Scoping the effectiveness and evolutionary obstacles in using plasmid-dependent phages to fight antibiotic resistance.

AIM: To investigate the potential evolutionary obstacles in the sustainable therapeutic use of plasmid-dependent phages to control the clinically important conjugative plasmid-mediated dissemination of antibiotic resistance genes to pathogenic bacteria. MATERIALS & METHODS: The lytic plasmid-dependent phage PRD1 and the multiresistance conferring plasmid RP4 in an Escherichia coli host were utilized to assess the genetic and phenotypic changes induced by combined phage and antibiotic selection. RESULTS & CONCLUSIONS: Resistance to PRD1 was always coupled with either completely lost or greatly reduced conjugation ability. Reversion to full conjugation efficiency was found to be rare, and it also restored the susceptibility to plasmid-dependent phages. Consequently, plasmid-dependent phages constitute an interesting candidate for development of sustainable anticonjugation/antiresistance therapeutic applications.

Conjugation is necessary for a bacterial plasmid to survive under protozoan predation.

Horizontal gene transfer by conjugative plasmids plays a critical role in the evolution of antibiotic resistance. Interactions between bacteria and other organisms can affect the persistence and spread of conjugative plasmids. Here we show that protozoan predation increased the persistence and spread of the antibiotic resistance plasmid RP4 in populations of the opportunist bacterial pathogen Serratia marcescens. A conjugation-defective mutant plasmid was unable to survive under predation, suggesting that conjugative transfer is required for plasmid persistence under the realistic condition of predation. These results indicate that multi-trophic interactions can affect the maintenance of conjugative plasmids with implications for bacterial evolution and the spread of antibiotic resistance genes.

Protist predation can select for bacteria with lowered susceptibility to infection by lytic phages.

BACKGROUND: Consumer-resource interactions constitute one of the most common types of interspecific antagonistic interaction. In natural communities, complex species interactions are likely to affect the outcomes of reciprocal co-evolution between consumers and their resource species. Individuals face multiple enemies simultaneously, and consequently they need to adapt to several different types of enemy pressures. In this study, we assessed how protist predation affects the susceptibility of bacterial populations to infection by viral parasites, and whether there is an associated cost of defence on the competitive ability of the bacteria. As a study system we used Serratia marcescens and its lytic bacteriophage, along with two bacteriovorous protists with distinct feeding modes: Tetrahymena thermophila (particle feeder) and Acanthamoeba castellanii (surface feeder). The results were further confirmed with another study system with Pseudomonas and Tetrahymena thermophila. RESULTS: We found that selection by protist predators lowered the susceptibility to infections by lytic phages in Serratia and Pseudomonas. In Serratia, concurrent selection by phages and protists led to lowered susceptibility to phage infections and this effect was independent from whether the bacteria shared a co-evolutionary history with the phage population or not. Bacteria that had evolved with phages were overall more susceptible to phage infection (compared to bacteria with history with multiple enemies) but they were less vulnerable to the phages they had co-evolved with than ancestral phages. Selection by bacterial enemies was costly in general and was seen as a lowered fitness in absence of phages, measured as a biomass yield. CONCLUSIONS: Our results show the significance of multiple species interactions on pairwise consumer-resource interaction, and suggest potential overlap in defending against predatory and parasitic enemies in microbial consumer-resource communities. Ultimately, our results could have larger scale effects on eco-evolutionary community dynamics.

Evolutionary rescue of bacteria via horizontal gene transfer under a lethal ß-lactam concentration.

ß-Lactams are a commonly used class of bactericidal antibiotics. The number of ß-lactam-resistant pathogens is constantly increasing in hospitals around the world. Interestingly, most of the ß-lactam-resistant bacteria carry mobile genetic elements, such as conjugative plasmids, that render the pathogen resistant. These elements mediate their own transfer from one bacterium to another, producing new resistant strains via horizontal gene transfer. Here we investigated whether it is possible that transfer of the resistance element from another bacterium may evolutionarily rescue a susceptible bacterium exposed to a lethal concentration of the ß-lactam ampicillin. Indeed, the rescuing occurs even at very high, clinically significant antibiotic levels, suggesting that pathogens may acquire the resistance 'on the fly' from commensal bacteria during treatment.

Fight evolution with evolution: plasmid-dependent phages with a wide host range prevent the spread of antibiotic resistance.

The emergence of pathogenic bacteria resistant to multiple antibiotics is a serious worldwide public health concern. Whenever antibiotics are applied, the genes encoding for antibiotic resistance are selected for within bacterial populations. This has led to the prevalence of conjugative plasmids that carry resistance genes and can transfer themselves between diverse bacterial groups. In this study, we investigated whether it is feasible to attempt to prevent the spread of antibiotic resistances with a lytic bacteriophage, which can replicate in a wide range of gram-negative bacteria harbouring conjugative drug resistance-conferring plasmids. The counter-selection against the plasmid was shown to be effective, reducing the frequency of multidrug-resistant bacteria that formed via horizontal transfer by several orders of magnitude. This was true also in the presence of an antibiotic against which the plasmid provided resistance. Majority of the multiresistant bacteria subjected to phage selection also lost their conjugation capability. Overall this study suggests that, while we are obligated to maintain the selection for the spread of the drug resistances, the 'fight evolution with evolution' approach could help us even out the outcome to our favour.