This is the second post in a three-part series on the microorganisms that live on our skin. In this post, we’ll be talking about the differences between resident bacteria and transient bacteria. Check out the first post The Types of Microorganisms That Live on Our Skin – the third and final post will be published next week.

Resident bacteria are sometimes known as colonising flora. This consists of the bacteria that live in your skin for a long time and typically has important functions. For example, the metabolism of these resident bacteria can stop the growth of non-resident bacteria (such as transient bacteria) and fungi that could be harmful to our body. Resident bacteria can also maintain a stable number of colonising microorganisms on our skin. This ensures that their numbers don’t get out of control. Also, resident microflora establishes a relationship with the host that is normally commensalistic or symbiotic.

It’s estimated that around 80% of the resident skin flora lives on the outermost layer of the epidermis. This is known as the stratum corneum. There are also certain parts of the skin that have higher concentrations of resident bacteria. Roughly 20% of the resident microorganisms in our body are found in areas that are rich in sebaceous glands. This includes the forehead, scalp, axillae, chest, between the shoulder blades, and also near the groin.

The dominant species of resident bacteria is Staphylococcus epidermidis, a gram-positive bacterium and one of the over 40 species that belong to the genus Staphylococcus. It is most commonly found in the skin and less commonly in the mucosal flora. Staphylococcus epidermidis isn’t usually pathogenic, but people with a weakened immune system may develop an infection if their numbers are not controlled. However, the majority of these infections related to Staphylococcus epidermidis come from hospitals.

Another common species of resident bacteria includes cutibacteria, corynebacteria, dermobacteria, and micrococci. These are Gram-positive and aerobic (apart from cutibacteria)  bacteria that are usually not pathogenic. However, they can be opportunistic and may take advantage if they have access to tissues or weakened host defences. This usually occurs via wounds such as cuts.

Transient bacteria refers to microorganisms that are usually not found in the body. Additionally, transient bacteria of the skin could mean bacteria that is not a common skin dweller but is transferred, temporally from other body sites. Transient bacteria generally refers to bacteria, but it can also mean fungi and viruses. Most transient bacteria spreads via direct skin contact or through indirectly touching and sharing different objects. Much like resident bacteria, transient bacteria isn’t inherently bad for us. However, given the opportunity, they will compromise our bodies and cause infections.

Transient bacteria tend to colonise the superficial layers of the skin. They don’t normally multiply on the skin like resident bacteria since they are inhibited by the resident bacteria, but they are known to occasionally multiply at a very irregular rate and will survive when the conditions are right. They usually struggle to compete when it comes to colonising certain parts of the body due to the presence of resident bacteria. However, if there are enough numbers or an opportunity for transient bacteria (such as an open wound or a weakened immune system) then they can quickly affect our bodies and cause an infection.

As mentioned, resident and transient bacteria aren’t inherently pathogenic, so what leads them to cause an infection?

The Staphylococcus genus is one of the most common resident bacteria in our bodies. However, the Staphylococcus aureus bacteria is more pathogenic than Staphylococcus epidermidis. This is because Staphylococcus aureus is an opportunistic pathogen with a higher virulence factor. Opportunistic pathogens take advantage of various conditions in order to infect the host. This can include a weakened immune system, an altered microbiome, or penetrating trauma such as a cut.

The Staphylococcus aureus bacterium is considered a transient bacteria of the skin and a common cause of skin infections such as abscesses. However, it can also cause respiratory infections such as sinusitis and even food poisoning. It also gave rise to MRSA (Methicillin-resistant-Staphylococcus-aureus) which is typically spread via hospitals and has additional virulence factors that produce potent protein toxins.

Despite the additional virulence factors that could cause Staphylococcus aureus to become pathogenic, a healthy host will have a much easier time fighting against it. Unfortunately, there are no vaccines for Staphylococcus aureus.

Other common examples of opportunistic infections include:

• Clostridioides difficile, a species of bacteria that can cause gastrointestinal infection.
• Streptococcus pneumoniae, a bacterium that causes respiratory infections.
• Streptococcus pyogenes (also known as group A Streptococcus), a bacterium that can cause a variety of pathologies such as impetigo and strep throat. It can also lead to more serious illnesses.
• Legionella pneumophila, a bacterium that causes a respiratory infection known as Legionnaire’s disease.
• Pseudomonas aeruginosa, a genus of bacteria that can cause folliculitis.

Most bacteria do not cause harm to the host, even if they are known to be more pathogenic. In most cases, a healthy body will do just fine repelling any attempts at colonisation thanks to resident bacteria that have much greater numbers and the immune system. In fact, many bacteria can live on your skin without negatively affecting it and will provide a variety of benefits.

A great example of this is the Cutibacterium genus. This group of bacteria is known for its unique metabolism. It’s able to synthesize propionic acid and they usually live in and around areas of the body with sebaceous glands and sweat glands. They essentially feed on sebum, an oily substance that is produced by the body’s sebaceous glands to lubricate our skin and hair.

If too much sebum is produced, it can lead to blockages in your pores which triggers inflammation, resulting in acne. Cutibacterium helps to regulate the levels of sebum on our skin to ensure that this doesn’t happen. However, it will still colonise the skin if conditions are right as it’s still an opportunistic bacteria. Too much sebum may also lead to excess growth of Cutibacterium acnes. As such, it’s important for your body to maintain a healthy mix of bacteria to ensure your skin stays healthy.