Skin and Soft Tissue Infections

We're already up to the infodumping stage of this unit! Yay!

Describe structure of skin

Skin, as you should hopefully know, is made up of several layers. The epidermis (outer layer) is derived from the ectoderm, and the dermis below is derived from the dermatomes of paraxial mesoderm. The dermis contains other structures, such as hair follicles and sweat glands, which, if I remember correctly, are epidermal derivatives. Below the dermis is the hypodermis, or fatty layer, which is actually considered to be soft tissue (rather than skin). Beneath all of this are muscles, surrounded by fascia.

The skin is a good barrier to infection, not only because it's a physical barrier, but also because it is covered in a lot of commensal flora that protect us (see this post). Most of the commensal bacteria on our skin are gram-positive, as the fatty acids of our skin tend to inhibit the growth of gram-negatives.

A note here about rashes (since I don't know where else to put this): rashes are not necessarily indicative of a skin infection. Some rashes are a symptom of a systemic disease (i.e. disease that affects the whole body). If the rash has eruptions, the skin is infectious; if the rash doesn't have eruptions, the skin is not infectious.

Outline major infections of the skin and soft tissue
Describe aetiological agents – bacteria, fungi, viruses
Explain virulence factors contributing to pathogenicity 

A lot of diseases are named after the condition, not the pathogen that causes them. Therefore, a pathogen can cause more than one disease, and a disease can be caused by more than one pathogen. In this post, I will discuss one pathogen at a time. Pathogens present in the game Microbe Invader (http://microbeinvader.com/) are written in red. (Microbe Invader's a great game, by the way. You should definitely play it :D)

Gram-positive bacteria

Staphylococcus spp (e.g. S. aureus)

(Note: spp is a microbiology abbreviation for "species.")

Staph species can cause a lot of suppurative, or pus-forming infections of the skin. If they manage to get through to the blood, they can cause more serious diseases, such as endocarditis or osteomyelitis. Staph species are very common in hospitals, where antibiotic resistant strains such as MRSA (methicillin-resistant S. aureus) present a major challenge. Most of the time, Staph infections are only a problem if your skin is broken, but more virulent strains can hurt you even if your skin is perfectly intact.

Staph can cause a range of skin problems, from impetigo ("school sores"- only affect the epidermis), ecthyma (affects the epidermis and dermis), cellulitis, folliculitis (inflammation of hair follicles), boils, scalded skin syndrome, and wound infections. Boils occur when sebaceous glands and hair follicles become infected, causing swelling under the skin. If boils join up, they produce carbuncles.

S. aureus' major virulence factors are toxins. Exfoliative toxins A and B (ETA and ETB) cause scalded skin syndrome (where your skin peels off). ETA and ETB are serine proteases which cleave desmosomal cadherins in the superficial layers of the skin, causing those layers to peel off.

Streptococcus pyrogenes

S. pyogenes, also known as Group A Strep, can cause skin infections (duh, otherwise I wouldn't be talking about them in this post), as well as pharyngitis (a.k.a. strep throat), and more severe diseases such as pneumonia and scarlet fever. S. pyogenes can also cause immune-mediated post-infection sequelae such as APSGN (Acute Post-Streptococcal GlomeruloNephritis) and rheumatic fever.

Skin infections caused by S. pyogenes include impetigo, ecthyma, erysipelas (only affects the dermis), cellulitis, necrotising fasciitis ("flesh-eating bacteria"), and wound infection. Necrotising fasciitis, which develops after minor trauma, is particularly nasty (though thankfully rare). It is associated with a highly virulent serotype called M1. M1 produces Streptococcal Pyrogenic Exotoxin A (SPE A) and a cysteine protease, which may contribute to the disease.

S. pyogenes' favourite battle strategy is colonisation. It has a polysaccharide capsule, which protects it from the immune system, cell wall lipoteichoic acid, which mediates attachments, and a bunch of surface proteins, mainly adhesins, which help in various other functions. These other functions include binding to fibronectin and degrading fibrin.

Clostridium perfringens

C. perfringens is one of several species that cause gas gangrene in the skin. It can also cause uterine gangrene. C. perfringens is a strict anaerobe (i.e. oxygen kills it), so in order to get it, you usually need soil contamination of a deep wound and some decrease in oxygen supply. Once it's in, though, its toxins can spread to adjacent tissues and put a swift end to you.

Propionibacterium acnes

P. acnes is a gram-positive rod that has a low virulence. It is associated with acne, though it is possible to get acne without P. acnes.

Corynebacterium spp

Corynebacterium are also gram-positive. They are part of our commensal flora. Corynebacterium can cause pitted ketarolysis ("smelly feet" associated with wearing your boots for too long) and erythrasma (inflammation between skin folds).

Gram-negative bacteria

Pseudomonas

Now we finally get into the gram-negatives! (There aren't that many gram-negative bacteria that cause skin infections because, as I mentioned earlier, the fatty acids in our skin deter gram-negatives.) Pseudomonas can cause a rash, also known as "hot tub rash," as it is associated with, well, hot tubs that haven't been chlorinated enough.

Vibrio spp

Some Vibrio species, particularly Vibrio vulnificus, can cause necrotising fasciitis. This usually only happens if the patient has some underlying disease, such as chronic alcoholism. Vibrio's main virulence factors include pili, cytolysins, cytotoxins, and metalloprotease.

Bite wounds

Okay, I lied, I didn't sort all of them according to pathogen type. Bite wounds can be caused by many different pathogens, depending on what bit you. Also, saliva carries a lot of pathogens, so you're likely to be infected by several different things at once. Virulence factors vary depending on the pathogen(s) involved, but may include formation of biofilms, enzymes, and toxins. These factors may be synergistic- one pathogen might secrete something that helps another pathogen. Bite wounds are of a particular problem if a patient has an underlying medical condition that might impede treatment, such as diabetes.

Fungi

Dermatophytes

Dermatophytes are a group of fungi including Epidermophyton, Microsporum, and Trichophyton. They cause diseases that are generally limited to the stratum corneum and epidermis, such as tinea, nail infections, and ringworm. Factors affecting their pathogenicity include adherence, proteases, and the immune status of the host.

Candida spp

Candida is a type of yeast that can cause paronychia (nail bed infection) and intertrigo (infection of skin folds). The most common species involved in Candida infections is C. albicans. As mentioned here, it is usually only a problem if the host's flora or immune system are disrupted in some way. Virulence factors include extracellular enzymes, formation of germ tubes involved in tissue penetration, and adhesion.

Malassezia spp

Malassezia species are lipophilic yeasts that form part of our commensal flora. The infections that they cause tend to be quite mild, such as pityriasis versicolor (discolouration of the skin), dandruff, and folliculitis.

Viruses

Herpes Simplex Virus (HSV) 

HSV, particularly HSV1, causes cold sores. Around 90% of us have HSV1 antibodies from acquiring it in infancy or childhood. The virus continues to live on in our neuronal ganglia, and can be reactivated, giving some people repeated cold sores. HSV is pretty sneaky and can coat itself with immunoglobulin, allowing it to hide from the immune system.

Varicella Zoster Virus (VZV)

VZV is the virus that causes chicken pox. It can be spread by secretions and by direct contact. VZV not only exists in the skin, but can also be found in the blood (viremia). After the illness is over, VZV continues to live on in the neuronal ganglia. When the virus is reactivated, it causes shingles. There are now vaccines against VZV.

Molluscum Contagiosum Virus (MCV)

MCV is a DNA Poxvirus that affects only the stratum corneum. It causes raised lesions on the skin that go away on their own after a while (though this may take months to years).

Papillomaviruses

Papillomaviruses cause warts. Common warts are warts on the hands and knees, and plantar warts are on the soles of the feet. Like molluscum, they often resolve without treatment, but this may take a long time.