[Control of the technic for recovery of E coli K12 after acquisition of R factors. Tests with various culture substrates under different mating conditions].

A survey was carried out in order to ascertain the reliable conditions for the recovery of E. coli K12 after acquisition of extrachromosomial resistance from polyresistant strains of Salmonella bredeney, responsible for a hospital outbreak of gastroenteritis. In particular in this study the length of mating, the amounts of partners in the cross-cultures, the availability of different media for selection of recipient E. coli, were controlled. It was possible to conclude that the incubation of cross-cultures during 18 hours at 37 degrees C was suitable for a greater transfer, whereas the amount of donor and recipient cells in the starting cultures was of little importance. Attempts to use synthetic media for the selective recovery of E. coli which had been made drug-resistant were not successful; instead some of lactosate media which are classically employed for the diagnosis of enterobacteria (Endo agar, Mac Conkey agar, Desoxycholate Lactose agar, Brilliant Green Bile agar, Eosin Methylene Blue agar, Violet Red Bile agar, Wurtz agar) gave satisfactory results. Among these media the two last were the best because they provided good differentiation between the prevalent Salmonella colonies and the few lactose-fermenting colonies, even in the crowded growth areas of the plates.