The most common pyogenic bacteria affecting skin, adnexa and subcutis are streptococci and staphylo- cocci. They are not visible in H&E-stained sections, but may be demonstrated with Gram stain (see Glos- sary).
12.1
Streptococcal Infections
Beta-hemolytic streptococci give rise to an acute, dif- fusely spreading infection associated with malaise and fever. Located in the dermis, the infection is called erysipelas, located in the subcutis cellulitis. Cellulitis may be extended erysipelas or originate in the sub- cutis. The histopathologic pattern is that of an acute inflammation with edema, dilated vessels and dense infiltrates of neutrophils.
However, β-hemolytic streptococci may also give rise to non-bullous impetigo (see below).
12.2
Staphylococcal Infections
Staphylococcus aureus causes furuncles and carbuncles (i.e., follicular and perifollicular abscesses of variable extension) and impetigo. There are two types of impe- tigo, non-bullous and bullous.
12.2.1
Non-Bullous Impetigo
Non-bullous impetigo is the most common form. The cause is either infection by Staphylococcus aureus only, or by Staphylococcus aureus together with a β-hemo- lytic streptococcus, or by a β-hemolytic streptococ- cus only. Children of all ages are the most commonly affected. The bacteria gain entry through slightly damaged skin; predilection sites are the face and ex- tremities. The lesions start as vesiculopustules, which rapidly transform into honey-colored crusts.
Histologically there are mainly unilocular pustules
in the superficial part of the epidermis, which at first are covered by stratum corneum and later by crusts, composed of fibrin and neutrophils.
12.2.2
Bullous Impetigo
Bullous impetigo is caused by Staphylococcus aureus.
It affects newborns and older infants, but is uncom- mon in adolescents and adults. The initial lesion is a clear vesicle that develops into a flaccid bulla, which bursts and gives rise to a honey-colored crust.
Histologic investigation shows a wide subcorneal vesicle. Thus the roof consists of the stratum corneum, to which a few cells from the stratum granulosum may be attached. The vesicle contains a small or moderate number of neutrophils and a few desquamated or ac- antholytic cells (see Glossary); it may even be empty or nearly empty, probably due to loss of the contents dur- ing processing. The floor includes the rest of the epi- dermis, which is edematous, but has a rather smooth surface. In the dermal papillae and upper dermis there are edema and inflammatory cell infiltrates.
12.3
Anonymous Bacterial Infections
Routine cultures for pathogenic bacteria are not al- ways helpful, as demonstrated in Case 2, in which re- peated cultures were negative.
12.4 Examples
Case 1. Bullous Impetigo
A 17-year-old boy without previous skin problems had had an eruption of blisters on the left buttock over a period of 3 weeks. He presented with small fresh ves- icles, pustules, and crusts on a slightly erythematous area twice the size of a palm. There were also small
Infections Caused
by Common Pyogenic Bacteria
12
Chapter 12
12 Infections Caused by Common Pyogenic Bacteria 84
Fig. 12.1 Bullous impetigo. a The micrograph shows a part of a wide subcorneal bulla, which contains a sparse number of inflammatory cells. The underlying part of the epidermis is edematous, is slightly penetrated by inflammatory cells, and has
several longitudinal clefts. b Close-up of the content of the bulla displays aggregates of neutrophils. Note the segmented nuclei, cut at different levels. H&E
Fig. 12.2 Anonymous, chronic, purulent infection. a The horny layer is markedly thickened and infiltrated by neutrophils, which at the outermost part form a pustule (arrowheads). In the papillary dermis there is a dense mixed cell infiltrate (ar- row).b Close-up of the dermal cell infiltrate shows a mixture
of lymphocytes, plasma cells, histiocytes and neutrophils with a segmented nucleus. The arrows indicate two newly formed ves- sels. c A vein at the subcutaneous border, the intima of which is partly thickened and infiltrated by lymphocytes. H&E
85
lesions on the right thigh, and the right cheek and ear. Suggested diagnoses were pityriasis lichenoides et varioliformis acuta and atypical pyoderma.
Histologic investigation showed a wide subcorneal vesicle, which contained a small number of neutrophils and scattered desquamated keratinocytes. The floor of the vesicle consisted of an acanthotic and edematous epidermis with a rather smooth surface, and showed small vertical clefts. In the papillae and upper dermis there were moderately dense perivascular infiltrates of lymphocytes.
Culture yielded massive growth of Staphylococcus aureus. After antibiotic treatment the lesions healed (Fig. 12.1).
Case 2. Anonymous, Chronic, Purulent Infection A 61-year-old man consulted for an erythematous and
scaling lesion, 15 mm in diameter, located on the tip of the nose and present for a year. Investigations in- cluding cultures for bacteria and fungi and serologic tests for syphilis were negative. Two biopsy specimens were taken at an interval of 2 months. One year later (i.e., two years after the onset of the lesion) it remained unchanged and resistant to treatment.
The two biopsy specimens were reexamined. They had a similar pattern. The epidermis was thickened, spongiotic and sparsely infiltrated by neutrophils. The conspicuously thickened horny layer was permeated with neutrophils, which formed superficial pustules.
In the dermis there was a confluent and very dense cell infiltrate, which mainly consisted of plasma cells and lymphocytes, but also contained many neutro- phils and scattered eosinophils. Newly formed vessels were prominent and in one of the specimens a single thick-walled vein with a subendothelial infiltrate of lymphocytes was observed at the dermal–subcutane- ous interface. Hair follicles were not involved. Fungal structures and bacteria were not found (Fig. 12.2).
Because of the dense mixed inflammatory cell infil- trate in the dermis combined with superficial pustules, some kind of bacterial infection was suspected in spite of repeated negative cultures. A broad-spectrum anti- biotic was tried and the lesion healed.
12.4.1 Comment
The clinical appearance as well as histopathologic pat- tern and course in Case 2 have similarities to so-called
blastomycosis-like pyoderma described by Su et al.
(1979). The observed patients had had purulent, ver- rucous lesions on an extremity or in the head and neck area for a long time, which were resistant to treatment and histologically showed epithelial hyperplasia and purulent inflammation. Investigation excluded fun- gal and mycobacterial infections. However, culture was positive for at least one of the pathogenic bacteria Staphylococcus aureus, β-hemolytic streptococci, and Pseudomonas aeruginosa. Following treatment with systemic antibiotics and a topical wet antibacterial dressing the lesions rapidly healed.
12.5
Differential Diagnosis
Biopsies are rarely taken from lesions caused by pyo- genic bacteria because of the well-known clinical pat- terns of these diseases, which are usually confirmed by successful treatment with antibiotics and/or by culture on material taken from the infected skin. Occasionally an atypical clinical appearance or failure in treatment leads to a biopsy.
• Folliculitis and perifolliculitis caused by fungal and mycobacterial infections may clinically be misin- terpreted as furuncles or carbuncles and may also be missed histologically if not in the mind of the pathologist (Figs. 13.4c, 13.6c and 15.3a).
• Intraepidermal IgA pustulosis. As in bullous impe- tigo and other bacterial pustules, there may be sub- corneal pustulosis and also some acantholytic cells.
Culture from intact vesicles and pustules is decisive (Sect. 25.2).
• Pemphigus foliaceus shows a discreet superficial acantholysis without neutrophils or other inflam- matory cells and very little inflammatory response in the upper dermis (Sect. 25.1.2).
• Staphylococcal scalded-skin syndrome (i.e., wide- spread bullae and exfoliation), seen in newborns and small children, is caused by exotoxins pro- duced by staphylococci in an infectious focus lo- cated somewhere else in the body. Intact lesions are sterile.
Reference
Su WPD, Duncan SC, Perry HO (1979) Blastomycosis-like pyoderma. Arch Dermatol 115:170–173
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Reference
