Microbial Diseases of the Skin

The Skin

•          The largest organ

•          Represents the boundary between the organism and the environment

•          Sometimes shows symptoms of illness not associated with skin (rash

Structure and Function of the Skin

•          Epidermis

–        Thin outer part, composed of layers of dead epithelial cells – stratum

–        Replaced every 25-45 days

–        Physical barrier against microbes

•          Dermis

–        Connective tissue; hair follicles, sweat glands, oil glands, neurons

•          Sebaceus glands – softens and lubricate the skin

Normal Microbiota of the Skin

•          Normally harmless, able to survive on the skin resistant to drying and high concentration of salt

–        These microbes cannot be completely removed through cleansing

–        Those in hair follicles and sweat glands reestablish rapidly – responsible for body odor

–        May be opportunistic pathogens

Representatives:

•          Gr+ cocci (Staphlycococcus, Micrococcus)

•          Gr + pleomorphic rods – diphteroids; Propionibacterium acnes – anaerobic bacteria, inhabit hair follicles, produce propionic acid  (maintains acidity of the skin pH 3-5)

•          The yeast, Malassezia – cause skin scaling dandruff

Acne

•          The most common skin disease caused by Propionibacterium acnes

•          Normally, dead cells and sebum are discharged through pores to skin surface

•          Overproduction of sebum leads to clogging the pores

•          Propionobacterium acnes digests the oily material – fatty acids are produced

•          Bacterial antigens + fatty acids causes local inflammation

Bacterial Infections

Staphylococcal skin infection

•          Staphylococci – spherical Gr+ bacteria; form irregular clusters

•          Two groups:      

–        coagulase “+” strains

–        coagulase “–“ strains

•          Coagulase positive strains coagulate (clot) fibrin in blood.

•          Coagulase negative strains make up 90% of normal microbiota. Pathogenic only when the skin is broken. (Staphylococcus epidermidis)

•          Staphylococcus aureus – the most pathogenic are coagulase positive (blood clot protects them against phagocytosis).

•          Found in hospital environments, resistant to antibiotics, difficult to treat.

•          Causes infection of hair follicles

–        Folliculitis – mild superficial inflammation of hair follicles

–        Furuncle - more serious folliculatis

–        Carbuncle – extensive damage of neighboring tissue

Scalded skin syndrome

·         Peeling off the skin - skin infection of newborns and children under age 2.

•          Caused by staphylococci that were lysogenized by certain phages

•          Toxins (A & B) are responsible for damage

•          The patient is vulnerable to secondary infection

Streptococcal skin infections

•          Spherical, Gr+ bacteria forming chains

•         

•          There are three groups of streptococci:

–        b-hemolytic (hemolysins lyse red blood cells and other cells)

–        a-hemolytic (partial-hemolysis)

–        g-hemolytic (non-hemolytic)

The most important Streptococcus belongs to group A (also called Streptococcus pyogenes

Impetigo

•          Caused either by Staphylococcus aureus or Streptococcus pyogenes

•          Skin lesions - small patches on the face and limbs

–        Develops into pus-filled vesicles that can itch intensely

–        Spread by contact – highly contagious

•          Virulence factor (S. aureus): exotoxins A and B – dissolve the cell cement

•          Treatment – antibiotics – protein inhibitors (cephalexin)

Streptococcus pyogenes

•          Sometimes the skin infection can spread to deeper tissue – Necrotizing fasciitis - “Flesh-eating bacteria”

•          Rapid destruction of tissue 

•          Hyaluronidase – dissolves the connective tissue

•          Exotoxin A – acts as superantigen - causes a massive immune response that is useless- immune system to contribute to damage

•          15,000 cases in US each year

Gangrene

•          Caused by anaerobic spore-forming bacterium Clostridium perfrigens

•          Occurs when the blood supply to the tissue is interrupted

•          The dying tissue provides anaerobic conditions for the growth of Clostridium

•          Toxin produced by the bacterium causes necrosis of neighboring tissue (muscles)

•          Gas is also produced that swells the tissue

•          Treatment: Penicillin, surgical removal of necrotic tissue, hyperbaric chamber

Infections by Pseudomonads

•          Aerobic, Gr – bacteria with polar flagella

•          Found in water and soil

•          Representative – Pseudomonas aeruginosa –opportunistic pathogen

•          Causes dermatitis – rash, associated with swimming in pools

•          P. aeruginosa is a common pathogen in burn patients

•          Treatment: antipseudomonal b-lactame antibiotics

Viral Diseases of the Skin

Warts

•          Caused by human papillomavirus - HPV (80 types)

•          Common in children

•          In 70% of cases disappear on their own

•          Transmitted by

–        contact 

–        sexual contact - genital warts

–        respiratory routs

•          Treatment by

–        liquid nitrogen, electrical current, burning with acids, laser treatment

Smallpox (Variola)

•          Caused by orthopoxvirus

•          Two forms:

–        Variola major - mortality 20%

–        Variola minor – mortality less than 1%

•          Transmitted by the respiratory rout and contact

•          Symptoms: skin lesions; postules after 10 days

•          The first vaccine – the one against smallpox

•          Incubation period: 10-20 days

•          Treatment: Immediate vaccination

•          The first disease that was eradicated

•          Prevention: Vaccination discontinued in 1972

Chickenpox (Varicella)

•          A mild childhood disease. Mortality rate is very low

•          The disease is caused by herpesviruse varicella-zoster (human herpesvirus 3)

•          Acquired through respiratory droplets or fluid of active skin lesions

•          Skin lesions manifest the disease

•          Virus moves from the skin to nerve cell near the spine

•          There it remains latent

•          Self-limiting

•          What enables the virus to remain latent?

•          The virus persist inside the nerve cell as viral DNA

•          Not recognized by the immune system

•          Later the virus may be reactivated (by stress) when it moves along the nerve to skin area

•          It causes a new outbreak – shingles (herpes zoster)

Herpes simplex

Two groups: HSV-1 and HSV-2

•          Herpes simplex virus type 1

•          Infection is manifested with lesions in the oral mucous membrane – “cold sores” or “fever blisters”

•          90% of population infected – many subclinical

Herpes simplex type 1

•          The virus is latent in the nerve that communicates between face and central nerve system (trigeminal nerve)

•          Sunburn, emotional stress and hormonal changes can cause reappearance of the infection

Herpes simplex virus type 2

•          Genital herpes

•          Can be differentiated from HSV-1 by its antigenic makeup

•          Transmitted by sexual contact

•          Latent in the sacral nerve ganglia

Measles (Rubeola)

•          Acquired by the respiratory rout

•          Symptoms similar to common cold, rash on the face and trunk, lesions in the mouth    

•          Complications - pneumonia, encephalitis

•          The law requires vaccination for school children – not earlier

•          Dangerous disease for very young and very old people

Rubella (German measles)

•          A mild viral disease

•          Manifestation - light fever, small red spots

•          If contracted by pregnant woman a fetal damage is possible (deafness, heart defects, mental retardation)

•          Rubella is prevented by effective vaccine (not recommended for pregnant women)

Fungal Diseases of the Skin

•          Dermatomycoses – fungal infection of skin, nails, hair – ringworm

•          Fungus can digest keratin (fibrous protein found in hair and nails)

•          Transmitted by contact with fomites

–        Ringworm of the groin – tinea cruris

–        Ringworm of the feed – tinea pedis – athlete’s foot

–        Ringworm of the scalp -Tinea capitis 

•          C Infection caused by the yeast Candida albicans

•          C. albicans can overgrow the mucosal tissue when the normal microflora is suppressed (antibiotic treatment)

•          In newborn infants – whitish overgrowth in the mouth cavity

•          Treatment: miconazole, nistatin

Candidiasis

•          Infection caused by the yeast Candida albicans

•          C. albicans can overgrow the mucosal tissue when the normal microflora is suppressed (antibiotic treatment)

•          In newborn infants – whitish overgrowth in the mouth cavity

•          Treatment: miconazole, nistatin

Microbial Diseases of the Eye

•          Conjunctivitis (red eye)– inflammation of conjuctiva (mucous membrane that lines the eyelids and covers the eye ball)

•          Can be caused by bacteria or viruses

•          Bacterial conjugtivitis is caused by Haemophilus influenzae and Pseudomonas

•          Source of infection – skin, upper respiratory tract and contact lenses

Neonatal gonorrheal ophtalmia

•          Conjunctivitis caused by Neisseria gonorrheae

•          Contracted as the infant passes through the birth canal

•          Newborns used to be treated with silver nitrate

Trachoma

•          Infectious disease caused by Chlamidia trachomatis (intracellular parasite)

•          The most often cause of blindness

•          C. trachimatis causes conjugtivitis – thickening of the upper eyelid

•          Mechanical damage of cornea caused by inturning of eyelashes

•          Treatment – tetracycline.

Acanthamoeba keratitis

•          A protozoa - causes inflammation, in later stage sever pain

•          Found in fresh water, tap water, hot tubs

•          Infections associated with contact lenses

•          Damaged cornea is susceptible to infection

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