BIOS Instant Notes in Medical Microbiology

William Irving,Tim Boswell,Dlawer Ala'Aldeen

• Publisher: Taylor & Francis
• Year: 2004
• ISBN:
  9781135321314
• Edition: 1
• Topic: Medicina
• Subtopic: Microbiología y parasitología médicas

Instant Notes in Medical Microbiology covers medical microbiology from the molecular biology of infectious agents right through to the clinical management of the infected patient, including disease pathogenesis, diagnosis, and the use of antimicrobial therapy.

The first section covers how micro-organisms spread and cause disease in humans, and how the human body responds to infection in general. The next three sections give a broad outline of the important properties of human infectious pathogens; split into viruses, bacteria, and eukaryotic organisms. The final sections cover laboratory diagnosis, antimicrobial chemotherapy, prevention strategies, and infection from the point of view of the patient.

Section F – Clinical manifestation of infection

F1 Skin and Soft Tissue Infections

DOI: 10.4324/9780203488157-61

Key Notes

Normal skin flora The normal skin flora comprises permanent, temporary and transient residents. Permanent residents include coagulase-negative staphylococci (including Staph.epidermidis), diphtheroids, micrococci and propionibacteria. Temporary residents may reside for several days or weeks, including Staph. aureus. Transient residents such as coliforms or pseudomonads, particularly on the hands, are readily removed by hand washing.

Damaged skin Damaged or diseased areas of skin are more prone to colonization, and subsequent infection, with potentially pathogenic bacteria such as Staph. aureus, hemolytic streptococci, pseudomonads and anaerobes. Enterococci and coliforms are also commonly found on areas of broken skin, particularly on areas below the perineum.

Bacterial skin and soft tissue infections A wide variety of bacteria can cause skin and soft tissue infections, but Staph. aureus and Strep. pyogenes are the most common. Staph. aureus typically causes spots, boils, carbuncles, impetigo, cellulitis, or wound infections. Strep. pyogenes (group A β-hemolytic streptococcus) typically causes impetigo, erysipelas, cellulitis, wound infections or necrotizing fasciitis (rare). Staph. aureus is the commonest cause of skin and soft tissue infection in hospitalized patients, often with strains that are methicillin-resistant (MRSA). In addition to direct infection, the skin may be affected by bacterial toxins such as staphylococcal scalded shin syndrome toxin or streptococcal erythrogenic toxin (scarlet fever), β-lactam antibiotics are the mainstay of antibiotic treatment.

Nonbacterial infections of the skin Viral infections involving the skin include herpesviruses (cold sores, chickenpox), enteroviruses (rashes) and papillomaviruses (warts). Common fungal infections include ringworm (dermatophyte infections or ‘tinea’) and Candida (intertrigo). Skin infestations with ectoparasites include mites (scabies) and lice (headlice, pubic lice).

Related topics

Staphylococci (Cl)

Streptococci and enterococci (C2)

Normal skin flora

Normal skin has a number of bacteria that permanently colonize the surface, or are found in deeper skin layers or around sebaceous or sweat glands. These permanent residents comprise mainly coagulase-negative staphylococci (e.g. Staphylococcus epidermidis), micrococci, corynebacteria (diphtheroids) and propionibacteria. Anaerobic streptococci and Acinetobacter may also be found.

Temporary residents are bacteria that may colonize areas of skin for several days or weeks but are not present in all individuals. Staph. aureus is an important temporary resident. It is estimated that 25–30% of individuals carry Staph. aureus in the nose, or other moist body sites, at any one time.

Transient residents are bacteria that can colonize the skin (including hands) for up to several hours, but are usually readily removed by washing. These include Staph. aureus, coliforms, pseudomonads and enterococci. This is important from an infection control perspective within hospitals, and emphasizes the need for regular hand decontamination.

Damaged skin

Damaged areas of skin, such as ulcers, burns, wounds, or areas affected by disease (e.g. eczema, psoriasis), are prone to colonization with potentially more pathogenic bacteria than are found on normal skin. Potential pathogens such as Staph. aureus, hemolytic streptococci, Pseudomonas and anaerobes can found. Enterococci and coliforms are also commonly found on areas of broken skin, particularly on the lower part of the body and legs (areas below the perineum).

Colonization may then lead to infection as the normal defense mechanisms of intact skin are breached.

Bacterial skin and soft tissue infections

A wide variety of bacteria can cause infection of the skin. Some of the main pathogens are listed in Table 1 , but of these, Staph. aureus and Strep. pyogenes are the most common by far.

Infections due to Staph, aureus

There is a variety of skin and soft tissue infections caused by Staph. aureus (see Topic C1):

• spots, boils, carbuncles (common);
• impetigo (common in children);
• cellulitis;
• infections of traumatic wounds, surgical wounds, and ulcers;
• infections around cannula sites or drains;
• scalded skin syndrome (a staphylococcal toxin-mediated disease).

Staph. aureus is the commonest cause of skin and soft tissue infection, including surgical wounds, for patients in hospital. Many of these infections are now due to methicillin-resistant strains (MRSA). Staphylococci are capable of survival on surfaces and within dust, and can cause endemic problems and cross-infection in hospital wards.

Microbiological investigations should include culture of pus swabs, as well as blood cultures for more serious infections (e.g. cellulitis). The mainstay of antibiotic therapy is with flucloxacillin or related antibiotics. Abscesses or carbuncles require surgical drainage of pus. MRSA is resistant to flucloxacillin and other (β-lactam antibiotics, and often other classes of antibiotics such as macrolides and quinolones. Antibiotic treatment of MRSA infections should be guided by sensitivity results (e.g. vancomycin, tetracyclines, fusidic acid, rifampicin, trimethoprim). Linezolid is a new antibiotic with good MRSA activity.

Recurrent staphylococcal infections can occur in some patients, particularly those with nasal carriage, or patients with diabetes mellitus.

Infections due to Strep. pyogenes

Strep. pyogenes β-hemolytic streptococcus group A) also causes a variety of skin and soft tissue infections (see Topic C2):

Table 1. Some bacterial causes of skin/soft tissue infection

Classification	Bacteria	Infection/comment
Gram positive	Staph. aureus Strep. pyogenes Other haemolytic streptococci	The commonest bacterial pathogens of skin and soft tissue (see text) Similar infections to Strep. pyogenes
	Corynebacterium diphtheriae	Cutaneous diphtheria – a cause of skin ulcers in tropical countries
	Bacillus anthracis	Cutaneous anthrax
	Erysipelothrix rhusiopathiae	Erysipeloid in meat or fish handlers
Gram negative	Pseudomonas aeruginosa	Secondary infection of burns or other wounds; common colonizer of leg ulcers; black necrotic ulcers (ecthyma gangrenosum) is a complication of pseudomonas bacteremia in immunosuppressed patients
	Coliforms	Common colonizer of leg ulcers; may be involved in invasive soft tissue infection along with anaerobes (necrotizing fasciitis)
	Neisseria meningitidis	Meningococcal septicemia with characteristic purpuric rash
	Neisseria gonorrhoeae	Disseminated gonococcal infection can produce pustular lesions on the limbs and body
	Pasteurella multocida	Wound infection and cellulitis following animal bites (cats, dogs)
	Streptobacillus moniliformis	Disseminated pustular lesions can occur in ‘rat-bite fever’
Anaerobes	Bacteroides Fusobacterium Anaerobic streptococci	Necrotising fasciitis (Fournier’s gangrene, Meleney’s synergistic infection) in conjunction with other bacteria
	Clostridium spp	Gas gangrene, tetanus, wound botulism
Higher bacteria	Actinomyces israelii Nocardia asteroides	Soft tissue abscesses
Spirochaetes	Treponema pallidum Borrelia burgdorferi	Syphilitic ulcers, rash, and gummas Lyme disease – erythema migrans
Mycobacteria	Mycobacterium tuberculosis Atypical mycobacteria	Lupus vulgaris, cold abscesses Fish tank granuloma (M. marinum)

• impetigo;
• erysipelas;
• cellulitis;
• wound infections;
• necrotizing fasciitis;
• scarlet fever (a streptococcal toxin-mediated disease).

Microbiological investigations comprise culture of swabs, pus or tissue, and blood cultures for more serious infections (e.g cellulitis, necrotizing fasciitis). Antibiotic treatment is mainly with penicillins or other β-lactam antibiotic.

Impetigo

Impetigo is a superficial skin infection caused by both Staph. aureus and Strep. pyogenes, sometimes in combination. Vesicular lesions occur, which then crust over.

Cellulitis and erysipelas

These are more serious infections. Cellulitis can occur at any part of the body, but often the lower limb is affected in patients with leg edema. Bacteria can gain entry often from only a minor lesion (e.g. athlete’s foot, or insect bite). This is a spreading infection of the skin with marked erythema. Inflammation of the lymphatic vessels and lymph nodes (lymphangitis, lymphadenitis) can occur. Systemic upset is common, and blood cultures may be positive. Erysipelas is similar to cellulitis, producing a characteristic blistering rash on the face.

Cellulitis may also complicate animal bites due to the bacterium Pasteurella multocida.

Necrotizing fasciitis

This is a rare but life-threatening infection of deep soft tissue (down to the fascial layers ov...