APPENDIX H 
(1) The probability (P ) that the EK2 cell will encounter a 
1 
bacterium that harbors a mobilizing plasmid and is capable of con- 
jugating under the prevailing environmental conditions. 
(2) The probability that the mobilizing plasmid will be trans- 
ferred to the EK2 cell. This can be measured experimentally under 
optimal mating conditions in vitro , and expressed as the transfer 
rate, R . 
1 
(3) The probability (P ) that an EK2 cell which has received a 
2 
mobilizing plasmid will encounter a competent recipient bacterium, 
and conjugate with it under prevailing environmental conditions. 
(4) The probability that the EK2 plasmid will be mobilized and 
transferred to the recipient. This can be measured under optimal 
mating conditions in vitro, and expressed as the transfer rate, R . 
2 
In actual experiments* on transfer between E. coli strains in 
vivo using host-plasmids for which R and R are both very high 
1 2 
-2 
(>10 ), the observed transfer frequencies (roughly corresponding 
-9 
to P X R , and P x R , respectively) were less than 1x10 , 
11 2 2 
-7 
suggesting that P and P are each less than 1x10 . Since, 
1 2 
however, much higher values might obtain under other conditions 
(e.g., in sewage, or in animals fed antibiotics), it would seem 
*S. Falkow, personal communication. 
Appendix H--7 
