22 
and should not be construed to encompass all possibilities . 
The probability of establishing X lysogeny in the normal 
laboratory host should be reduced by removal of the phage 
att site j the Int function 3 the repressor gene(s) and adding 
virulence -enhancing mutations. The frequency of plasmid for- 
mation 3 although normally already less than 10~ 3 could be 
further reduced by defects in the p ^-Q_ region 3 including 
mutations such as vir- s 3 cro (TS) 3 cl? 3 ri° , OJTS) 3 P_(TS) 3 
and nin . Moreover 3 chloroform treatment used, routinely 
following cell lysis would reduce the number of surviving 
cells 3 including possible lysogens or plasmid carriers 3 by 
O 
more than 10 . The host may also be modified by deletion of 
the host X att site and inclusion of one or more of the muta- 
tions described above for plasmid-host systems to further 
reduce the chance of formation and survival of any lysogen 
or plasmid carrier cell . 
The survival of escaping phage and the chance of 
encountering a sensitive host in nature are very low 3 as 
discussed for EK1 systems. The infectivity of the phage 
particles could be further reduced by introducing mutations 
(e.g. 3 suppressed ambers) which would make the phage particles 
extremely unstable except under special laboratory conditions 
(e.g. 3 high concentrations of salts or putrescine) . Another 
means would be to make the phage itself a two-component 
system 3 by eliminating the tail genes and reproducing the 
[ 93 ] 
