Efficacy of a ciprofloxacin/amikacin combination against planktonic and biofilm cultures of susceptible and low-level resistant Pseudomonas aeruginosa

Effect of colistin-based antibiotic combinations on the eradication of persister cells in Pseudomonas aeruginosa

OBJECTIVES

Persister cells are responsible for antibiotic treatment failure and the emergence of antibiotic resistance. The synergistic lethal effects of antibiotic combinations on persister cells were investigated using Pseudomonas aeruginosa isolates.

METHODS

Persister assays were performed on P. aeruginosa clinical isolates using colistin, amikacin, ciprofloxacin and cefepime, individually and in combination. ATP concentrations were measured and morphological changes in persister cells were observed using transmission electron microscopy (TEM). The expression of relA, spoT and obg genes was evaluated and persister-cell formation was investigated in a relA and spoT double mutant (ΔrelAΔspoT).

RESULTS

The P. aeruginosa persister cells were eradicated upon exposure to the colistin-based antibiotic combination colistin + ciprofloxacin. Simultaneous treatment with both antibiotics, rather than sequential treatment, caused more effective eradication. The intracellular ATP concentration was most reduced in colistin persisters. While the spoT gene was only overexpressed in colistin-persister cells, the relA gene was overexpressed in all persister cells compared with untreated parent cells. TEM analysis suggested the possibility that persister cells might be formed by different mechanisms depending on the antibiotic. Cell elongation and cell wall or membrane damage in colistin persisters, DNA condensation in amikacin persisters and outer membrane vesicles in ciprofloxacin persisters were identified.

CONCLUSIONS

In P. aeruginosa, the colistin-based antibiotic combination (colistin + ciprofloxacin) was effective for the eradication of persister cells, probably due to the different persister cell-formation mechanisms between the two antibiotics. Simultaneous, rather than sequential, treatment with two antibiotics could be more effective for eradicating persister P. aeruginosa cells.

Co-delivery of ciprofloxacin and colistin using microcontainers for bacterial biofilm treatment

In many infected patients, bacterial biofilms represent a mode of growth that significantly enhances the tolerance to antimicrobials, leaving the patients with difficult-to-cure infections. Therefore, there is a growing need for effective treatment strategies to combat biofilm infections. In this work, reservoir-based microdevices, also known as microcontainers (MCs), are co-loaded with two antibiotics: ciprofloxacin hydrochloride (CIP) and colistin sulfate (COL), targeting both metabolically active and dormant subpopulations of the biofilm. We assess the effect of the two drugs in a time-kill study of planktonic P. aeruginosa and find that co-loaded MCs are superior to monotherapy, resulting in complete killing of the entire population. Biofilm consortia of P. aeruginosa grown in flow chambers were not fully eradicated. However, antibiotics in MCs work significantly faster than simple perfusion of antibiotics (62.5 ± 8.3% versus 10.6 ± 10.1% after 5 h) in biofilm consortia, showing the potential of the MC-based treatment to minimize the use of antimicrobials in future therapies.

Tolerance and Persistence of Pseudomonas aeruginosa in Biofilms Exposed to Antibiotics: Molecular Mechanisms, Antibiotic Strategies and Therapeutic Perspectives

Pseudomonas aeruginosa biofilm-related infections are difficult to treat with antibiotics. Along the different layers of the biofilm, the P. aeruginosa population is heterogeneous, exhibiting an extreme ability to adapt his metabolic activity to the local microenvironment. At the deepest layers of the biofilm is a subset of dormant cells, called persister cells. Though antimicrobial failure might be multifactorial, it is now demonstrated that these persister cells, genetically identical to a fully susceptible strain, but phenotypically divergent, are highly tolerant to antibiotics, and contribute to antimicrobial failure. By eradicating susceptible, metabolically active cells, antibiotics bring out pre-existing persister cells. The biofilm mode of growth creates microenvironment conditions that activate stringent response mechanisms, SOS response and toxin-antitoxin systems that render the bacterial population highly tolerant to antibiotics. Using diverse, not standardized, models of biofilm infection, a large panel of antibiotic regimen has been evaluated. They demonstrated that biofilm growth had an unequal impact of antibiotic activity, colistin and meropenem being the less impacted antibiotics. Different combination and sequential antimicrobial therapies were also evaluated, and could be partially efficient, but none succeeded in eradicating persister cells, so that non-antibiotic alternative strategies are currently under development. This article reviews the molecular mechanisms involved in antibiotic tolerance and persistence in P. aeruginosa biofilm infections. A review of the antimicrobial regimen evaluated for the treatment of P. aeruginosa biofilm infection is also presented. While tremendous progress has been made in the understanding of biofilm-related infections, alternative non-antibiotic strategies are now urgently needed.

A Quantitative Survey of Bacterial Persistence in the Presence of Antibiotics: Towards Antipersister Antimicrobial Discovery

Background: Bacterial persistence to antibiotics relates to the phenotypic ability to survive lethal concentrations of otherwise bactericidal antibiotics. The quantitative nature of the time-kill assay, which is the sector's standard for the study of antibiotic bacterial persistence, is an invaluable asset for global, unbiased, and cross-species analyses. Methods: We compiled the results of antibiotic persistence from antibiotic-sensitive bacteria during planktonic growth. The data were extracted from a sample of 187 publications over the last 50 years. The antibiotics used in this compilation were also compared in terms of structural similarity to fluorescent molecules known to accumulate in Escherichia coli. Results: We reviewed in detail data from 54 antibiotics and 36 bacterial species. Persistence varies widely as a function of the type of antibiotic (membrane-active antibiotics admit the fewest), the nature of the growth phase and medium (persistence is less common in exponential phase and rich media), and the Gram staining of the target organism (persistence is more common in Gram positives). Some antibiotics bear strong structural similarity to fluorophores known to be taken up by E. coli, potentially allowing competitive assays. Some antibiotics also, paradoxically, seem to allow more persisters at higher antibiotic concentrations. Conclusions: We consolidated an actionable knowledge base to support a rational development of antipersister antimicrobials. Persistence is seen as a step on the pathway to antimicrobial resistance, and we found no organisms that failed to exhibit it. Novel antibiotics need to have antipersister activity. Discovery strategies should include persister-specific approaches that could find antibiotics that preferably target the membrane structure and permeability of slow-growing cells.