Small Animal Dermatology, 3rd edition

144 BASICS

Increased MICs associated with thickening of the cell wall possibly associated

with trapping of the antibiotic within the cell wall and reduced penetration

Thickening is not mediated by a gene known to be related to macrolide

resistance.

 Efflux pump:
MRS organisms often have proteins involved in active removal of antibiotics

through a membrane-bound pump, which effectively limits the activity of many

antibiotics that focus on internal cellular structures.

 Biofilms:
Biofilms are three-dimensional clusters of bacteria that are not derived from a

single specific organism: they are not colonies

The cluster is surrounded by a protective polysaccharide coating

Most commonly occur in the ear canal but may be prevalent in more generalized

infections and contribute to treatment failures

Biofilm production has become of prime importance in treatingPseudomonas

infection in the ear canal

Both staphylococcal and pseudomonal organisms have the capability to produce

protective biofilms that inhibit antimicrobial penetration and allow for protected

reservoirs of bacteria

Partial antimicrobial penetration into the biofilm may increase selection pressure

for mutations and resistance (treatment failure and development of therapeutic

resistance)

One report states that 40% of canine oticPseudomonasisolates, 30% of canine

oticStaphylococcusstrains, and 95% ofMalasseziacan produce biofilms

A second study found thatS. pseudintermediusswabbed from the abdomen

(100%) and interdigital spaces (85%) of 36 healthy dogs were able to produce

biofilms.

STAPHYLOCOCCAL VIRULENCE FACTORS

Staphylococcal virulence factors aid in production of disease.

 Hyaluronidase degrades hyaluronic acid, damaging connective tissue.

 Coagulase increases production of fibrin thrombi in tissue.

 Kinase converts plasminogen to plasmin, which digests fibrin.

 Streptolysin disrupts phagocytosis.

 Streptokinase activates plasmin-like proteolytic activity.

 Beta-hemolysin destroys cells by lysis; high affinity for precursor lipids of the stratum

corneum ceramides and may help penetration into the skin.

 Protein A binds IgG in a dysfunction pattern and thus avoids phagocytosis (adhesion

molecule).

 Exfoliative toxin targets desmoglein 1 (a desmosomal cell-to-cell adhesion

molecule) causing blistering; dermonecrotic toxin (i.e., staphylococcal scalded skin

syndrome).

 Exotoxin causes local tissue damage and incites inflammation.