Much has changed in the management of bacterial skin diseases in veterinary patients since the recognition of methicillin
resistant (β-lactam antibiotic resistant) staphylococcal skin infections and their significance in both the human and veterinary
medical communities. Awareness and responsible use of antibiotic medications is a must when dealing with these emerging pathogens.
Bacterial flora of the skin: an overview
Normal canine and feline skin is host to various species and strains of bacteria that are considered normal inhabitants.
Traditionally, these organisms have been considered resident transient organisms based on their ability to replicate on normal
skin and hair. While resident bacteria are in fact capable of multiplication on normal skin, transient organisms are acquired
from the environment and are not capable of multiplying on the normal skin of most animals. Thus transient organisms are
not considered pathogenic unless isolated from lesional skin. In addition, a nomadic population also exists. Nomads are
capable of brief adherence and colonization and their isolation is thought to reflect environmental contamination. Thus the
current classification scheme identifies residents as those organisms that are isolated >75% of the time the skin is cultured,
nomads are isolated <75% but >25% of the time, and transients can only be isolated <25% of the time the skin is cultured.
An alternative method of classification involves the number of colonies isolated. One method utilized defined resident organisms
as those with ≥10 colonies isolated from a specimen and transient organisms as those with <10 colonies per specimen. This
method however, is more restrictive in that species that are consistently isolated with high frequency from multiple sites,
regardless of the colony count, should be considered to have colonized the individual. By evaluating colony count only, less
common but important species may not be considered relevant. Therefore both frequency and persistence of isolation should
be considered when differentiating normal inhabitants from those organisms causing secondary colonization or contamination
rather than secondary infections. This differentiation is crucial in generating an effective therapeutic plan for the appropriate
pathogenic organism based on culture and sensitivity data, especially when multiple organisms are reported.
Frequently, additional organisms may be cultured from the skin. Upon isolating one such organism from the skin of a patient
with skin disease, the clinical picture may become confused. The significance of the isolation of a transient organism must
be questioned as a pathogen unless it is involved with a pathologic process as a secondary invader. Therefore, although it
may not be the expected organism, when it is present in high numbers in a relatively pure culture, it is important to evaluate
the site of culture as well as the method of collection. A culture obtained from the surface of the skin in an area that
is pruritic or painful to the patient, for example, will be expected to contain transient contaminant organisms from the oral
cavity as the animal licks or chews that site. However, if the lesion was a closed pustule or nodule, the result may be accurate.
It has been demonstrated that culturing an epidermal collarette is also a reliable method if performed correctly.
Colonization of the skin
It is well documented that the oral, nasal and anal mucocutaneous sites serve as a reservoir for S. pseudintermedius colonization in the dog. In addition, the ear is another site where S. pseudintermedius can be frequently isolated. These locations appear to serve as carrier sites for seeding the rest of the skin and hair through
normal grooming activity. It is not clear however, why some dogs become resident carriers of S. pseudintermedius with higher numbers of bacteria than others. Puppies of dogs that had been classified as resident carriers also had significantly
higher populations of S. pseudintermedius, and it appeared that a dominant strain was consistently isolated from both the adult dogs and their puppies. This continued
transfer of strains of S. pseudintermedius between generations has been suggested as a potential reservoir for retention of genes for staphylococcal antibiotic resistance.
This concept needs to be investigated further as exchange and acquisition of new strains occurs between dogs with normal daily
contact. Animals receiving antibiotic therapy may be especially at risk for acquisition of resistant organisms as this situation
may in fact encourage the transfer of organisms via reduction of the normal resident population of staphylococcus.