Which FISH probe was used for total bacterial counts?

• A. Eub338-FITC
• B. Eub338-PE
• C. Uni338-FITC
• D. Bacteroides-FISH

Answer: (A)

Total bacterial counts in FISH are typically assessed with a universal bacterial probe that binds to a conserved region of bacterial 16S rRNA, allowing detection of nearly all bacterial cells in a sample. The Eub338 probe is designed to target that broad range of bacteria, so labeling it with a fluorophore like FITC provides a green signal that marks the entire bacterial population. This makes it the standard choice for counting all bacteria in the sample.

In contrast, a probe labeled for Bacteroides would only highlight the Bacteroides group, not the whole bacterial community, so it wouldn’t give a total count. Another universal probe variant might exist, but the commonly used combination for total counts, and the one indicated here, is Eub338 labeled with FITC. The choice with a different fluorophore (such as PE) would detect the same target but in a different fluorescence channel; the fundamental concept—universal bacterial detection—remains the same.