The process of infection involves the interaction between two organisms the host and the invader.
The clinical changes result from mechanisms involved in this process, notably the micro-organism, its virulence, and the patient’s immune defenses.
The lesions produced often have a well defined appearance, such as impetigo or tinea cruris, but the changes may be less specific.
Several features enable us to recognise that infection is a possible cause of the patient’s condition.
Acute bacterial infections generally produce some or all of the classical characteristics of acute inflammation.
Clinical presentation
The woman shown in the photographs had acute erysipelas due to streptococcal infection, and all four features of inflammation were present.
She was referred to the clinic with a diagnosis of an acute allergic response, which, from the appearance alone, was understandable.
However, malaise and fever were also present and the lesions were warm and tender.
The condition responded well to antibiotic treatment.
The point of entry in such cases is thought to be a small erosion on the face.
Erysipelas of the leg or foot may follow the development of a small fissure between the toes, but often there is no discernible portal of entry.
Erysipelas is the local manifestation of a Group A streptococcal infection, in the case illustrated the infection is confined to deep dermis as a form of cellulitis.
However the same organism at distal sites, through the production of toxins or superantigens, can cause other skin lesions such as:
(a) the rash of scarlet fever;
(b) erythema nodosum;
(c) guttate psoriasis; and
(d) an acute generalised vasculitis.
Other forms of local bacterial infection include impetigo, folliculitis, and furuncles (boils).
These conditions are caused by Staphylococcus aureus and in the case of folliculitis or boils the infection is associated with a local abscess.
Staph. aureus colonises the anterior nares or perineum of normal people; it also commonly colonises eczema and may cause an acute exacerbation of atopic dermatitis. Impetigo is a superficial infection of the skin of which there are two forms.
In the non-bullous form the affected skin is covered with crusts.
Both staphylococci and streptococci are responsible. However the bullous form which presents with blisters is due to staphylococci.
Folliculitis, an inflammation of the hair follicle, is commonly caused by Staph. aureus.
Infection of the scalp or beard hair (sycosis barbae) is uncommon but may become chronic.
Abscess formation around the hair follicles may result in furuncles or boils; where several furuncles coalesce the lesion is known as a carbuncle.
Ecthyma, which is most common on the leg, is due to bacterial infection penetrating through the epidermis to the dermis causing a necrotic lesion with a superficial crust and surrounding inflammation.
Both streptococci and staphylococci are responsible.
Mycobacterial disease
The clinical presentation of infections due to mycobacteria, a specific group of organisms that includes the causes of tuberculosis and leprosy, reflects the success of the host’s response in eradicating organisms.
There are clear differences, for instance, between disseminated miliary tuberculosis and lupus vulgaris or, for example, tuberculoid and lepromatous leprosy.
As these infections are not common only lupus vulgaris and non-tuberculous or “atypical” mycobacterial infection are described.
Tuberculous mycobacterial infections
Lupus vulgaris presents as a very slowly growing indolent plaque.
It usually represents a localised skin infection disseminated from a deep focus of infection.
Squamous carcinomas may develop in long standing cases.
Non-tuberculous mycobacterial infections
The most common is “fishtank” or “swimming pool” granuloma, acquired from tropical fish or rarely swimming pools, respectively, and caused by Mycobacterium marinum.
Nodular lesions develop slowly with ulceration and may spread along local lymphatics to give a chain of nodules (sporotrichoid spread). Injection abscesses may be caused by mycobacteria such as M. chelonei.
Buruli ulcer, an extensive ulcerating condition due to M. ulcerans, is confined to the tropics.
Other infections
Rochalimea infections include bacillary angiomatosis, which presents in AIDS patients with small haemangioma-like papules, and cat scratch disease where crusted nodules appear at the site of the scratch associated with the development of regional lymphadenopathy one or two months later.
A maculopapular eruption on the face and limbs or erythema multiforme may occur.
Psittacosis and ornithosis may be associated with a rash.
Rickettsial infections, including typhus, Rocky Mountain spotted fever, and rickettsial pox, are all associated with rashes, often purpuric.
Syphilis
There is a disseminated erythematous rash in secondary syphilis that is followed by a papulosquamous eruption, which affects the trunk, limbs, and mucous membranes.
The palms and soles may be involved. There are also small clustered mouth ulcers.
In patients with AIDS the rash of secondary syphilis is florid and often crusted or scaly.
