Acinetobacter calcoaceticus subsp. anitratus, which is unusually resistant to multiple antibiotics, was the cause of an epidemic of respiratory tract infections in patients in an intensive care unit. A representative isolate of the epidemic strain was found to contain the aminoglycoside-modifying enzymes 3-N-acetyltransferase, 3'-phosphotransferase, and 3"-adenylyltransferase, which confer resistance to gentamicin, kanamycin, and streptomycin, respectively. In addition, the strain produced a cephalosporinase and was resistant to penicillins due to the production of a TEM-2 beta-lactamase. The bacterial isolate also exhibited resistance to chloramphenicol, tetracycline, and sulfonamides. The resistant phenotype of this strain was similar to resistance patterns frequently observed in endemic hospital flora, suggesting that the transfer of an R plasmid into Acinetobacter sp. may have occurred. However, antibiotic resistance could not be transferred to any recipient by various mating procedures. After plasmid RP4 was transferred into an ampicillin- and kanamycin-susceptible derivative of the epidemic strain, mobilization of resistance to chloramphenicol, gentamicin, streptomycin, sulfonamides, and possibly tetracycline could be achieved. This mobilization was due to the transposition of a 16-megadalton DNA sequence from the Acinetobacter chromosome into plasmid RP4. Insertion of the transposable sequence occurred near the PstI and SmaI sites around position 22.5 on the physical map of plasmid RP4. We suggest that a plasmid resistant to multiple antibiotics was transferred from the hospital flora into Acinetobacter sp. but could not be maintained stably in this host. Instead, a multiply resistant DNA sequence was transposed and stably integrated into the Acinetobacter chromosome. The occurrence of such multiply resistant transposons on conjugative plasmids contributes greatly to the genetic variability of bacteria and may sometimes have serious epidemiological and therapeutic consequences.
A cleavage map of bacteriophage P1 DNA was established by reciprocal double digestion with various restriction endonucleases. The enzymes used and, in parenthesis, the number of their cleavage sites on the P1clts genome are: PstI (1), HindIII(3), BglII (11), BamHI (14) and EcoRI (26). The relative order of the PstI, HindIII and BglII sites, as well as the order of 13 out of the 14 BamHI sites and of 17 out of the 26 EcoRI sites was determined. The P1 genome was divided into 100 map units and the PstI site was arbitrarily chosen as reference point at map unit 20. DNA packaging into phage heads starts preferentially at map unit 92 and it proceeds towards higher map units. The two inverted repeat sequences of P1 DNA map about at units 30 and 34.
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