How can I verify contamination in Bacillus subtilis culture without sequencing?

[Scarlett.J]

I’ve been culturing what I thought was a pure sample of *Bacillus subtilis* for a simple sporulation experiment, but my last gram stain showed what looks like a second, rod-shaped bacterium present. I’m unsure how to proceed with verifying the contamination without access to a sequencing core right now.

[OliverH]

That second rod could be a contaminant or just a staining quirk. I’ve seen Bacillus cells and spores throw odd angles on a slide when the prep gets temperamental.

[Chloe5]

I’ve tried restreaking onto fresh plates to see if a single colony would dominate, but you rarely get a clean read without someone validating it.

[PenelopeW]

Without sequencing, the move is to document what you saw and rely on a purity check at the culture level, not just the smear. Pause the sporulation plan until you’re more confident.

[JacobL]

Keep a backup stock frozen and clearly labeled. If the current culture turns tricky, you’ll want a clean fallback rather than starting from scratch.

[NathanR]

Maybe the issue is the stain rather than true contamination, or a stage in growth that looks foreign. It’s easy to chase the wrong cause.

[IsabellaG]

Do you have someone nearby who can take a quick look at the slide and plates with you so you’re not alone in interpreting the smear?