Reduced antimicrobial potencies of oxytetracycline, tylosin, sulfadiazin, streptomycin, ciprofloxacin, and olaquindox due to environmental processes

The stability of oxytetracycline (OTC), tylosin (TYL), sulfadiazin (SDZ), streptomycin (ST), ciprofloxacin (CF) and olaquindox (O) was examined in environmentally relevant matrices, such as soil interstitial water and sewage sludge water. Compounds were assessed in both aerobic (OTC, TYL, SDZ, ST, and CF) and anaerobic experiments (OTC, TYL, and O) using analytical measurements (UV spectrophotometry or HPLC) combined with a growth inhibition pour plate assay using activated sludge bacteria. (OTC was additionally assessed using a soil bacterial assay.) This combination of results enabled the assessment of whether a loss in antibacterial potency was reflected in a similar reduction of substance concentration. If a potency reduction is not reflected in a decreased substance concentration, the results may indicate the formation of less potent degradation products possessing the same chromophoric system (same UV absorbance maximum) as the parent compound. With the exception of ST and CF, the antimicrobial agents generally lost a considerable amount of their antimicrobial potency in aerobic experiments. In the anaerobic experiments having either an experimental duration of 21 or 100 days only OTC retained potency. These results correspond well with the fact that several degradation products were encountered in the study for this compound

Toxicity of tetracyclines and tetracycline degradation products to environmentally relevant bacteria, including selected tetracycline-resistant bacteria

Tetracyclines used in veterinary therapy invariably will find their way as parent compound and degradation products to the agricultural field. Major degradation products formed due to the limited stability of parent tetracyclines (tetracycline, chlortetracycline, and oxytetracycline) in aqueous solution were theoretically identified at various environmental conditions, such as pH, presence of chelating metals, and light. Their potency was assessed on sludge bacteria, tetracycline-sensitive soil bacteria, and tetracycline-resistant strains. Several of the degradation products had potency at the same concentration level as tetracycline, chlortetracycline, and oxytetracycline on both the sludge and the tetracycline-sensitive soil bacteria. Further, both 5a,6-anhydrotetracycline and 5a,6-anhydrochlortetracycline had potency on tetracycline-resistant bacteria supporting a mode of action different from that of the parent compounds.

Effect of genomic location on horizontal transfer of a recombinant gene cassette between Pseudomonas strains in the rhizosphere and spermosphere of barley seedlings

The use of genetically engineered bacteria in natural environments constitutes a risk of transfer of recombinant DNA to the indigenous bacteria. However, chromosomal genes are believed to be less likely to transfer than genes on mobilizable and conjugative plasmids. To study this assumption, horizontal transfer of a recombinant gene cassette inserted into the chromosome of a Pseudomonas stutzeri strain, into a mobilizable plasmid (pAGM42), and into a conjugative plasmid (pKJK5) isolated from barley rhizosphere was investigated. Horizontal transfer efficiencies of the gene cassette inserted into a conjugative plasmid was 8.20 x 10(-3) transconjugants/(donors x recipients)1/2 in the rhizosphere and 4.57 x 10(-2) transconjugants/(donors x recipients)1/2 in the spermosphere. Mobilization of the plasmid pAGM42 by the plasmids RP4 and pKJK5 was also detected at high levels in the microcosms, transfer efficiencies were up to 4.36 x 10(-3) transconjugants/(donors x recipients)1/2. Transfer of chromosomal encoded genes could not be detected in the microcosms by conjugation or transformation. However, transformation did occur by using the same bacterial strains under laboratory conditions. The rhizosphere and especially the spermosphere thus proved to be hot spot environments providing favorable conditions for gene transfer by mobilization and conjugation, but these environments did not support transformation at a detectable level.

Methods for detection of conjugative plasmid transfer in aquatic environments

Donor and recipient counter selection was evaluated by selecting bacteria that received plasmid RP4 by conjugation on filters and in lake water microcosms. Three counter selection systems were compared; (i) Use of antibiotic-resistant recipients, (ii) use of an auxotrophic donor, and (iii) use of a donor with chromosomal suicide genes. Transfer efficiencies of transconjugants per recipient obtained with the three different counter selection systems in filter-matings were not significantly different. Some nalidixic acid-resistant recipients became partly sensitive to nalidixic acid after receiving the plasmid. Use of an auxotrophic donor was a feasible and easy way to recover indigenous transconjugants. A strain with two copies of the suicide gene gef was successfully eliminated in filter-matings, but elimination of the donor in microcosms by induction of the suicide genes did not succeed. Thus, this counter selection system was not usable in microcosm experiments.