Use of peracetic acid as a disinfectant in a water-treatment plant: effect on the plasmid contents of Escherichia coli strains.

Total thermotolerant coliforms (TTC) and Escherichia coli strains were isolated from sewage from a treatment plant before and after peracetic acid (PAA) disinfection. The plasmid profiles of 120 E. coli strains were analyzed. Although PAA disinfection effectively reduced the number of TTC and E. coli strains, the percentage of E. coli strains containing plasmids was not statistically different among water samples. The sizes of the plasmids found ranged from < 3 kb to > 56 kb, but plasmids of between 3 and 5 kb were encountered most frequently.

Conjugal transfer of natural plasmids between Escherichia coli strains in sterile environmental water.

Seven antibiotic-multiresistant Escherichia coli strains, possessing three or four plasmids, capable of transferring their resistance marker at a high frequency, were selected among a total of 300 antibiotic-resistant E. coli strains isolated from natural water--raw and treated wastewater, and brackish water (collected 1 km downstream). These strains were mated with E. coli K-12 C600 nalr, both in sterilized natural water and LB medium at 25 degrees C. Conjugation did occur in all the systems tested, although fewer transconjugants were recovered from raw and treated wastewater experiments. In contrast, in brackish and seawater, the transfer frequency did not significantly decrease in spite of salt contents. In 100% of the cases, transfer of the high-molecular-weight plasmids (> or = 20 kb) was observed, but the small plasmids (2.6-7.5 kb) were only cotransferred in raw or treated wastewater and in brackish water. Moreover, genotypic variation occurred more frequently in natural water than in LB medium.

Two-dimensional electrophoresis method used for determination of plasmid profiles of Escherichia coli isolated from a sewage treatment plant.

The plasmid contents of 306 strains of Escherichia coli isolated from raw and treated sewage from a treatment plant as well as from the water 1 km downstream were determined. The number and molecular weight of plasmids isolated from a representative sample of these strains were also measured. It was observed that biological treatment did not significantly alter these parameters. In contrast, statistically significant differences in plasmid number and size were observed when strains from sewage (raw and treated) were compared with strains isolated from brackish water. In raw and treated sewage, more than 96% of the strains contained plasmids, compared with 85% in brackish water. Nine to 13% of the strains from sewage contained plasmids with more than 56 kilobases, while only 5% of the strains from brackish water reached this size.

Susceptibilities of coagulase-negative staphylococci to aminoglycosides antibiotics as compared by early bactericidy assays.

Twenty four strains of coagulase-negative staphylococci from clinically significant blood cultures were separated in four groups on the basis of oxacillin susceptibility and phenotype of resistance to aminoglycoside antibiotics. Susceptibility testing included disk diffusion testing, M.I.C. and M.B.C. determinations and finally, measurements, at sequential times (from 0 to 6 hours), of the bactericidal effect of aminoglycosides alone or combined with oxacillin or vancomycin. Aminoglycosides clinical use (gentamicin, netilmicin, tobramycin and amikacin) exhibited an efficient early bactericidy over the sensitive strains to any tested aminoglycosides and to oxacillin (group 1, 13 strains). When strains having a phenotype of resistance to kanamycin and neomycin (group 2, 4 strains) or kanamycin and tobramycin (group 3, 2 strains) were considered, amikacin inefficiency was assessed by time-killing assay even though disk diffusion testing as well as M.I.C. and M.B.C. determinations failed to demonstrate it. In the same way, only bactericidy measurements indicated the lack of activity of amikacin or netilmicin over strains exhibiting a phenotype of resistance to gentamicin plus kanamycin and tobramycin (group 4, 5 strains). Netilmicin appeared as the most active aminoglycoside over the strains studied. Crude results of disk diffusion testing might appear as an oversimplification, especially when coagulase-negative staphylococci are considered; the early-bactericidy assay findings were stressing for the need of interpreting the results of antibiotic testing on the ground of resistance mechanisms.