Effects of ethidium bromide on growth and on loss of the penicillinase plasmid of Staphylococcus aureus.

Ethidium bromide (EB) was more efficient than ethyl violet or rifampin as a curing agent for the penicillinase plasmids of Staphylococcus aureus strains. The effects of EB on growth and on the loss of the penicillinase plasmid of PS 81 were studied in detail. The growth rates of PS 81 and an EB-cured derivative were identical in broth, but the cured derivative had a shorter lag in the presence of added 6 x 10(-6)m EB. The shortened lag was due to prior exposure to EB as the cured derivative and an EB-treated but uncured strain of PS 81 gave identical growth lag and growth rates in the presence of EB. The curing of PS 81 by EB occurs in three phases. After a 4 to 5 hr lag, there is a 100-fold increase in the number of penicillinase-negative cells, and the proportion of cured cells continues to rise until 10 to 12 hr. Thereafter, the population becomes refractory to further curing, and the proportion of penicillinase-negative cells remains constant at about 20% of the total. Penicillinase-positive survivors of EB treatment showed increased EB resistance and were cured at lower rates upon subsequent EB treatment. Isolated colonies of the parental strain PS 81 were heterogeneous in their EB sensitivity. Thus, EB does not competitively favor spontaneously cured penicillinase-negative cells but appears to act in a manner analogous to acridine orange on the plasmids of enteric bacteria.