The skin is the main physical barrier that keeps the delicate inner tissues and organs separate from the environment which is often teeming with bugs and germs. Despite its protective ability, the skin itself can become diseased by these organisms. Some microbes live on the skin and play an important role in human health. It does not pose any threat. However, when the skin is damaged or diseased, this very same naturally occurring bacteria can pose a problem. Here are some of the bugs that can infest and infect human skin.
Natural Skin Microbes
A number of different bacterial and fungal species live on human skin and do not pose any threat to healthy skin. It is known as the normal skin flora. There is some variation among people as to the number of actual bacteria and fungi on the skin. This means that some people tend to have more microbes on the skin while others may have less. Personal hygiene and environmental factors play the main roles in determining the species and quantity of bacteria and fungi on the skin.
The normal skin flora can vary from the skin at one part of the body to another. It is important to note that almost all of the normal skin flora can cause disease. Some have to be ‘transformed’ into disease-causing (pathogenic) microbes. Others have the ability to cause disease but will only do so once the skin is compromised. Another important factor is the location. The normal skin flora can cause disease if it is elsewhere in the body other than the skin.
In terms of bacteria, staphylococci and corynebacteria are predominant on human skin. With fungi, it is the Malassezia (Pityrosporum) and Candida yeasts that are often found on healthy human skin.
Skin Infections and Infestations
The outer layers of the skin are made up of keratin, a tough and waterproof protein. It forms the first line of defense. Some microorganisms can consume this protein and generally remain as a superficial infection. Others can only cause an infection once there is a break in the outer layer of the skin, and can even spread throughout the body The skin can be infected by viruses, fungi and bacteria while parasites like lice and mites infest the skin. Many of the bugs and germs that infect or infest the skin may also affect the hair and nails.
There are a number of different bacteria that can infect human skin. Usually the bacteria cannot infect the outer layers of healthy undamaged skin. However, when there is break in the skin, an infection can set in quickly. There are two types of skin-related infections referred to as impetigo and cellulitis.
- Impetigo is a contagious skin infection which is usually limited to the skin and does not typically extend into deeper tissue. The lesions starts as small red spots which become blisters and tends to ooze.
- Cellulitis is a deeper infection of the skin and tissue underneath the skin (subcutaneous tissue). It is a serious infection because it can easily spread throughout the body with life-threatening consequences.
Although many different types of bacteria can cause impetigo and cellulitis, staphylococcal and streptococcal infections are the most common. These days drug resistant strains like MRSA (methicillin-resistant Staphylococcus aureus) pose a serious threat as it is difficult to treat.
Another type of infection is necrotizing fasciitis infects the connective tissue below the skin (fascia). It can spread rapidly through the skin, fat underneath the skin (subcutaneous fat) and muscles.
The yeasts that naturally occur on human skin usually cannot cause an infection unless there is a break in the skin. Even in these instances, an yeast infection may not always occur. Most fungal infections of the skin are due to dermatophytes. These fungi are not yeasts. It causes a typical ring-like rash and is often referred to as a ringworm infection.
Dermatophytes have the ability to digest keratin and tend to cause superficial infections of the skin, meaning that it is limited to the outermost layers. These fungi can be spread from one person to another, from animals to humans (zoonotic) or acquired from the soil (geophilic).
- Candidiasis refers to yeast infections while dermatophytosis refers to infections with dermatophytes.
- Tinea pedis and tinea cruris are among the two most common fungal skin infections caused by dermatophytes which affect the feet (pedis) and groin (cruris) respectively.
- The other sites are referred to as:
– Tinea capitis: scalp
– Tinea faciei: face
– Tinea manuum: hand
– Tinea unguium: nail
– Tinea barbae: mustache and beard area
– Tinea corporis: body
- Tinea versicolor is an yeast infection and not a dermatophyte infection as is often thought. It tends to affect the neck, back, chest and arms.
Overgrowth of Malassezia fungi which naturally occur on the skin or an immune reaction to it may also cause certain skin diseases although it does not infect the skin.
As with bacteria and fungi, there are many types of viruses that can cause skin infections. Three types of viral skin infections that are relatively common include:
- Warts are small rough outgrowths of the skin caused by the human papilloma virus (HPV). Common warts on the skin are caused by one type of HPV and there are about 100 types of this virus.
- Shingles is caused by the reactivation of the chickenpox virus (varicella zoster). It often affects the skin in segments (dermatomes) according to the distribution of nerves.
- Herpes simplex is one of the common viruses to cause skin infections. There are two types – herpes simplex virus type 1 (HSV-1) which is mainly responsible for cold sores and herpes simplex virus type 2 (HSV) which mainly causes genital herpes.
There are many other types of viral skin infections. Some viruses cause systemic infections or infect certain organs and may present with a skin rash. This is often not a viral skin infection but a symptom of the underlying viral disease.
Several bugs can infest the skin but the two most significant of these skin infestations are body lice and scabies. These bugs are essentially insects and can cause bacterial infections of the skin once it infests it.
- Body lice are a parasitic insect known as Pediculus humanus corporis. It is similar to head lice and pubic lice (crabs) although all three types of lice are different. Body lice is often related to poverty and poor hygiene.
- Scabies is an infestation of the skin caused by a tiny mite, Sarcoptes scabiei. The mite burrows into the skin where it lays its eggs. There are different scabies mites that infest different animals. The human mite easily spreads from one person to another.
Here is a list of body parasites which may infest the skin or live in the human habitat to feed on skin and blood.