The Rapid Differentiation of
Meticillin-Resistant
Staphylococcus aureus (MRSA) from Meticillin-Resistant
Staphylococcus intermedius
in Canines:

The recovery of meticillin (oxacillin) resistant Staphylococcus aureus (MRSA) from human sources is well documented. Recently, there have been reports of this organism being isolated from multiple veterinary species, prompting zoonotic concern. In canines, Staph. aureus is rarely recovered, and, indeed, recent studies have shown poor adherence of Staph. aureus to canine keratocytes.

Consequently, Staph. aureus is not con-sidered to be a primary pathogen of the dog, and the coagulase-positive Staphylococcus most usually recovered from the dog is Staph. intermedius, strains of which may also be resistant to meticillin (MRSI).

Nevertheless, Staph. aureus, and particularly MRSA, remains an important opportunist, with zoonotic implications, making the identification of it, in veterinary cultures, important.

Veterinary samples may contain both Staph. intermedius and Staph. aureus. Since both species demonstrate similar cultural characteristics, particularly on routine media, their differentiation may prove difficult.

Definitive differentiation relies on biochemical profiling, using commercial kits, though even these produce similar reaction patterns. The principle differences lie in the Vogues-Proskauer (VP) reaction (production of acetoin), anaerobic fermentation of carbohydrates, notably mannitol and maltose, and the presence of beta-galactosidase. However, to evaluate all coagulase-positive Staphylococcal isolates in this way is time-consuming and costly.

There are many published methods for the routine recovery of MRSA from human cultures, though it is recognised that no single media can be used for the recovery of every strain. Moreover, these specific media may prove less helpful in routine veterinary investigations, where they may not be available, or because they may support the growth of both species. The differentiation of MRSA from MRSI is, therefore, an additional, and important, problem not encountered in human clinical microbiology.

Oxoid recently introduced a chromogenic media (containing oxacillin and a chromogenic substrate), which can be used for the rapid isolation and identification of MRSA. We conducted a pilot study to determine whether this media would also support the growth of MRSI, and, if so, if it could be used to differentiate MRSA from MRSI.

Our initial findings show that oxacillin-sensitive Staph. intermedius strains do not grow on this media whilst MRSI strains do, though with different colonial characteristics to those of MRSA. Typically, colonies of MRSA appear a deep blue colour while MRSI colonies appear colourless or very pale blue, and are generally more slow-growing.

This not only permits the rapid differentiation of MRSA from MRSI, but also allows the identification of both species in mixed culture .

We conclude that this media shows promise for the routine differentiation of MRSA from MRSI in veterinary samples. Importantly, it may also prove to be of value in the screening of large populations (e.g. carriage studies) for either, or both, organisms.

Use of this medium is cost effective, when compared to the cost of using commercially available identification systems (e.g. API Staph., or similar) most importantly missing an isolate altogether.

References:
Cowan and Steel. Manual for the identification of medical bacteria. Guidelines for the Laboratory Diagnosis and Susceptibility Testing of Meticillin-Resistant Staphylococcus. aureus BSAC ESVD Workshop on cutaneous immunology.

Last updated: 28/03/2007