The Grass Doctor

Why did I get bacterial lawns on my agar dishes?

We did bacterial transformation in bio ab last week. We transformed E. Coli with the pGEX-2T plasmid DNA which enables ampicillin resistance. Our bacteria grew, but we did not get colonies, we got lawns of bacteria. Why is this so, and what exactly is the difference between colonies and lawns?

Public Comments

1. A "lawn" means that there are so many Amp resistant colonies that they have melted into one big layer of cells. There are two common causes: You could maybe have had a very extremely efficient transformation, or poor quality plates. If you did not do a ligation, but just transformed E. coli with a circular, ampR plasmid, the transformation will be very efficient. All molecules will make the E. coli AmpR. So usually one will plate out a dilution series (1 ul, 10 ul, 100 ul or even 1/10 ul). If you did a ligation (insert + linearized plasmid) the chance of getting a plasmid + insert is pretty small. So in that case you need to plate more transformants to see a couple of colonies. The second most common cause is poor quality plates. Ampicillin is not very stable, the plates could have been poured too hot (> 60 deg Celcius) or too old (I would not use anything older than 2 weeks).