Date of Award

December 2015

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biomedical and Chemical Engineering

Advisor(s)

Dacheng Ren

Keywords

antibiotic resistance, antimicrobial peptides, biofilm, Escherichia coli, persister cells, Pseudomonas aeruginosa

Subject Categories

Engineering

Abstract

The rapid increase in antibiotic resistant infections and the slowing pace of antibiotic development emphasize the need for alternative therapeutic agents to cure infectious diseases especially those caused by multidrug-resistant (MDR) strains. Bacteria obtain resistance to antibiotics through multiple mechanisms. One of intrinsic mechanisms of drug resistance is persister formation, by which bacterial cells enter a metabolically inactive stage and become highly tolerant to essentially all antibiotics, even at the concentrations that are hundreds of times higher than the lethal dose required to kill normal planktonic cells of the same strain. Persister cells in biofilms are even more difficult to kill due to the presence of an extracellular matrix that can block or retard the penetration of antibiotics. Thus new antimicrobials that are effective against these drug tolerant cells are urgently needed for infection control.

In this study, we characterized the antimicrobial activities of newly designed synthetic peptides on Escherichia coli and Pseudomonas aeruginosa strains including regular planktonic cells and those in biofilms and at the persister stages. Our results revealed that 2D-24, an RW-rich dendrimeric peptide, can kill planktonic cells of both P. aeruginosa PAO1 and PDO300 (a mucoid strain) in a dose-dependent manner. Killing effect on biofilm and persister cells was observed at the concentrations without significant toxicity to IB3-1 cells originated from human lung tissues.

We also demonstrated that TN-5, a 1,3,5-triazine derivative, has antimicrobial effects on E. coli RP437, P. aeruginosa PAO1 and PDO300 cells, with a minimum inhibitory concentration (MIC) of 12.8 µM, and kills regular planktonic cells of both species dose dependently. TN-5 was also found effective against persister and biofilm cells of both E. coli and P. aeruginosa; and the killing of biofilm cells of the mucoid PDO300 was enhanced by alginate lyase.

To understand the effects of AMP charge on the killing effects, we modified the net charge of calcitermin originated from human airway secretions, and tested the effects on E. coli and P. aeruginosa planktonic and persister cells at different pH values. The neutral derivative of calcitermin showed better killing effect on persister cells at pH 7.4.

Along with synthetic peptides, we also studied the membrane potential of persister cells with cell sorting and flow cytometry techniques using potentiometric dyes. Persister cells showed lower membrane potential along with lower efflux pump activities compared to normal cells. Based on these findings, we tested the hypothesis that persister cells can be effectively killed by antibiotics that are substrates of efflux pumps. Consistent with this hypothesis, erythromycin was found effective in killing persister cells of E. coli while normal cells are resistant to it. This higher killing activity of erythromycin was corroborated with higher erythromycin accumulation in persister cells based on the results of Mass Spectrometry analysis.

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