Every experiment in biology rests on one skill: keeping unwanted microbes out. Master this first and half of all "why did it fail?" problems disappear.
The air, your skin, and every surface around you carry microorganisms. When you culture cells, you're offering those hitchhikers a warm, nutrient-rich home too. Sterile (aseptic) technique is the set of habits that keep your cultures pure — so what grows on your plate is what you put there.
Why 70% and not 100%? A little water helps alcohol penetrate and denature microbial proteins — 70% actually disinfects better than pure alcohol.
Remove everything you don't need. Spray the surface with 70% alcohol and wipe outward from the center with a paper towel. Let it air-dry — the contact time is what disinfects.
Put on nitrile gloves, then spray or wipe them with alcohol too. Re-disinfect any time you touch your face, phone, or a non-sterile surface.
Work inside a still-air box, or within the rising warm air near a flame (the "sterile cone"). Arrange items so you never have to reach over an open plate or tube.
Lift a petri dish lid only partway, like a clamshell, and only while you're actively working. Keep tubes angled and capped until the moment you need them.
Pass a metal inoculation loop through a flame until it glows, then let it cool for a few seconds before touching cells. With single-use plastic loops, take a fresh sterile one each time.
Don't let the mouth of a tube, the inside of a lid, or the tip of a pipette touch your fingers, the bench, or the air any longer than necessary. When in doubt, discard and start with a fresh sterile item.
Reseal everything, wipe the bench with alcohol again, and dispose of or decontaminate anything that touched live culture (see disposal, below).
| You see… | Likely cause | Fix |
|---|---|---|
| Fuzzy or coloured spots (mold) on plates | Airborne contamination | Reduce open time; work in a still-air box; check plates aren't left ajar. |
| A "lawn" of unexpected growth everywhere | Tool or reagent not sterile | Flame/replace loops each use. Confirm plates and media were properly sterilized. |
| Contamination only near where you reached | Reaching over open containers | Rearrange your layout. Keep hands to the side, never above. |
Treat anything that touched live culture as biohazardous even though these organisms are harmless (BSL-1). Submerge used plates and tubes in a 10% household bleach solution for 20+ minutes, or pressure-cook/autoclave them, before binning. Wipe surfaces with alcohol when you finish.
The Microbiology Starter Kit is the gentlest way to build these habits — plates, loops, and a first organism to culture.