Restriction Digestion of Phage DNA

[Jay Mone']

As a laboratory experiment, I and some students have isolated several 
E. coli specific bacteriophages from untreated sewage.  We have 
isolated the phage DNA by treatment of purified phage preps by 
treatment with proteinase K and SDS, followed by phenol-chloroform 
extraction and precipitation with alcohol.  The problem is that all of 
the phage DNA preps appear to be completely resistant to restriction 
digestion.  I have tried several enzymes, including EcoRI, HindIII, 
PstI and KpnI, and never a hint of digestion?  What is going in here?  
I generally run lambda DNA as a control to ensure that the enzymes 
cut, so thats not the problem.  The 260/280 is consistently > 1.5.
Help!!

Jay Mone''