Common pathogenic bacteria that live on humans

This was originally published in Maeflowers on Medium Oct. 14, 2024 as "Common Pathogenic Bacteria That Are Probably Living on You Right Now"

Photo of E. coli by CDC on Unsplash

Germophobes are not going to like this. But I’m not being sensational. We all have bacteria living on us, and in us, that time has cultivated a balanced co-existence with. Unfortunately, some of these organisms are pathogenic — they cause disease.

Of course, if they live on your skin, they can’t be causing disease all the time. It’s when the balance is tipped to favor certain species over another (such as antibiotic overuse) or when the barrier that keeps them out is broken (such as a cut or surgery) that they take the opportunity to wreak havoc. They can’t help it! The bacteria are just living their lives — replicating and pushing out other cells is what they do to survive.

So, what are these microscopic gremlins that call our bodies home? You’d be surprised how many you recognize.

Staphylococcus

Staphylococcus species are extremely common bacteria in the human nose and on the skin. The common strain to cause disease is Staphylococcus aureus. Approximately one-fourth to one-third of adults have S. aureus among their skin flora. The other commensal inhabitants, such as Staphylococcus epidermidis, keep the numbers of these bacteria in check. However, this locale allows easy access to open wounds or cracked, dry skin. Skin infections with this genus are simply called staph.

With the rise of antibiotic resistance, there is now also resistant staph infections. MRSA (methicillin-resistant Staphylococcus aureus) is well known, and it is treated with vancomycin due to the resistance to synthetic penicillin. But now there is also VRSA (vancomycin-resistant Staphylococcus aureus), or VISA (vancomycin-intermediate S. aureus), making treatment more difficult.

In addition, to the potential for skin infection, Staphylococcus is among the most common pathogens underlying hospital-acquired pneumonia, with causes including S. pneumoniae, S. aureus, and MRSA. Unfortunately, another major cause of nosocomial infection is S. epidermidis, which infiltrates the body via medical devices and during the surgery to implant them.

Streptococcus

The most common site of pathogenic invasion is considered to be the respiratory tract. The bacteria and viruses that live in, or infect, the airways and nose are constantly being breathed out to other people. Though the main pathogens of respiratory illness are viruses, bacteria are responsible for the development of secondary infections, such as pneumonia, as well as some primary infections.

Most people carry Streptococcus pneumoniae in their nose and throat. However, this strain is also the most common cause of lower respiratory tract infection in adults and children (i.e., pneumococcal pneumonia). It is usually an opportunistic secondary infection after a viral infection, such as the flu. The immune system tends to keep the Strep bacteria in check, but getting sick can tax the system, allowing the pneumococcal pneumonia to set in. S. pneumoniae can also spread to the blood, causing sepsis.

A non-commensal strain of Streptococcus that can cause disease is group A strep. When a group A Streptococcus infection, such as strep throat, isn’t treated, it can lead to additional complications.

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E. coli

The most common cause of food-related illness is bacteria. Commonly called food poisoning, when these pathogenic bacteria are ingested they cause gastrointestinal disorders, such as diarrhea and vomiting, putting you at risk for dehydration. One such bacterium is Escherichia coli.

Most people have E. coli biofilm in their intestines as a normal part of the gut flora. But when the bacteria overgrows, due to diet, mutation, or antibiotic use, it can cause mild intestinal distress. There is a mutated strain that causes more severe disease, and we introduce it into the gut biome by ingesting contaminated food (e.g., unwashed vegetables, undercooked beef) and water. It’s called E. coli O157:H7.

Research on how exactly the gut flora balance is maintained is ongoing.

Living with Bacteria

Although they can be opportunistic, commensal bacteria have been found to protect against more pathogenic strains on the skin and to be a necessary part of the digestive process. From 2007 to 2016, the Human Microbiome Project attempted to sort out the positives and negatives in living with the species we host. Protecting the food and water supply from contamination, using appropriate antibiotics only when necessary, and quickly treating infections (potentially with probiotics) can help protect the balance between exposure and co-existence.