With the rapid rise of methicillin- and multi-drug-resistant bacterial pyodermas in veterinary dermatology, bacterial cultures are becoming more necessary in daily clinical practice. There are many factors, including sample quality, types of cultures, and clinical breakpoints, that make evaluating the results of a bacterial culture more than just looking for an “S” versus an “R.” Understanding how to maximize culture results to make the best antibiotic selection can help improve clinical outcomes for patients and reduce risk of progressive development of resistant pyodermas. Recommending a culture A 2014 guidelines document outlines five situations that should signal a clinician to recommend a bacterial culture:1 Less than 50 percent improvement in skin lesions within two weeks of starting an appropriate antibiotic Emergence of new lesions while on antibiotics Persistence of lesions and cytological evidence of bacteria after six weeks on antibiotics Intracellular rods on cytology A history of a prior methicillin, multi-drug-resistant pyoderma Other patient factors that may increase the risk of resistant infection and the subsequent need for skin cultures include a history of frequent antibiotic therapy, history of resistant pyodermas, a history of or concurrent use of glucocorticoids, and recent hospitalization or veterinary contact.2 While understanding risk factors and when to do cultures is important, sample selection is also necessary for maximizing your culture results. Intact pustules, crusts, and epidermal collarettes are ideal lesions for swab sampling for superficial pyodermas.1 For deep pyodermas, deep tissue cultures may provide more accurate results compared to superficial swabs. Once the culture swab is taken, it is recommended to collect cytology samples from the same sites to confirm the presence of bacteria and the type of bacteria that was sampled. Cytological evidence of bacteria at the sampled sites will reduce the risk of negative cultures and the number and shape of bacteria can be used to compare to the culture to ensure the results are representative of the active infection. Types of cultures Bacterial cultures are commonly performed to determine specific species of bacteria that may be involved in an infection, as well as determine which susceptible antibiotics will work against that bacteria. There are two main types of bacterial cultures: diffusion-based methods and dilution-based methods. Diffusion-based testing will give categorical results for each antibiotic tested, including susceptible, intermediate, or resistant.3 Dilution-based testing will provide the lowest concentration that inhibits bacterial growth or the minimum inhibitory concentration (MIC).3 Both culture types can provide information to guide antibiotic selection. In addition to antibiotic sensitivities, some labs may include a clinical breakpoint with the culture results. Clinical breakpoints are concentrations in which the bacteria are susceptible to successful antibiotic treatment.4 These are established for many of the commonly used antibiotics in veterinary medicine.3 Evaluating the MIC in light of the clinical breakpoint, also known as the breakpoint to MIC quotient, can further improve antibiotic selection. Antibiotic options with higher breakpoint to MIC quotients will have improved therapeutic efficacy in clearing the infection.5 Further, if an antibiotic is graded as susceptible, but has a low breakpoint to MIC quotient there is still a higher risk of therapeutic failures when using that antibiotic for treatment.5 When available, evaluating both the MIC and breakpoint can help improve therapeutic success in the management of resistant skin infections. Evaluating results When evaluating the results, the presence of environmental contaminants is common, and it is important to focus on only treating the cutaneous pathogens. The skin flora is made up of resident and transient microbes. When culturing, it is not uncommon to sample and grow a mixture of bacteria. However, focusing antibiotic selection to cover all species of bacteria grown on a culture can result in falsely limited antibiotic options and/or the use of an unnecessary class of antibiotic or a combination of antibiotics for complete coverage. The most common strain isolated from canine and feline skin infections is Staphylococcus pseudintermedius. Other potential pathogenic staphylococcal species include S. aureus, S. schleiferi, and other coagulase negative staphylococci.6 Less common cutaneous pathogens that can be considered are Pseudomonas aeruginosa and Corynebacterium spp.1,6 Common environmental contaminants seen on skin cultures include E. coli, Enterococcus spp., Enterobacter spp., Proteus spp., and Bacillus spp. Therefore, to streamline antibiotic therapy, antibiotic selection should focus on the primary cutaneous pathogens only. Antibiotic selection Culture results are an important tool to direct antibiotic selection. However, clinicians should still consider judicious use of antibiotic classes when selecting from the susceptible antibiotic options. A 2014 paper published in Veterinary Dermatology outlined a three-tier system for appropriate antibiotic selection:1 Tier 1: These antibiotics are considered the first-line empirical choices for bacterial pyodermas. Clindamycin, first-generation cephalosporins, amoxicillin-clavulanate, and sulfonamides make up the first-tier options. Cefpodoxime and cefovecin are considered first or second tier pending the source. Tier 2: This tier of antibiotics should be reserved for when there is culture evidence that there are no tier one options. It includes doxycycline/minocycline, chloramphenicol, fluoroquinolones, rifampin, and aminoglycosides. Tier 3: This tier should only be considered if there are no other susceptible options on a culture. These medications are reserved for treatment of resistant infections in humans and use of these antibiotics is strongly discouraged in veterinary medicine.1 The third-tier antibiotics are linezolid, teicoplanin, and vancomycin. To reduce the risk of progressive bacterial resistance, when selecting an antibiotic based on your culture results, it is important to also consider the appropriate dosing and duration of the selected antibiotic. The ideal duration of antimicrobial therapies is one week past clinical improvement in lesions and resolution of cytological evidence of bacteria. In most cases, superficial pyodermas require three to four weeks of antibiotic therapy, while deep pyodermas need four to six weeks of treatment.1,6 Antibiotic dosing may vary from commonly used doses for other systemic infections. Using specific dose ranges outlined for superficial and deep skin infections is recommended, and the outlined table in the 2014 paper is a great reference for skin-specific dosing.1 Finally, in addition to systemic medications for treatment of a resistant pyoderma, topical antimicrobial therapy should be considered to both speed resolution of the active infection and long-term to reduce the risk of reoccurrence. In the age of resistance, bacterial cultures should be a cornerstone in the management of canine and feline pyodermas. Further, it is important to fully understand the culture process and outcomes to have increased confidence in making the best clinical recommendations based on the results. Ultimately, bacterial cultures are imperative for both successful clinical improvement for the patient and to promote continued judicious use of antibiotic therapy in veterinary medicine. Rebecca Mount, DVM, DACVD, earned her bachelor of science degree in biology from the University of New Mexico in 2005, and her doctorate of veterinary medicine from Colorado State University College of Veterinary and Biomedical Sciences in 2009. Dr. Mount completed an internship in small animal medicine and surgery at Garden State Veterinary Specialist in New Jersey. She began her residency with Dermatology for Animals in 2010, and became a diplomate of the American College of Veterinary Dermatology in 2014. References Hillier, A., Lloyd, DH., Weese, JS., Blondeau, JM., et al. Guidelines for the diagnosis and antimicrobial therapy of canine superficial bacterial folliculitis (Anitmicrobial Guidelines Working Group of the International Society for Companion Animal Infectious Diseases. Veterinary Dermatology. 2014;25: 163-e43 Nienhoff, U., Kadlec, K., Chabern, IF., Verspohl, J., et al. Methicillin-resistant Staphylococcus pseudintermeidus among dogs admitted to a small animal hospital. Veterinary Microbiology. 2011;150: 191-197. Rubin, J.E. (2013). Antimicrobial Susceptibility Testing Methods and Interpretation of Results. In Antimicrobial Therapy in Veterinary Medicine (eds S. Giguère, J.F. Prescott and P.M. Dowling). Pg 11-20. Mitka, Mike. Antibiotic breakpoints. Journal of American Medical Association. 20212; 307 (10) Dhandapani, S., Priyadarshi, K., Rajshekar, D., et al. The role of cascade reporting integrated with breakpoint to minimum inhibitory concentration quotient (minimum inhibitory concentration therapeutic index) and minimum inhibitory concentration guiding table on clinical microbiology reporting of culture-proven bloodstream infections. Journal of Current Research in Scientific Medicine 9(2):p 104-112 Miller, WH., Campbell, KL., Griffin, CE. Mueller and Kirk’s Small Animal Dermatology 7th edition (various pages). St. Louis, Missouri: Elsevier.
