“…It is necessary to postulate only one transfer event, since each of the septic incidents was probably due to staphylococci that were all essentially the same strain. Changes in the antibiotic sensitivity in this strain were almost certainly due to variations in the carriage of plasmids (Lacey, Lewis, and Harrison, 1974). Consistent with the proposal that enhancement of virulence had resulted from acquisition of a relatively large piece of DNA is the finding that in vitro the decrease of virulence associated with carriage of several plasmids was probably caused by the loss of such DNA (Lacey and Chopra, 1974).…”