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    Understanding Veterinary Diagnostics: Culture And Sensitivity

    On the left side, a gloved hand hold a red Petri dish with white dots indicating bacteria growth. On the right side, a gloved hand hold a red Petri dish with white lines indicating bacteria growth.
    Agar plates with bacteria growth. Photo by National Cancer Institute on Unsplash
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    Veterinary Review Initiative
    This resource has been reviewed for accuracy and clarity by a qualified Doctor of Veterinary Medicine as of May 2023.

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    Have you ever wondered how veterinarians decide which medications to prescribe when an individual has an infection? While there are a number of tools that a veterinarian may use to make a diagnosis and subsequent treatment decisions, two important tools that you should be familiar with are microbial culture and antimicrobial susceptibility testing (together, these are often referred to as “culture and sensitivity”). Culture and sensitivity testing can play an important role in the identification of bacteria and fungi as well as in determining their susceptibility to antimicrobials, which is especially important given the growing issue of antimicrobial resistance worldwide.  

    Before we dive in, it’s important to stress that while microbial culture and antimicrobial susceptibility testing can play an important role in the diagnosis of an infection as well as in determining effective antimicrobial treatment options, this is just one piece of the puzzle. Different situations are going to require a different set of tools, which may or may not include culture and sensitivity tests. The person best equipped to identify the most appropriate combination of tools for each specific case will be your veterinarian. Letting your veterinarian know that you’re interested in discussing all available diagnostic options when residents are ill is a good idea. And, if you’re ever wondering if a culture and sensitivity test would be appropriate in a specific situation, just ask! To learn more about the importance of setting expectations upfront and building a good relationship with your veterinarian, check out our resource here.

    What Is A Microbial Culture?

    A microbial culture (sometimes referred to as a microbiological culture) is a laboratory procedure in which a diagnostic sample is kept under very specific conditions in an attempt to grow and identify microorganisms present in the sample. Microbial culture tests are often referred to in more specific terms based on the type of microorganism of interest (i.e. aerobic cultures, anaerobic cultures, or fungal cultures).

    What Types Microorganisms Can Be Identified With A Microbial Culture?

    Microbial cultures are used to identify certain bacteria and fungi. Because viruses require a living cell in order to replicate, growing and isolating viruses is quite a bit different than growing and isolating many bacteria and fungi, most of which can be grown in a special media (such as an agar plate). While viruses can be cultured using specialized techniques, other tools are often used to diagnose viral diseases. In this resource, we will be focusing on bacterial and fungal cultures. 

    When a sample is submitted for culture, there is more to it than simply requesting that the sample be cultured. Your veterinarian will request a certain test (or tests) based on what pathogens (or types of pathogens) they suspect could be causing the issue. Types of culture tests include fungal culture, aerobic bacterial culture, anaerobic bacterial culture, and mycoplasma culture. While these types of cultures can be used to grow and identify various species of fungi and bacteria, culture may not always be the best way to determine if a certain infection is present, as there are other more specific tests available (ELISA tests, latex agglutination, antibody tests). While cultures are used to grow, isolate, and identify bacteria and fungi, not all bacteria and fungi are easily grown and isolated. Your veterinarian will be able to identify the most appropriate type(s) of tests based on the specifics of the situation.

    What Is Sensitivity Testing?

    Sensitivity testing (also called antimicrobial susceptibility testing, or AST) may be recommended after certain bacteria or fungi have been grown, isolated, and identified. As the name suggests, antimicrobial susceptibility testing is used to assess a pathogen’s susceptibility to certain antimicrobials (sensitive, intermediate, or resistant). Your veterinarian can then use this information, in conjunction with other important information, to determine the best treatment option(s).

    In some situations, susceptibility testing may not be necessary, particularly if it is well-known what the pathogen is susceptible to (i.e. certain groups of bacteria are known to be susceptible to penicillins, whereas E. coli can have different strains, and susceptibility to antibiotics varies). If a bacteria identified is a normal inhabitant of the area sampled, antibiotics may not be recommended. 

    You may also wonder why a sensitivity test needs to be performed versus just automatically starting on a broad-spectrum antibiotic in every situation where infection is suspected. Because of the increase of antimicrobial resistance, veterinarians want to make sure they are treating with the appropriate antibiotic versus just starting on a broad-spectrum antibiotic. Starting an antibiotic without knowing if the pathogen is present or is truly susceptible to it can not only contribute to antimicrobial resistance, but it may be detrimental to your resident’s health if a resistant bacteria develops in your resident due to antibiotic use, or if the antibiotic doesn’t work and your resident continues to have an uncontrolled infection. 

    How Long Does It Take To Get Culture And Sensitivity Results?

    The turnaround time for results will depend on the diagnostic lab used, the specific test requested, and when the sample is submitted. For many bacterial cultures, turnaround time ranges from 2-7 days, with antimicrobial susceptibility testing taking an additional 2-5 days. Some bacteria take significantly more time to grow, so certain tests that are attempting to culture a specific bacteria may have longer turnaround times (for example, the causative agent of Johne’s disease is very slow growing and, therefore, fecal culture results can take as long as 16 weeks). Negative results may take longer than most positive results because the lab needs to wait for a certain amount of time to see if any microorganisms will eventually grow. Fungal cultures tend to take a bit longer than the average bacterial culture, with a turnaround time ranging from about 1-3 weeks.

    If you’re worried that waiting on culture and sensitivity results will mean that initiating necessary treatment will be delayed, this does not have to be the case. Remember that culture and sensitivity testing is just one part of the equation. Veterinarians make treatment decisions based on other information as well, such as findings from a physical examination, the individual’s history, current symptoms, severity of symptoms, most likely causes of symptoms, etc. If they deem immediate treatment necessary, they will consider the pathogen(s) most likely to be the cause of the issue and then will choose a treatment that is likely to be effective. Then, based on culture and sensitivity results, they can change the treatment as needed (for example, switching from a potentially less effective medication to one that should be more effective based on AST results, or switching from a broad-spectrum antimicrobial to one that is more targeted). 

    How Are Microbial Culture And Sensitivity Results Interpreted?

    We recommend that all diagnostic results be interpreted by an experienced veterinarian and that any subsequent treatment decisions are also made by or in consultation with your vet. However, this is particularly true when it comes to interpreting culture and sensitivity results. The following information is intended to help caregivers better understand how their veterinarian interprets culture and sensitivity results and is not meant to encourage folks to attempt to interpret results themselves.

    Microbial Culture Results

    Folks other than veterinarians can certainly look at a diagnostic report and see what, if any, microorganisms were grown. However, when your veterinarian interprets these results, they draw on their knowledge and experience, as well as the specifics of the case, and interpret lab results through this lens. If the sample did not grow any microorganisms, but the individual is clearly showing signs of infection, they may consider factors that could have affected the results, such as improper collection and handling technique or collecting the sample too late in the disease process. They may also consider if the causative agent might be a virus or a bacteria or yeast that is difficult to culture. Depending on the specifics of the situation, your veterinarian may suggest collecting another sample to submit for microbial culture, or they may suggest pursuing different diagnostics such as molecular testing. In other cases, if no microorganisms are grown, they may conclude that there likely is not an infectious process involved.

    Similarly, when interpreting culture results in which certain bacteria or fungi were cultured, your veterinarian will consider how these results fit in with the bigger picture of what’s going on. The results in and of themselves cannot differentiate between microorganisms that are causing an active infection (which may warrant antimicrobial treatment) and microbial colonization (which may not warrant treatment). Similarly, there is always a possibility that a sample has been contaminated, particularly if proper collection technique was not followed. Starting antimicrobial treatment without contextualizing the findings could lead to unnecessary antimicrobial use (which contributes to the development of antimicrobial resistance). 

    Antimicrobial Susceptibility Interpretation

    Just as interpreting microbial culture results is not straightforward, neither is interpreting sensitivity results. The lab report will list the antimicrobials tested as well as the pathogen’s susceptibility to each (sensitive, intermediate, or resistant), but these should not be confused with treatment recommendations. While your veterinarian will want to avoid antimicrobials that the pathogen is resistant to, within the category of antimicrobials it is sensitive to, some drugs may be a better choice than others. (Also, while we’re not going to get into minimum inhibitory concentration, or MIC, if you’re looking at results and wondering if you can simply compare the listed MICs to determine which drug is most effective, you can’t.) 

    In addition to susceptibility, your veterinarian will also consider factors pertaining to the host, the medication, and the pathogen. For example, the location of an infection may make one medication a better choice than others. There are also certain antibiotics listed on a sensitivity assay that are prohibited in certain animals (due to their designation as “food producing animals,” for example). Your veterinarian will take this into account when making treatment recommendations. The individual’s overall health may also make certain treatments a better option than others, with some medications possibly being contraindicated (for example, certain antibiotics should not be used if a patient has kidney dysfunction). Overall, you should know it may not be as simple as looking at a sensitivity result and choosing the antibiotic(s) that say “sensitive.” 

    What Types Of Samples Can Be Used For Culture And Sensitivity Testing?

    Culture and sensitivity can be performed on a host of different diagnostic samples such bodily secretions, excretions, and fluids. The type of sample that is most appropriate will depend on factors such as the area of the body/body system suspected to be affected, the pathogen(s) suspected, and the suspected pathogen’s transmission route. For example:

    • A urine culture can help diagnose a urinary tract infection;
    • A fecal culture can help diagnose certain gastrointestinal infections;
    • A blood culture can help diagnose septicemia (blood infection);
    • A milk sample (or, in non-lactating individuals, a fluid sample from the udder) can be used to help diagnose mastitis (infection of the udder);
    • A fluid sample taken from the crop of an avian resident can help diagnose a crop infection;
    • A hair sample or skin scraping may be used to help diagnose a skin infection;
    • A nasal swab or trachea swab can help diagnose an upper respiratory tract infection;
    • A swab taken from a wound can help diagnose a localized infection in the wound;
    • A pus sample taken from an abscess can help diagnose a localized infection or an abscess-causing disease such as caseous lymphadenitis

    And so on… 

    The type of sample and the type of culture being done will dictate how the sample must be collected. Always work with your veterinarian to determine the most appropriate type of sample and how it should be collected. If you or your staff are collecting a sample to submit for culture, make sure you know how it must be collected (including what type of container is appropriate), how it should be stored, and also how soon it must be submitted to the lab to ensure accurate results.

    Don’t Forget To Take Good Notes And Keep Accurate Records

    We recommend always asking your veterinarian for a copy of diagnostic results so that you can include them in your residents’ records. It’s also important to take thorough notes about their interpretation of those results. If your veterinarian provides their interpretation in writing, simply print this out and file it along with the results. If they offer their interpretation in person or over the phone, be sure to take good notes so that this information is not lost. Regardless of how they provide their interpretation, be sure to ask questions if you don’t understand something.

    Culture and sensitivity testing can play an important role in disease diagnosis and treatment decisions, but it’s imperative that you have a good relationship with an experienced veterinarian so that you can make the most of these diagnostic tools. While we hope this resource has given you a basic understanding of how these tools are used, your veterinarian is going to be your best resource when it comes to assessing your residents’ health, recommending the most appropriate diagnostics, interpreting results, and making treatment decisions.


    Importance Of Antimicrobial Susceptibility Testing | Michigan State University Veterinary Diagnostic Laboratory 

    Types Of Veterinary Medical Tests | Merck Veterinary Manual

    Bacterial Culture And Antibiotic Susceptibility Testing | VetFolio 

    Interpretation of a Culture and Sensitivity Report | Jessica Thompson, PHARMD, BCPS (AQ-ID)       

    Microbiology Guide To Interpreting Minimum Inhibitory Concentration (MIC) | IDEXX 

    Antimicrobial Susceptibility Testing (AST) Resources | Michigan State University Veterinary Diagnostic Laboratory 

    Use Of Antimicrobial Susceptibility Data To Guide Therapy | Michigan State University Veterinary Diagnostic Laboratory

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