Impact of co-carriage of IncA/C plasmids with additional plasmids on the transfer of antimicrobial resistance in Salmonella enterica isolates.

BACKGROUND: Antimicrobial resistance in Salmonella enterica is often plasmid encoded. A key resistance plasmid group is the incompatibility group (Inc) A/C plasmids that often carry multiple resistance determinants. Previous studies showed that IncA/C plasmids were often co-located with other plasmids. The current study was undertaken to evaluate the impact of plasmid co-carriage on antimicrobial resistance and plasmid transfer. METHODS: A total of 1267 Salmonella isolates, representing multiple serotypes and sources were previously subjected to susceptibility testing and 251 isolates with resistance to at least 5 antimicrobial agents were identified for further study. Each isolate was subjected to PCR-based replicon typing, and those with IncA/C plasmids were selected for plasmid isolation, PCR-based mapping of IncA/C plasmid backbone genes, and conjugation assays to evaluate resistance plasmid transferability. RESULTS: Of the 87 identified IncA/C positive isolates, approximately 75% carried a plasmid with another identified replicon type, with the most common being I1 (39%), FIA, FIIA, FIB and HI2 (each 15%). PCR-based mapping indicated significant diversity in IncA/C backbone content, especially in regions encoding transfer-associated and hypothetical proteins. Conjugation experiments showed that nearly 68% of the isolates transferred resistance plasmids, with 90% containing additional identified plasmids or larger (>50 kb) non-typeable plasmids. CONCLUSIONS: The majority of IncA/C-positive strains were able to conjugally transfer antimicrobial resistance to the recipient, encoded by IncA/C and/or co-carried plasmids. These findings highlight the importance of co-located plasmids for resistance dissemination either by directly transferring resistance genes or by potentially providing the needed conjugation machinery for IncA/C plasmid transfer.

Potential of Plant Essential Oils and Their Components in Animal Agriculture - in vitro Studies on Antibacterial Mode of Action.

The antimicrobial activity of essential oils and their components has been recognized for several years. Essential oils are produced as secondary metabolites by many plants and can be distilled from all different portions of plants. The recent emergence of bacteria resistant to multiple antibiotics has spurred research into the use of essential oils as alternatives. Recent research has demonstrated that many of these essential oils have beneficial effects for livestock, including reduction of foodborne pathogens in these animals. Numerous studies have been made into the mode of action of essential oils, and the resulting elucidation of bacterial cell targets has contributed to new perspectives on countering antimicrobial resistance and pathogenicity of these bacteria. In this review, an overview of the current knowledge about the antibacterial mode of action of essential oils and their constituents is provided.

Inhibition of beef isolates of E. coli O157:H7 by orange oil at various temperatures.

UNLABELLED: Plant essential oils have previously been shown to exhibit antimicrobial activities against various microorganisms. In this study, cold pressed terpeneless Valencia orange oil (CPTVO) was examined at various temperatures (37, 10, and 4 °C) to determine its antimicrobial activity against 3 strains of E. coli O157:H7 recovered from beef products. A micro broth dilution method using 96 well microtiter plates was used with trypticase soy broth with 0.15% agar and 2,3,5 tetrazolium chloride as a growth indicator as the medium. Serial dilutions of CPTVO were made, resulting in final concentrations of oil ranging from 0.2% to 25% or 0.1% to 10%. Plates were incubated statically at 4, 10, or 37 °C, and sampled hourly. After 6 h at 37 °C, all strains were inhibited at concentrations ranging from 0.2% to 0.6%, with a mean of 0.4 ± 0.01%. At 10 °C, all strains were inhibited at concentrations ranging from 0.8% to 6.3%, with a mean of 1.1% ± 0.2%, after 6 h. At 4 °C, all strains were inhibited after 6 h at concentrations ranging from 2.3% to 4.6%, with a mean of 3.5% ± 2.1%. After 24 h at 4 °C the strains were inhibited at concentrations ranging from 0.7% to 1% with a mean of 0.8% ± 0.3%. The ranges appear to be the result of effects from the variable nature of a complex media and an antimicrobial that presents potential multiple mechanisms for inhibition. It appears CPTVO is a viable option to inhibit E. coli O157:H7 growth at refrigeration temperatures. PRACTICAL APPLICATION: Beef products are often the source of foodborne illness from the organism E. coli O157:H7. Orange essential oils have been in the human diet for centuries, and the research reported here indicates that some of these oils may be used as surface applications during cold temperatures to inhibit the foodborne pathogen E. coli O157:H7.