acidic. Even sebum, your skin’s lubricant, is antimicrobial. And, ideally, your skin has a bountiful amount of microbiota to combat all the bad bacteria you come into contact with.

A healthy skin microbiome, which prefers the acidic environment your skin provides, helps your immune system out. This likely starts by skin flora overcrowding pathogen overgrowth. Also, your skin’s immune system and microbiome communicate and respond to one another’s needs.

But your skin could be left vulnerable if your skin’s microbiome has been damaged in one of many ways:

• soaps
• incorrect or overuse of antibiotics
• harsh skincare products
• environmental factors

Unfortunately, the diversity in many modern societies’ microbiomes is as much as half as diverse as it once was. The culprits of the dwindling number of microbiota? Modern hygiene practices—such as daily showers or baths and the use of aggressive soaps and detergents—along with less healthful diets. Also, a lack of interactions with plants, soil, and the microbiomes of livestock and other wildlife, may have an impact.

On the individual level, many factors can shape the diversity of your skin flora. Your job, age, lifestyle, clothing, hygiene habits, and even how much time you spend in the sunlight can all affect the types and amount of microorganisms inhabiting your microbiome.

The lack of diversity can become obvious, even to the naked eye. It can lead to dryness, overproduction of sebum, breakouts, redness, or other afflictions. Therefore, keeping the proper balance of microbiota, and maintaining proper pH, can help protect your skin and microbiota from undesirable conditions.

The relationship between your skin’s appearance and the microbiome isn’t completely clear. That’s partial because the vast majority of skin flora haven’t been cultured or extensively studied yet. But more research and information is likely coming. That’s because the subject of the skin microbiome has caught the attention of many large beauty and skincare brands. It has even inspired the creation of some startup cosmetic brands that are experimenting with adding microbes to their products.