Hi there ^^
I want to share with you this protocol. It's quite effective, so you can prepare bacteria as competent as 5x10^6 cfu/ug DNA (it's the most common value I've got with this protocol), with little material, and get plenty of single use aliquots, so you can avoid freezing/thawing cycles which lower the efficiency.
CaCl2 0.1M, ice cold and filtered
MgCl2 0.1M, ice cold and filtered
Storage solution: 85% CaCl2 0.1M, 15% Glycerol, ice cold and filtered
You'll be needing a centrifuge capable of 4000 rpm at 4ºC, and 50mL falcon adapters
Some advice before starting: Gather ice... plenty of ice, because it's a protocol that lasts 3-4 hours to complete, and the whole time you MUST have the bacteria and the solutions on ice
1.- Culture your bacteria overnight in 4mL of your growing medium. A 1:1000 bacteria:medium inoculation works great.
2.- Inoculate 200 mL of new, sterile growing medium with 2 mL of your overnight culture and grow it until OD600 ~ 0.6
*From now on, everything must be done on ice. Falcons and Eppendorfs must be ice cold when used*
3.- Put the 200mL of the culture in four 50mL falcon and place them on ice for 10 minutes.
4.- Centrifuge 10 minutes at 3000 rpm at 4ºC. Discard supernatant and place the pellet quickly on ice.
[From now on, every volume given for resuspension must be used for every 50mL of culture you had]
5.- Gently resuspend the pellet in 12.5 mL ice cold MgCl2 0.1M. The more glently you resuspend them, the greater the efficiency.
6.- Incubate on ice for 5 minutes
7.-Centrifuge 10 minutes at 4000 rpm at 4º C. Discard supernatant and place the pellet quicky on ice.
8.- Gently resuspend the pellet in 2.5 mL ice cold CaCl2 0.1M
9.- Incubate on ice for 20 minutes
10.- Centrifuge 10 minutes at 4000 rpm at 4ºC. Discard supernatant and place the pellet quicky on ice.
11.- Gently resuspend the pellet in 1 mL storage solution. Aliquote in 100 uL fractions in 1.5 mL ice cold eppendorfs, and store directly at -80 ºC.
Hope it helps!