Resistance to cationic antimicrobial peptides (e.g. polymyxin) by Gram negative bacteria is associated with chemical changes in the composition of the outer membrane lipopolysaccharide (LPS) that make it less negatively charged, such as by the addition of an amino sugar or phosphoethanolamine. This is determined by isolating the LPS and analyzing it chemically, and by genetic studies. Another mechanism by which some Gram negative bacteria become resistant is by ceasing to produce LPS entirely, replacing it with phospholipids. Efflux pumps can also be involved.

Here is a review article:

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4244539/

Thanks Adam!

     I am thankfully for your suggestion .But actually I am working on Gram positive bacteria so it is my humble request to give  me suggestion in this regard.

Dear Ram

We have analysed Gram+ cell walls and membrane. I particular we found the cationic cytochrome C as negative charge indicator handy. See attached article.

Hope it helps.

Marina

Article Cell-surface alterations in class IIa bacteriocin-resistant ...

Dear Ram and Marina,

Hopefully, you can use DLS (dynamic light scattering) for measuring zeta potential as well as FTIR for surface characterization. 

Hi Ram,

maybe you can measure the isoelectric point (IEP) of bacteria. I didn't do it but I came across the topic when I was studying a role of cell adhesion during biofilm formation. Search for these papers:

Rijnaarts, H.H.M., Norde, W., Lyklema, J., Zehnder, A.J.B.

The isoelectric point of bacteria as an indicator for the presence of cell surface polymers that inhibit adhesion

(1995) Colloids and Surfaces B: Biointerfaces, 4 (4), pp. 191-197

Van Der Wal, A., Norde, W., Zehnder, A.J.B., Lyklema, J.

Determination of the total charge in the cell walls of Gram-positive bacteria

(1997) Colloids and Surfaces B: Biointerfaces, 9 (1-2), pp. 81-100