Lecture 3

Physical Barriers

• Hair/Fur- prevent direct skin contact

• Stratum corneum- inert and impermeable layer

• Temperature and pH- too cold/unfavorable pH for optimal growth of many

Chemical Barriers

• Fatty acids- bacteriostatic

• Inorganic salts- high salts conc.

• Transferrins- bind iron needed for bacterial growth

Immunological Defense

• Site for interactions between immune cells and antigens

• Prevent deeper invasion

Normal Flora

• Bacteria and fungi

• Protective

What is normal flora comprised of?

Resident flora and Transient flora

Resident Flora

• Live and multiply on the skin (obligate parasites)

• Form a permanent population

• Cannot be eliminated

• Normally are harmless

EX: Coagulase-negative Staphylococcus, alpha-hemolytic Streptococcus, Micrococcus spp.

Transient Flora

• Acquired from environment or mucous membranes

• Most do NOT multiply efficiently on skin- just there

• Are transient- CAN be removed or eliminated

• May be involved in pathogenic processes

EX: Coagulase-positive Staphylococcus, E.coli, Proteus

Bacterial Infections of the Skin

• Primary Infections

• Secondary Infections

Primary Skin Infections

• Bacteria initiate and cause most of the pathology

• Single bacteria species is dorminant

• Occurs in “healthy” skin

• Antibacterial therapy is effective

EX: Greasy pig disease, Dermatophilosis

Secondary Skin Infections

• Bacterial infections associated with other infections and/or conditions

• Predisposing conditions:

- Skin infected with parasites, viruses, fungi

- Other local predisposing factors: moisture, skin folds

- Systemic disease resulting in skin pathology: hypothyroidism, Cushing’s disease

- Physical or chemical trauma- surgical incision, catheter placement

- Immunosuppression- corticosteroid therapy

Bacterial Culture

• Must distinguish between colonization and infection

• Must identify if primary or secondary

Colonization

Present, but not causing disease

Infection

Present, associated with pathology

Surface Pyoderma

• Involves epidermis only

• Often secondary to self-trauma or allergic skin disease

EX: hot spots and early skin fold dermatitis

Superficial Pyoderma

• Involves the epidermis and dermis

• Pustules are present

• Often secondary to other disease (hormonal, metabolic, or parasitic)

• Reoccurrence is common and long-term management is difficult

Deep Pyoderma

• Involves epidermis, dermis, and subcutaneous layers

• Almost ALWAYS secondary to other contributing factors

• Not common, but VERY difficult to treat