S U M M A R YR factors transmissible to Escherichia coli from naturally occurring strains of Serratia marcescens belonged either to compatibility groups ( S and L) not represented amongst plasmids reported in other genera or to groups (C, FII, P and M) notable for their wide host range. We have collected strains of S. marcescens, most, but not all, clinical isolates, from widely separate geographical areas; each strain was tested for antibiotic resistance and for R factors transmissible to E. coli K 1 2 . The R factors were classified by compatibility in K 1 2 (Datta, 1974). Our purpose was to find out how much of the antibiotic resistance observed in
M E T H O D SSerratia marcescens. The sources of the 236 strains of S. marcescens studied are shown in Antibiotic resistance patterns were scored using a disc method (Hedges, 1974). Transfer of resistance to Escherichia coli ~1 2 .A rifampicin-resistant mutant of E. coli ~1 2 , 562-2 (Hedges, 1974) was used as recipient in tests for transfer. Methods were as described by Coetzee et al. (1972).
Transfer of plasmids of group S is much more efficient at low temperatures (e.g. 22 "C) than at 37 "C. This is due to failure of the donor strain to produce the transfer system during growth at the higher temperature.
I N T R O D U C T I O NA new class of transmissible plasmids has been identified in strains of Serratia marcescens. The plasmids constitute the compatibility class S (Hedges, Rodriguez-Lemoine & Datta, They were transferred from wild-type S. marcescens isolates into strains of E. coli K I~ by conjugation. There was little or no transfer of these plasmids between strains of E. coli K I~ when incubation was at 37 "C. When, however, the incubation was at a lower temperature transfer proceeded at a much higher rate. We describe the identification of the temperaturesensitive step in the mating process.
1975). METHODSBacteria. Both strains used in these experiments were nutritionally distinguishable derivatives of E. coli K12. They were ~5 3 (F-, pro, met) and 562.2 (F-, pro, his, trp, lac, r f ) . Plasmids. R477 is an R factor of group S which determines resistance to streptomycin, tetracycline, chloramphenicol, kanamycin and sulphonamides. It and other S plasmids were described by Hedges et al. (1975).Temperature shvt experiments. The donor culture, ~62.2(R477), was grown overnight in nutrient broth at 37 "C. It was diluted, usually I : 100, in nutrient broth and incubated at the desired temperature for at least 2 h. Cultures were diluted with fresh broth at appropriate intervals in order to maintain the cell concentration at approximately 108 cellslml, as measured by a Celloscope 302 particle counter (Lars Ljungberg and Co., Stockholm, Sweden).After preliminary experiments, which showed that the conditions under which the recipient culture was grown before mating had little effect on the efficiency of transfer, recipient cultures were routinely grown to a concentration of approximately 109 cellslml at the temperature to be used during conjugation.Mating mixtures were set up by mixing 4-5 ml of the recipient culture with 0.5 ml of the donor culture and adding to this mixture 5 ml of fresh, pre-warmed broth. Mating mixtures
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.