Most agar contamination doesn’t come from bad technique during the pour itself – it comes from what was sitting on the surfaces and in the air around you beforehand. A still air box or flow hood helps, but it isn’t a substitute for actually sterilising your workspace properly first. Here’s the routine worth running every time, not just when something’s gone wrong before.
Clear and wipe down first
Remove everything from your working surface that isn’t needed for this pour – clutter is more surface area for spores to settle on. Wipe the entire work area, including the inside of a still air box or flow hood, with isopropyl alcohol before you bring any sterile equipment into the space. Let it fully evaporate rather than wiping it dry – the contact time is what does the work.
Sterilise your hands and gloves
Wash your hands before gloving up, then wipe gloved hands with isopropyl alcohol once your sterile surgical gloves are on – the outside of even a sterile glove picks up airborne contaminants the moment it’s exposed to the room. See our Nitrile Glove Buying Guide if you’re not sure which grade is right for this stage of the process.
Wipe down every item that enters the space
Petri dishes, your agar container, syringes, anything you’re bringing into the sterile zone should be wiped with alcohol immediately before it goes in – not assumed clean because it was clean the last time you used it. This includes the outside of sealed agar bottles and the exterior of any packaging.
Minimise movement and talking
Airflow disturbs settled spores and pushes them toward your open plates. Work with slow, deliberate movements, keep your face away from open dishes, and avoid talking directly over an open pour – all genuine, if unglamorous, sources of contamination.
Seal immediately after pouring
Once plates are poured and the agar has set, seal them straight away with parafilm rather than leaving them open any longer than necessary. Parafilm stretches to form a real seal around the dish rim, which keeps airborne contaminants out during the incubation period far better than an unsealed lid alone.
A pre-pour checklist
- Work surface and hood interior wiped with isopropyl alcohol, allowed to evaporate
- Sterile gloves on, wiped down after gloving
- All items entering the space wiped immediately before use
- Movement and talking kept to a minimum during the pour
- Plates sealed with parafilm as soon as they’ve set