Which technique was used to visualize the feline colonic biopsy and quantify the bacterial signals in the image described?

• A. FISH
• B. Immunohistochemistry
• C. Electron microscopy
• D. Gram stain

Answer: (A)

This item tests visualization of bacteria in tissue using a nucleic acid–based in situ technique that yields fluorescent signals suitable for quantification. Fluorescent in situ hybridization uses labeled probes that bind to conserved bacterial rRNA within the biopsy, producing bright fluorescent dots that can be seen with a fluorescence microscope. Because the probes target bacterial RNA, you can both visualize the exact location of bacteria in the colonic tissue and count the signals to quantify bacterial presence, all while keeping the tissue architecture intact to show relationship to the mucosa.

Immunohistochemistry could detect bacteria with antibodies, but it depends on antigen availability and may be less reliable for broad or precise quantification across diverse bacteria. Electron microscopy provides ultrastructural detail but does not rely on fluorescent labeling and is not typically used for routine in situ quantification of bacterial signals. Gram staining can reveal bacteria and give Gram status, but is less specific in fixed tissue and not as amenable to accurate, systematic quantification of bacterial burden.