The Gram stain, step by step
Thick peptidoglycan traps the violet–iodine complex, so gram-positives stay purple. Gram-negatives wash out at the alcohol step and pick up the pink counterstain. The whole thing takes about a minute.
Thick peptidoglycan traps the violet–iodine complex, so gram-positives stay purple. Gram-negatives wash out at the alcohol step and pick up the pink counterstain. The whole thing takes about a minute.
Each quadrant dilutes the inoculum further. By the last one you get single colonies — and a single colony is what you need for identification and susceptibility.
Most routine bacteria are readable the next morning. Identification and susceptibility add roughly another day — which is why empiric therapy exists.
The instrument watches every bottle around the clock and alarms the moment growth is detected. Then it is Gram stain first, phone the team, molecular panel next.
The bigger the clear zone, the more the organism was inhibited. Zone diameters are read against breakpoint tables — this method gives S/I/R, not a minimum inhibitory concentration.
Fluorescence doubles each cycle. The cycle where it crosses the threshold is the Ct — earlier Ct means more starting target. Fast and sensitive, but it detects nucleic acid, not whether the organism is alive.