Dermatological infections can be caused by a variety of organisms including bacteria, viruses, fungi, yeasts and parasites. Common presentations include cellulitis, impetigo, warts, herpes infections, ringworm, pityriasis versicolor and scabies. Identification of the infecting organism is important and can be done through skin scrapings, swabs or biopsies examined microscopically or cultured. Treatment involves antibiotics, antivirals or antifungals and may be topical or systemic depending on the organism and site of infection.

1. Dermatological infections
Dr Daniel Hewitt
Dermatologist
Skin and Cancer Foundation Westmead

2. Objectives
To understand the different types of
organisms that can cause skin pathology
To appreciate the common clinical
presentations
To understand the important investigations
To briefly appreciate the ways in which
infections and infestations are treated

3. Introduction
There are many types of micro-organisms
that exist on the skin and in the
environment. The normal bacteria and
yeasts on the skin are termed commensal
organisms.
Some organisms can cause skin pathology.
The infections or infestations of these
organisms have many different clinical
presentations.
Micro-organisms may also complicate pre-
existing skin diseases, such as dermatitis.

4. The most common organisms causing skin pathology are
Bacteria Gram positives eg Staphylococcus,
Streptococcus
Gram negatives eg Pseudomonas
Viruses eg Herpes viruses
Fungi eg Dermatophyte organisms
Yeasts eg Candida, Malassezia
Parasites eg Lice, scabies

5. Investigations
In all infections, identification of the organism provides confirmation of a
diagnosis. There are many ways to do this.
Wood’s light – a UV light that causes certain organisms to fluoresce.
eg erythrasma shows “coral pink” fluorescence and some fungal
infections of the scalp flouoresce green.
Scrapings from scaly areas and abnormal nails can be examined
microscopically at the bedside or sent to the laboratory to look for
yeasts or fungi.
Swabs can be examined for bacteria by microscopy and culture. Swabs
can also be examined by polymerase chain reaction (PCR) for
viruses such as herpes virus.
Fresh tissue samples can also be sent for these investigations. Less
common organisms such as mycobacteria need special growth
media and techniques so the laboratory must be informed of the full
range of organisms possibly present.

6. Bacterial skin swab
Method of skin scraping

7. Erythrasma and the coral pink
fluorescence seen under
Wood’s light

8. Bacterial infections
Cellulitis
This is infection of the skin and cutaneous connective
tissue.
Tender, warm, poorly defined erythema is seen. There may
have been a portal of entry eg a cut or break in the skin.
Patients may be unwell with fever and raised inflammatory
markers.
The abdomen and lower legs are common sites.
There are many possible causative organisms, but
Streptococcus and Staphylococcus are most common

9. Swollen erythema characteristic
of cellulitis

10. Cellulitis

11. Erysipelas
This is more superficial than cellulitis, causing well defined
hot erythema.
The face and lower leg are common sites.
Streptococcus pyogenes is the most common cause.

12. Superficial, blistering erythema
characteristic of erysipelas

13. Impetigo
This is a superficial infection of the skin. There is a non-
bullous form with yellow crusty erytematous patches and
a bullous form with erythema and bullae (blisters.)
It is more common in children. The face and limbs are the
most common sites.
Staphylococcus aureus is the most common cause of
bullous impetigo. Streptococcus pyogenes and
staphylococcus aureus can cause non-bullous impetigo.

14. Classic examples of bullous and non-bullous impetigo

15. Folliculitis
This is inflammation of the follicular unit. It may be due to
Gram positive or Gram negative bacteria, yeasts and
fungi or non-infective.
Staphylococcus aurues and Propionibacterium acnes are
the most common organisms grown on swabs.
More severe inflammatory nodules may develop – termed
furuncles (or boils.) Several of these can coalesce to
form a carbuncle.

16. Folliculitis

17. Virus infections
Warts
These are caused by the human papilloma virus. There are
many different types, such as common, plane and
genital warts.
They are common in children, but can occur at all ages.
Treatments are numerous but generally involve either
chemically or physically “burning” the wart or applying a
cream (eg imiquimod) to stimulate the patient’s immune
system to react against the virus and clear the wart.
Treatment is frequently not required, as warts do eventually
self-resolve.

18. Common warts and plantar warts

19. Molluscum contagiosum
This is a very common pox virus infection causing
umbilicated, flesh coloured papules, most common in
infants. They can be spread by close contact but are not
highly contagious.
Ocassionally they itch, especially in children with eczema.
Uncontrolled eczema can also contribute to the lesions
spreading.
Treatment is not usually required as they self-resolve and
cause no medical concerns.
In adults they can be a sign of immunodeficiency eg AIDS.

20. Molluscum contagiosum

21. Herpes viruses
There may be itching and pain prior to a vesicle or erosion
forming.
The virus resides in the nerves and can re-activate to give
the cutaneous changes. Crops of punched out ulcers are
chracteristic.
On the lips this change is commonly known as a “coldsore,”
on the fingers it is termed a “herpetic whitlow.”
Type I herpes simplex virus is most often associated with
facial and finger lesions and Type II is almost exclusively
associated with genital lesions.
Herpes zoster virus, the cause of chickenpox, can re-
activate to cause painful erythematous papules which
develop into vesicles and then dried crusts. This eruption
is usually dermatomal, most commonly in the thoracic
area. It is also known as shingles.

22. Herpes simplex Type I infections

23. Genital herpes infections

24. Herpes zoster infection
- chickenpox

25. Herpes zoster infection
- shingles

26. Many other viruses can cause skin changes.
An exanthem is a fever characterised by a skin eruption.
These are caused by virsues and usually have
distinguishing features. Some of these are now much
less common due to immunization. Examples inlcude
Measles
Rubella
Pityriasis rosea
Erythema infectiosum
Roseola infantum
Gianotti-Crosti syndrome
Hand, foot and mouth disease

27. Measles

28. Rubella

29. Pityriasis rosea

30. Erythema infectiosum

31. Hand, foot and mouth disease

32. Gianotti Crosti Syndrome

33. Fungal infections
Fungal infections are common and easily missed. They
should be considered in any scaling or itchy rash where
no other reason (eg dermatitis) is identified.
Skin scrapings should be taken from such lesions and sent
for fungal microscopy and culture. A blade is scraped
across the skin and skin scales collected – this is a very
simple but important test that is under-utilised.
If the nails are involved, clippings of these should also be
sent.

34. Fungi can infect any part of the skin, the hair and the nails.
Infections of the body, commonly known as “ringworm” are
not worm infections and more precisely known as Tinea.
The location is specified in the diagnosis.
eg Tinea capitis Scalp infection
Tinea corporis Body infection
TInea pedis Foot infection
Tinea manuum Hand infection
Tinea of the nails is known as onychomycosis

35. Infections can be acquired from pets, from the environment
or from other people.
The most characterisitic feature is a annular plaque with a
raised, red border and a trailing scale. Itch is usual.
Sometimes, especially if there has been treatment with
topical steroids, the appearance is much less specific.
In the scalp there is usually less characteristic scaling and
erythema.
In the nails there is often thickening and crumbling of the
nail with scaling of the nail bed –
termed “subungual hyperkeratosis.”

36. Tinea corporis

37. Tinea corporis Tinea manuum

38. Tinea pedis

39. Onychomycosis

40. Treatment involves either topical or systemic antifungals.
Skin infections are often cleared over four weeks with
topical terbinafine.
Systemic treatment is generally required for up to six
months to clear onychomycosis and for six weeks for
tinea capitis.

41. Yeast infections
Pityriasis versicolor
This is a very common rash due to overgrowth of a yeast –
Malassezia furfur.
It is most common in young adult males and usually arises
in hotter weather, especially after the patient has been
sweating.
Brown or light reg scaly macules develop on the trunk. It
darker skins, hypopigmented macules develop.
It may be slightly itchy.
There are many effective treatments such as pevaryl
foaming lotion (econazole) or selsun shampoo. The
pigmentary changes can last for several months even
when the active yeast proliferation is effecetively treated.

42. Pityriasis versicolor

43. Pityriasis versicolor

44. Candida infections
Candida commonly infects intertriginous areas (areas
where the skin is moist and rubbing together) such as
the groin and under the breasts.
There is weeping erythema, and often separate, peripheral
erythematous “satellite” pustules.
It may be a superimposed factor in other causes of
intertrigo eg dermatitis or seborrheic dermatitis.
Candida can also infect the nails, but is most frequently
isolated in the nails as a commensal organism.

45. Cutaneous candidiasis

46. Parasite infestations
The two most commonly seen are lice and
scabies

47. Pediculosis (Lice)
These are wingless insects that infect the head, the body or
the pubic area.
Head lice are very common and transmitted via
combs, brushes, hats and close contact. Lice feed on the
scalp and can produce irritation and itch.
Over the counter treatments are effective but are ideally
combined with frequent combing to remove the lice eggs
(“nits”) and may need to be repeated.

48. A head louse and the eggs (nits)

49. Scabies
This produces burrows (5-10mm long ridges) and papules,
often around the hands, nipples and genitalia. The
diagnosis is easily missed as these signs can be very
subtle or absent.
Itching occurs two weeks after initial infestation. It is often
widespread and severe, especially at night. The scabies
mite requires a human host, so it can only be acquired
via contact with an infected person.
It is particularly prevalent in crowded communities such as
nursing homes and is endemic throughout many
Aboriginal communities.
All household members must be treated simultaneously to
prevent re-infection.

50. Scabies

51. Classic features of scabies - burrows

52. Scabies

53. Scabies

54. Conclusions
Infections of the skin are numerous and
varied, involving bacteria, viruses, fungi, yeasts
and parasites.
They can be easily missed if not considered, but
once diagnosed, treatment can be curative and
highly satisfying.