I do not know of another method, but could you use a more specific chelator such as EGTA and then quench with excess Ca2+ after treatment?

it might be too exotic, but Amp (or CTX) sensitive

bacteria can still grow in presence of such antibiotics,

resulting in L form - very little or no cell wall.

http://en.wikipedia.org/wiki/L-form_bacteria

We used to make spheroplasts , on the way to right-side-out membranes, by treating the E. coli cells with lysozyme 10mg/mL in 20% (w/v) sucrose.

Shalom Yoram :)

Sorry, EDTA was added... Memory fading...

The original reference was  Kaback H. (1971) Methods Enzymol. 22, 99–120.

Looks like I have no choice but to use EDTA...

Thank you all, for your answers