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In the Literature

Walker M, Singh A, Nazarali A, Gibson TWG, Rousseau JD, Weese JS. Evaluation of the impact of methicillin-resistant Staphylococcus pseudintermedius biofilm formation on antimicrobial susceptibility. Vet Surg. 2016;45(7):968-971.

From the Page …

Antimicrobial resistance has grown dramatically in both human and veterinary medicine and is a major cause of patient morbidity and increasing healthcare costs.1,2 Staphylococcus spp—particularly S pseudintermedius, which can display methicillin resistance—are common isolates in dogs and cats. Staphylococcus spp are known biofilm formers, which can further complicate bacterial eradication. Biofilms are particularly problematic in surgical site infections related to orthopedic implants.

This study investigated the impact of methicillin resistance and biofilm formation on determination of minimum inhibitory concentration (MIC) in S pseudintermedius isolates. Amikacin, enrofloxacin, cefazolin, and gentamicin—4 antimicrobials commonly used for treating staphylococcal infections—were tested. Antimicrobial MICs were examined using standard laboratory methods for both planktonic and biofilm bacterial isolates. 

The MIC for all antimicrobials was significantly higher for all biofilm-associated vs planktonic bacteria. The presence of biofilm resulted in a >667-fold to >4000-fold increase in antimicrobial MIC, which was beyond the upper limit of the antimicrobial dilution tests. For planktonic bacteria, the MIC for all antimicrobials was significantly higher in methicillin-resistant S pseudintermedius (MRSP) as compared with methicillin-susceptible isolates. For biofilm bacteria, the MIC was not different between MRSP and methicillin-susceptible isolates; however, the MIC was greater than the tested dilutions for all antimicrobials.

Topical Agent Mechanism Uses
Chlorhexidine, 0.05% Antiseptic Open wounds, pyoderma
Tris-EDTA Alkalizing, potentiates antimicrobial efficacy Open wounds, otitis
Silver (nanoparticle, sulfadiazine) Protein inactivation, inhibiting cell division Open wounds, implant coatings
Honey, sugar Hyperosmotic agents Open wounds

The results of this in vitro study corroborated clinical findings in MRSP infections in small animals.3 The MIC patterns are often increased, which results in fewer antimicrobial choices that remain susceptible to bacterial isolates. Although methods of MIC determination with biofilm-forming bacteria are not robustly established, these data suggest that standard systemic antimicrobial treatment of biofilm bacterial infections may have little, if any, effect on eradication.

… To Your Patients

Key pearls to put into practice:


It is imperative to perform culture and susceptibility testing when there is concern for antimicrobial resistance, including in cases with recurrent or persistent infections and/or high-risk sites (eg, ears, surgical sites, implant-associated sites).


In Staphylococcus spp infections, the potential for biofilm formation should be considered.


Biofilm formation may render results of antimicrobial MIC test results invalid.


Local strategies (Table) should be considered to augment or even replace systemic antimicrobial therapy whenever possible.

References and author information Show
  1. World Health Organization. Antimicrobial resistance global report on surveillance. Published April 2014. Accessed January 6, 2017. 
  2. Weese JS. A review of multidrug resistant surgical site infections. Vet Comp Orthop Traumatol. 2008;21(1):1-7.
  3. Frank LA, Loeffler A. Meticillin-resistant Staphylococcus pseudintermedius: clinical challenge and treatment options. Vet Dermatol. 2012;23(4):283-291, e56.

Jason Bleedorn

DVM, DACVS University of Wisconsin–Madison

JASON BLEEDORN, DVM, MS, DACVS (Small Animal), is a clinical assistant professor in small animal orthopedics at University of Wisconsin–Madison, where he instructs residents, interns, and veterinary students and conducts clinical research. He earned his DVM from University of Illinois and his MS in biomedical sciences from University of Wisconsin–Madison. He completed a rotating internship at Purdue University, a surgery internship at Dallas Veterinary Surgical Center, and a small animal surgery residency at University of Wisconsin–Madison. His research interests include biomechanics and augmentation of fracture healing, imaging assessment and treatment of bone deformities, 3-dimensional printing for surgical planning, limb salvage of bone tumors, and mechanisms of cruciate ligament disease in dogs.

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