Due to the rapid rise of multidrug-resistant bacterial pathogens over the past two decades, the U.S. Centers for Disease Control and the World Health Organization both recently issued major reports warning of the entry of human medicine into a “post antibiotic era”. This growing list of pathogens now includes methicillin-resistant Staphylococcus aureus (MRSA), Vancomycin-resistant Enterococcus (VRE), Carbapenem-resistant Enterobacteriaceae (CRE), multi-drug-resistant tuberculosis (MDR-TB), and many others. When treating patients with drug-resistant infections, clinicians have to resort to second and third tier antimicrobials which often have reduced efficacy, increased toxicity, or both, often leading to poorer outcomes.
However, long before a clinician diagnoses an infection and antibiotic treatment is started, our innate immune system responds to pathogens by producing potent endogenous antimicrobial peptides (AMPs) with a broad spectrum of activity. These AMPs are expressed on epithelial cell surfaces and by leukocytes in response to injury or infection. Well-characterized AMPs include cathelicidins, α- and β-defensins, and thrombocidins.
Due to the historic reliance on a single bioassay, the minimal inhibitory concentration (MIC), for testing bacterial pathogen antimicrobial susceptibility, the complex interaction between the innate immune system, antibiotics, and bacterial pathogens is often not well studied: the MIC assay contains only bacteria, bacteriologic broth, and antibiotics without any component of innate immunity.
For this PhD dissertation project, I examined the interactions between components of innate immunity system in combination with conventional antibiotics in the treatment of drug resistant bacterial pathogens. First, I discovered that the most commonly prescribed antibiotic in the United States, azithromycin, has striking efficacy in-vitro and in-vivo against extremely drug resistant Gram-negative pathogens including carbapenem resistant Pseudomonas aeruginosa, Klebsiella pneumoniae, and Acinetobacter baumannii. This activity has been overlooked because azithromycin has no activity against these pathogens in standard MIC testing, but becomes extremely potent when tested in eukaryotic media or in synergy with cationic antimicrobial peptides. Secondly, I worked closely with an infectious disease fellow, Monika Kumaraswamy, who spearheaded a logical follow-up to our initial studies and discovered that azithromycin also has potent activity against another emerging multidrug-resistant pathogen, Stenotrophomonas maltophilia. Finally, since preventing infection by drug resistant organisms is even better than finding a good therapy for them, I worked closely with Janie Kim, and discovered a novel formulation of multipurpose contact lens solution that has more antimicrobial efficacy against both the plaktonic and biofilm forms of Pseudomonas aeruginosa and Staphylococcus aureus than anything available to contact lens wearers today. Altogether, this dissertation highlights the importance of studying the interaction between bacterial pathogens and antimicrobial therapy in more physiologic settings, especially in the context of innate immunity, and the importance of trying novel combination therapies in this era of rapidly increasing drug resistance.