In vivo development of amoxicillin and ceftriaxone resistance in Salmonella enterica serovar Typhi

Overcoming reduced antibiotic susceptibility in intracellular Salmonella enterica serovar Typhimurium using AR-12

Host-directed therapies (HDTs) could enhance the activity of traditional antibiotics. AR-12 is a promising HDT against intracellular pathogens including Salmonella enterica serovar Typhimurium, and has been shown to act through modulation of autophagy and the Akt kinase pathway. Since AR-12 does not inhibit the growth of planktonic bacteria but only works in conjunction with the infected host-cell, we hypothesized that AR-12 could enhance the activity of antibiotics in less-susceptible strains in the intracellular host environment. We found that repetitive passaging of S. typhimurium in macrophages in the absence of antibiotics led to a 4-fold reduction in their intracellular susceptibility to streptomycin (STR), but had no effect on the bacteria's sensitivity to AR-12. Moreover, when the host-passaged strains were treated with a combined therapy of AR-12 and STR, there was a significant reduction of intracellular bacterial burden compared to STR monotherapy. Additionally, co-treatment of macrophages infected with multi-drug resistant S. typhimurium with AR-12 and STR or ampicillin showed enhanced clearance of the intracellular bacteria. The drug combination did not elicit this effect on planktonic bacteria. Overall, AR-12 enhanced the clearance of less susceptible S. typhimurium in an intracellular environment.

Molecular characterization of plasmids encoding blaCTX-M from faecal Escherichia coli in travellers returning to the UK from South Asia

BACKGROUND

The global prevalence of extended-spectrum beta-lactamase-producing Escherichia coli is rising and is dominated by bla_CTX-M spread by plasmids. Travellers to South Asia from Western Europe have high rates of acquisition of faecal CTX-M-producing E. coli (CTX-M-EC).

AIMS

To determine the conjugative ability of CTX-M-EC acquired by healthy volunteers after travel to South Asia, the proportion of travel-acquired CTX-M-EC where bla_CTX-M is encoded on a plasmid vs on the bacterial chromosome, and the relatedness of travel-acquired CTX-M-EC plasmids to previously sequenced plasmids.

METHODS

Faecal samples were collected pre- and post-travel from 23 volunteers who visited South Asia, and CTX-M-EC were cultured. After short- and long-read sequencing, 10 plasmid sequences were identified and compared with previously sequenced plasmids in GenBank. Conjugation to E. coli K-12 was undertaken using filter mating.

FINDINGS

Thirty-five percent of CTX-M-EC isolates tested transferred the bla_CTX-M plasmid by conjugation. Travel-acquired CTX-M-EC carried bla_CTX-M on a plasmid in 62% of isolates, whereas 38% of isolates had bla_CTX-M on the chromosome. CTX-M-EC plasmids acquired after travel to South Asia had close homology to previously described epidemic plasmids which are widely disseminated in humans, animals and the natural environment.

CONCLUSION

Globally successful epidemic plasmids are involved in the spread of CTX-M-EC. Targeted strategies may be used to displace such plasmids from the host strain as part of efforts in infection prevention and control in healthcare settings. Bacteria with bla_CTX-M plasmids were readily acquired by healthy volunteers, and were carried on return to the UK, providing opportunities for onward dissemination.