Understanding Your Dog's Skin Culture and Sensitivity Results

By Emiel Maddens  ·  Reviewed in consultation with licensed veterinary professionals  ·  Updated June 2026  ·  10 min read

Photo by Alexander Mass on Unsplash

When your veterinarian hands you a culture and sensitivity report for your dog's skin infection, the document can look like an intimidating wall of scientific terminology, abbreviations, and unfamiliar organism names. But this report contains critical information that directly determines whether your dog's treatment will succeed or fail. Learning to read and understand these results empowers you to have more productive conversations with your veterinarian and to advocate effectively for your dog's care.

Skin cultures are laboratory tests that identify the specific microorganisms causing an infection by growing them in controlled conditions. Once the organism is identified, the laboratory tests it against a panel of antibiotics to determine which drugs will kill it (susceptible), which might work at higher doses (intermediate), and which the organism can resist (resistant). This sensitivity data transforms antibiotic selection from an educated guess into a targeted, evidence-based decision with a much higher likelihood of success.

Despite their value, skin cultures are underutilized in veterinary dermatology. Many infections are treated empirically, meaning the veterinarian selects an antibiotic based on the most likely pathogen and the most commonly effective drug without laboratory confirmation. While this approach works well for first-time, uncomplicated infections, it fails with alarming frequency for recurrent, chronic, or resistant infections. Understanding when cultures are essential and what the results mean helps you work more effectively with your veterinary team to resolve stubborn skin infections.

How Skin Cultures Work

The process begins with your veterinarian collecting a sample from your dog's infected skin. The collection technique matters significantly because improper sampling can produce misleading results. For bacterial cultures, the veterinarian typically ruptures an intact pustule (if present) and swabs the contents, or swabs beneath a crust or at the leading edge of a spreading lesion. The goal is to collect the actual infecting organisms rather than surface contaminants. For deeper infections like abscesses, a sample may be obtained by needle aspiration or tissue biopsy. The sample is placed in a sterile transport medium and sent to a diagnostic laboratory.

Bacterial Culture Process

At the laboratory, the sample is inoculated onto specialized growth media (agar plates) that provide nutrients supporting bacterial growth. Different media types select for different organisms. Blood agar supports most bacteria, MacConkey agar selects for gram-negative organisms, and specialized chromogenic media can identify specific species like Staphylococcus. The plates are incubated at body temperature (37 degrees Celsius) for 24 to 72 hours, during which time bacterial colonies grow and become visible. Laboratory technicians examine the colony characteristics (size, shape, color, hemolysis pattern) and perform additional biochemical tests to identify the specific bacterial species.

Fungal Culture Considerations

When fungal infection is suspected, particularly dermatophytosis (ringworm), separate fungal culture media such as dermatophyte test medium (DTM) or Sabouraud dextrose agar are used. Fungal cultures require longer incubation periods, often two to four weeks, because dermatophyte fungi grow much more slowly than bacteria. The color change on DTM and the microscopic appearance of fungal elements help identify the specific species. Understanding that fungal cultures take longer than bacterial cultures helps set appropriate expectations for results timing.

Reading the Sensitivity Report: S, I, and R Explained

The sensitivity portion of the report lists each antibiotic tested alongside a designation of S (susceptible), I (intermediate), or R (resistant). Understanding these categories is essential for interpreting the results correctly. An S designation means that the standard dose of that antibiotic achieves sufficient concentration at the infection site to kill or inhibit the identified organism. The infection is expected to respond to treatment with that drug at normal doses and administration routes.

Intermediate and Resistant Designations

An I (intermediate) designation means that the organism shows reduced sensitivity to the antibiotic. Treatment with this drug might work if higher doses are used, if the drug achieves particularly high concentrations at the infection site (such as in urine for urinary tract infections), or if the antibiotic is applied topically rather than given systemically. Your veterinarian may choose an intermediate drug when susceptible options are limited, but close monitoring for treatment response is essential. An R (resistant) designation means the organism is not expected to respond to that antibiotic at achievable concentrations, and using it would likely fail and could promote further resistance development.

MIC Values and What They Mean

Some culture reports include minimum inhibitory concentration (MIC) values, which indicate the lowest concentration of an antibiotic needed to prevent visible bacterial growth. Lower MIC values generally indicate greater sensitivity. The laboratory compares these MIC values against established breakpoints, which are concentration thresholds that determine whether an organism is classified as S, I, or R for each antibiotic. Breakpoints take into account the drug's pharmacokinetics (how the body processes it) and are set so that an S designation means achievable tissue concentrations will exceed the MIC, supporting clinical effectiveness.

Choosing the Right Antibiotic from Multiple Options

When several antibiotics show S designations, your veterinarian considers additional factors to select the optimal choice. These include the drug's ability to penetrate skin tissue, oral bioavailability, dosing frequency (once daily versus multiple times daily affects compliance), potential side effects, cost, and whether the drug is considered first-line or reserve-status. Veterinary guidelines recommend using the narrowest-spectrum effective antibiotic to minimize disruption of normal bacterial flora and reduce selection pressure for resistance. Your vet may also consider practical factors like whether your dog will accept pills or requires a liquid formulation.

Understanding culture and sensitivity results helps you advocate for targeted, effective treatment.

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MRSP: Understanding Methicillin-Resistant Infections

Methicillin-resistant Staphylococcus pseudintermedius (MRSP) is an increasingly common finding on canine skin cultures. S. pseudintermedius is the most common bacterial pathogen causing skin infections in dogs, and the methicillin-resistant variant carries a gene (mecA) that confers resistance to all beta-lactam antibiotics, including penicillins, cephalosporins, and carbapenems. This eliminates an entire class of commonly used, well-tolerated antibiotics from the treatment options and typically narrows the available choices to drugs that may have more side effects or practical limitations.

Treatment Options for MRSP

When an MRSP infection is identified, the sensitivity report becomes especially critical because the remaining effective antibiotics vary between MRSP strains. Common options that retain activity against many MRSP strains include chloramphenicol, fluoroquinolones (though resistance is increasing), tetracyclines (doxycycline or minocycline), trimethoprim-sulfonamide, and rifampin (often used in combination with another agent). Topical therapy with chlorhexidine-containing products plays a particularly important role in MRSP management because topical application achieves concentrations at the infection site that far exceed anything achievable with systemic antibiotics, effectively bypassing resistance mechanisms.

Preventing MRSP Development

MRSP infections are strongly associated with prior antibiotic use, particularly repeated or prolonged courses of beta-lactam antibiotics. Reducing unnecessary antibiotic prescribing, completing prescribed courses fully, and using culture-guided therapy from the outset are the most effective strategies for preventing MRSP emergence. When topical therapy can adequately manage a superficial infection, avoiding systemic antibiotics entirely removes the selection pressure that drives resistance. This is why many veterinary dermatologists now recommend topical antimicrobial therapy as a first-line approach for superficial pyoderma, reserving systemic antibiotics for deep or widespread infections.

Why Empirical Treatment Sometimes Fails

Empirical treatment, where an antibiotic is selected without culture data based on the most likely pathogen and typical susceptibility patterns, works well for first-occurrence, straightforward skin infections. First-line empirical choices like cephalexin and amoxicillin-clavulanate are effective against the majority of methicillin-susceptible S. pseudintermedius strains, which still represent the majority of canine skin infections. However, each time a dog receives antibiotics, the surviving bacteria include those with the greatest natural resistance, creating selection pressure that shifts the population toward less susceptible organisms.

The Downward Spiral of Repeated Empirical Treatment

A common scenario illustrates the problem. A dog develops a skin infection and responds well to a first course of cephalexin. Months later, the infection recurs and is again treated empirically with cephalexin, but this time the response is slower or incomplete. A third recurrence is treated with a different empirical antibiotic, which also fails. By this point, the dog has received multiple antibiotic courses that have selected for increasingly resistant bacteria, and the infection has become chronic. A culture performed at this stage often reveals MRSP or multi-drug resistant organisms that could have been identified and targeted earlier in the process.

When to Insist on a Culture

Veterinary dermatologists recommend performing cultures in several specific situations: any infection that has failed to respond to a first course of appropriate empirical therapy, any recurrent infection (three or more episodes in twelve months), any deep pyoderma, any infection in a dog with a history of MRSP or multi-drug resistant infection, and any infection in a dog currently or recently treated with immunosuppressive medications. If your dog falls into any of these categories and a culture has not been performed, it is entirely appropriate to request one from your veterinarian.

Asking Your Vet the Right Questions

Armed with an understanding of culture and sensitivity results, you can have more productive conversations with your veterinary team. When your vet presents culture results, asking specific questions demonstrates engagement and helps ensure optimal treatment decisions. Start by asking what organism was identified and whether it is a common canine skin pathogen or an unusual finding. The most frequently isolated organism, S. pseudintermedius, behaves differently from less common pathogens like Pseudomonas, E. coli, or beta-hemolytic Streptococcus, and treatment approaches differ accordingly.

Questions About Antibiotic Selection

Ask your veterinarian which antibiotic they have chosen from the susceptible options and why. Understanding the reasoning behind the selection helps you appreciate factors you might not have considered, such as tissue penetration, dosing convenience, or the desire to preserve reserve antibiotics for more serious infections. Ask about the expected duration of treatment, which is typically three to four weeks for superficial pyoderma and six to eight weeks for deep infections, extending one to two weeks beyond clinical resolution. Ask what signs of improvement you should expect and by when, so you know whether the treatment is working as anticipated.

Questions About Resistance and Recurrence Prevention

If resistance was identified, ask your vet about strategies to prevent further resistance development. This might include incorporating topical antimicrobial therapy to reduce reliance on systemic antibiotics, addressing underlying conditions that predispose to infections (such as allergies or hormonal disorders), and discussing whether immunotherapy or other long-term management strategies could reduce the frequency of infections. Understanding the full picture helps you view each infection not as an isolated event but as part of a larger pattern that may need a comprehensive management approach.

When Culture Results Are Surprising or Confusing

Sometimes culture results reveal unexpected findings. The identified organism might not match what was clinically suspected, multiple organisms might grow from a single sample, or the sensitivity pattern might seem unusual. Multiple organisms growing from a single culture can indicate polymicrobial infection, surface contamination during sample collection, or a primary pathogen accompanied by opportunistic secondary invaders. Your veterinarian will evaluate the clinical significance of each organism based on its identity, quantity, and the clinical presentation.

No Growth Results

A culture that returns no growth (no bacteria isolated) does not necessarily mean the infection is not bacterial. False-negative results can occur when the dog is already receiving antibiotics at the time of sampling, when the sample was improperly collected or stored, or when the organism requires specialized growth conditions that the standard culture media do not provide. If a no-growth result does not match the clinical picture (an obviously infected-looking lesion), your vet may recommend repeating the culture with careful attention to collection technique and timing relative to any antibiotic treatment.

Mixed Growth and Contamination

When multiple bacterial species grow from a culture, distinguishing between true pathogens and contaminants requires clinical judgment. Coagulase-positive Staphylococci found in moderate to heavy growth from a purulent lesion are almost certainly the causative pathogen. However, common skin commensals like coagulase-negative Staphylococci or Bacillus species growing in low numbers may represent surface contamination rather than true infection. Your veterinarian will correlate the laboratory findings with the clinical presentation to determine which organisms need targeted treatment and which can be disregarded.

The Role of Topical Therapy Alongside Culture-Guided Systemic Treatment

Culture and sensitivity testing primarily guides systemic (oral) antibiotic selection, but topical antimicrobial therapy plays a complementary and sometimes primary role in managing skin infections. Topical products containing chlorhexidine achieve concentrations at the skin surface that are hundreds to thousands of times higher than anything achievable through oral administration, making them effective even against organisms that show resistance to systemic antibiotics on culture. This is because resistance mechanisms, such as efflux pumps and altered binding targets, have limits that are overwhelmed by the extremely high local concentrations delivered topically.

When Topical-Only Treatment Is Appropriate

For superficial pyoderma limited to a small body area, veterinary dermatologists increasingly recommend topical-only treatment as the first approach, reserving systemic antibiotics for cases that fail to respond or are too widespread for practical topical application. This approach, supported by guidelines from the International Society for Companion Animal Infectious Diseases (ISCAID), reduces unnecessary antibiotic exposure and the associated resistance pressure. Topical antimicrobial sprays are particularly practical because they can be applied quickly to affected areas without the need for contact time that shampoos require.

Combining Topical and Systemic Approaches

For deep pyoderma, widespread infections, and MRSP cases, the most effective approach combines culture-guided systemic antibiotics with topical antimicrobial therapy. The systemic antibiotic addresses bacteria deep within tissues that topical products cannot reach, while topical therapy eliminates surface organisms and reduces the total bacterial burden. This combination often produces faster clinical improvement than either approach alone and may allow shorter systemic antibiotic courses, further reducing resistance risk. Discuss with your vet whether a combined approach is appropriate for your dog's specific infection.

Long-Term Strategies to Reduce the Need for Cultures

While cultures are invaluable when needed, the ultimate goal is to reduce the frequency of infections that necessitate them. This requires identifying and managing the underlying conditions that predispose dogs to recurrent skin infections. Allergic skin disease is the most common predisposing factor, and effective allergy management through immunotherapy, anti-itch medications, and environmental controls can dramatically reduce infection frequency. Hormonal disorders like hypothyroidism and Cushing's disease impair immune function and skin barrier integrity, and treating these conditions often resolves recurrent infection cycles.

Skin Barrier Support

Supporting skin barrier function through regular bathing with appropriate products, omega-3 fatty acid supplementation, and topical moisturizers helps create an environment that resists bacterial colonization. Healthy skin with an intact barrier is far less susceptible to infection than the compromised, inflamed skin that characterizes allergic or endocrine disease. Probiotics that support a healthy skin microbiome are an area of active research, with some evidence suggesting that maintaining diverse, balanced skin flora reduces the ability of pathogenic organisms to establish infections.

Proactive Monitoring

Regular at-home skin checks allow early detection of emerging infections before they become established and difficult to treat. Learn to recognize the early signs of pyoderma, including small red bumps (papules), pus-filled bumps (pustules), circular areas of hair loss with crusting (epidermal collarettes), and changes in skin odor. Initiating topical treatment at the first sign of infection can prevent progression to the point where systemic antibiotics and culture-guided therapy become necessary. This proactive approach reduces antibiotic use, resistance development, and the overall cost and complexity of managing your dog's skin health.

Understanding Your Dog's Skin Culture and Sensitivity Results FAQ

Q: What do S, I, and R mean on a culture report?

S (susceptible) means the antibiotic is expected to effectively treat the infection at standard doses. I (intermediate) means the antibiotic might work at higher doses or when applied topically where concentrations are higher. R (resistant) means the organism is not expected to respond to that antibiotic, and using it would likely fail while potentially promoting further resistance.

Q: When should my vet perform a skin culture?

Cultures should be performed when an infection fails to respond to a first appropriate antibiotic course, for any recurrent infection (three or more episodes in a year), for all deep pyoderma, when MRSP is suspected or the dog has a history of resistant infections, and when the dog is on immunosuppressive medications. Requesting a culture early saves time and money compared to multiple failed empirical treatment courses.

Q: What is MRSP and why does it matter?

MRSP stands for methicillin-resistant Staphylococcus pseudintermedius. It is a form of the most common canine skin pathogen that carries a gene making it resistant to all beta-lactam antibiotics, including penicillins and cephalosporins. MRSP infections require culture-guided treatment because the effective antibiotic options are limited and vary between strains. Topical chlorhexidine therapy plays a particularly important role in MRSP management.

Q: How long does it take to get culture results?

Bacterial culture results typically take three to five business days. If sensitivity testing is included, which it should be, results may take an additional one to two days. Fungal cultures require significantly longer, often two to four weeks, because fungi grow much more slowly than bacteria. Your veterinarian may start empirical treatment while waiting for results and then adjust based on the findings.

Q: Can topical therapy work against resistant bacteria?

Yes, topical antimicrobial products like chlorhexidine achieve concentrations at the skin surface that are hundreds to thousands of times higher than systemic antibiotics. These extremely high local concentrations can overwhelm many resistance mechanisms that make bacteria resistant to oral antibiotics. This is why topical therapy is a cornerstone of MRSP management and why veterinary dermatologists recommend it as a first-line approach for many superficial skin infections.

Sources

Hillier, A., Lloyd, D. H., Weese, J. S., Blondeau, J. M., Boothe, D., Breitschwerdt, E., ... & Sykes, J. E. (2014). Guidelines for the diagnosis and antimicrobial therapy of canine superficial bacterial folliculitis (ISBCAID). Veterinary Dermatology, 25(3), 163-e43.

Bryan, J., Frank, L. A., Rohrbach, B. W., Burgette, L. J., Cain, C. L., & Bemis, D. A. (2012). Treatment outcome of dogs with meticillin-resistant and meticillin-susceptible Staphylococcus pseudintermedius pyoderma. Veterinary Dermatology, 23(4), 361-e65.

Morris, D. O., Loeffler, A., Davis, M. F., Guardabassi, L., & Weese, J. S. (2017). Recommendations for approaches to meticillin-resistant staphylococcal infections of small animals. Journal of Small Animal Practice, 58(1), 2-11.

Beco, L., Guaguere, E., Lorente Mendez, C., Noli, C., Nuttall, T., & Vroom, M. (2013). Suggested guidelines for using systemic antimicrobials in bacterial skin infections. Veterinary Record, 172(13), 72.

Loeffler, A., & Lloyd, D. H. (2018). What has changed in canine pyoderma? A narrative review. The Veterinary Journal, 235, 73-82.

Related Reading

• Dog Skin Microbiome, How bacteria balance affects infection risk
• Managing Chronic Skin Allergies, Addressing the underlying cause of recurrent infections
• Newly Diagnosed Atopic Dermatitis, Understanding the allergy-infection connection

Emiel Maddens

Founder of Vetified. Develops topical antifungal and antimicrobial formulations for companion animals. Vetified products are listed on DailyMed and manufactured through FDA-registered facilities in the United States.