Biofilms, antibiotic resistance and immune evasion
Biofilms are dense aggregates of bacteria that are enmeshed in an extracellular matrix of exopolysaccharides and extracellular DNA. Given the increased antibiotic tolerance of biofilms, and the ability to evade the immune response, biofilms are frequently the cause of chronic infections. My interest has been to examine the role of extracellular DNA in the biofilm matrix and in infection sites. We have shown that DNA has the ability to bind and sequester divalent metal cations and to acidify biofilms. These environmental conditions lead to the increased expression of bacterial surface modifications that protect bacteria from antibiotics, including antimicrobial peptides and aminoglycosides. DNA induces the expression of many genes and their functions are currently being explored.
Extracellular DNA chelates cations and induces antibiotic resistance in Pseudomonas aeruginosa biofilms. Mulcahy H, Charron-Mazenod L, Lewenza S. PLoS Pathog. 2008 Nov;4(11):e1000213. doi: 10.1371/journal.ppat.1000213. Epub 2008 Nov 21.
Surface-localized spermidine protects the Pseudomonas aeruginosa outer membrane from antibiotic treatment and oxidative stress. Johnson L, Mulcahy H, Kanevets U, Shi Y, Lewenza S. J Bacteriol. 2012 Feb;194(4):813-26. doi: 10.1128/JB.05230-11. Epub 2011 Dec 9.
Extracellular DNA Acidifies Biofilms and Induces Aminoglycoside Resistance in Pseudomonas aeruginosa. Wilton M, Charron-Mazenod L, Moore R, Lewenza S. Antimicrob Agents Chemother. 2015 Nov 9;60(1):544-53. doi: 10.1128/AAC.01650-15.
Extracellular DNA-induced antimicrobial peptide resistance mechanisms in Pseudomonas aeruginosa. Lewenza S. Front Microbiol. 2013 Feb 14;4:21. doi: 10.3389/fmicb.2013.00021. eCollection 2013.
We have also shown that DNA itself is antimicrobial. The ability to chelate cations from the outer bacterial surface, which is highly concentrated in Mg2+ bound to the surface, results in disrupting the outer membrane integrity, and ultimately cell lysis and death. Neutrophil extracellular traps are ejected from neutrophils as a novel mechanism of trapping and killing microbes. While the DNA lattice of NETs are bound to numerous antimicrobial proteins, the DNA itself in a NET is antimicrobial. Therefore, bacteria must also protect themselves from the innate immune killing from DNA. This typically involves secreted deoxyribonucleases, and we are exploring other protective mechanisms to survive NET killing.
DNA is an antimicrobial component of neutrophil extracellular traps. Halverson TW, Wilton M, Poon KK, Petri B, Lewenza S. PLoS Pathog. 2015 Jan 15;11(1):e1004593. doi: 10.1371/journal.ppat.1004593. eCollection 2015 Jan.
Given the urgency for discovering new antibiotics, we are exploring ideas to discover new antimicrobials that do not select for intense resistance. ‘Antivirulence’ compounds are drugs that repress or block essential virulence factors, which limits their ability to cause disease and promotes the immune response to clear an infection. We have developed ‘antibiofilm’ compounds that reduce the production of exopolysaccharides, and therefore block biofilm. This kind of therapeutic drug decreases bacterial virulence and promotes conventional antibiotic killing.
Exopolysaccharide-Repressing Small Molecules with Antibiofilm and Antivirulence Activity against Pseudomonas aeruginosa. van Tilburg Bernardes E, Charron-Mazenod L, Reading DJ, Reckseidler-Zenteno SL, Lewenza S. Antimicrob Agents Chemother. 2017 Apr 24;61(5). pii: e01997-16. doi: 10.1128/AAC.01997-16. Print 2017 May.
We are also developing biosensor approaches as tools for the discovery of compounds that specifically disrupt the outer membrane of bacteria.
Biosensors for wastewater pollutants
Whole bacterial biosensors are engineered bacteria that express a genetic circuit comprised of a promoter and a reporter of gene expression. The hallmark feature of bacteria is to express genes in response to changing environmental conditions. By identifying the genes expressed in response to pollutant exposure, these genes can be used to construct biosensors. Decades of research has demonstrated the sensitivity and specificity of biosensors for numerous organic compounds, heavy metals, antibiotics and others. We are constructing biosensors to detect naphthenic acids, the primary toxic pollutant in oilsands tailings ponds.