What happens to a sample once it is submitted for culture?

Check out our process: Part 1 and Part 2.

Why is it important to include case history?

Including a brief clinical history on laboratory submissions is critical for a multitude of reasons. One of the most important being that it enables diagnosticians to assemble a complete differential diagnosis list and thus facilitates appropriate test selection up front to ensure rapid turnaround of results.  Safety and rapid diagnosis of zoonotic agents is another reason: For instance, tissues submitted from a cat with acute illness, high fever and history of contact with rabbits would alert the pathologist to request Francisella tularensis (tularemia) PCR testing, a test which would not be routinely ordered in the absence of case history or lesions (if tissues were included for histopathology). If a select agent is suspected (Brucella spp., Bacillus anthracis, F. tularensis, Y. pestis) it is important to contact the laboratory to ensure the sample is handled properly. See our Anthrax, Plague, and Tularemia Submission Guidelines document for more information.

Along with appropriate test selection clinical history is also necessary for interpretation of some results. It is particularly important for bacteriology where the clinical history is used to ensure the specimen is set up on the appropriate type of media(s) and culture condition(s). Many bacteria require special media for growth and unless adequate clinical history is submitted a specimen may not be cultured for specific bacteria. For more examples of how clinical history informs culture setup and interpretation, check out this article from the American Society for Microbiology (ASM).  Thorough clinical case histories help the lab help you make the right diagnosis.

An organism was isolated, but there was a comment stating “Isolate not viable for antimicrobial susceptibility testing (AST)”. What does this mean?

It means that the isolate was able to grow in the culture, well enough that we could identify it, but when we tried to perform AST, the isolate would not grow by our standard methods. It could mean that the bacteria was fastidious or had potentially been damaged by previous treatment or transport.

You isolated a bacteria but did not perform AST. Why wasn’t it performed?

Certain bacteria do not grow well by standard broth dilution methods used for AST or standards for interpretation of results have not been developed by a reputable source, such as the Clinical and Laboratory Standards Institute, meaning we have no way to interpret the growth of the organism.

Alternatively, the organism may have predictable susceptibility patterns to commonly used antimicrobials, such as beta-hemolytic Streptococcus spp. (eg. S. canis, S. equi subsp. equi, S. equi subsp. zooepidemicus, S. felis) to beta-lactam class antibiotics. In other cases, such as Corynebacterium pseudotuberculosis, the causative agent of caseous lymphadenitis, treatment is not effective.

Why didn’t you perform antimicrobial susceptibility testing on the E. coli from an intestinal/fecal source?

E. coli is normal flora in the gastrointestinal tract. Current practice methods recommend supportive therapy for diarrhea in most cases for humans, food animals, and companion animals. This includes diarrhea caused by Salmonella sp. or toxigenic E. coli. Antimicrobial treatment not only wipes out normal gut flora which makes it harder for the animal to recover from illness, but also contributes to antimicrobial resistance in our normal flora which can make later treatment of significant infection more challenging. Additionally, interpretations for AST have not been developed by the Clinical and Laboratory Standards Institute (due to lack of clinical efficacy), meaning we have no way to interpret the growth of the organism so the results have little value and cannot be used for treatment planning.

My report says “2 morphotypes”. What does this mean?

When we isolate colonies of bacteria that look distinctly different from each other but are in significant amounts and identify as the same organism, eg., Escherichia coli, we perform AST separately on each colony type. Sometimes the AST results are the same, in which case we only report the one result and do not charge the client for both tests. Often the isolates have different susceptibility results. We cannot accurately mix the colonies for AST and ensure that the isolate does not appear more or less resistant because of differing amounts of each type used for testing. Inaccurate AST results could mean treatment of the infection with a broader spectrum than necessary or treating it with an antimicrobial that is ineffective against that isolate; both possibilities contribute to the ongoing public health issue of antimicrobial resistance.