Biofilms – the bane of dentists and cargo ships alike – are a form of surface-attached microbial growth that is especially hardy. The cellular community not only changes its genetic expression patterns to increase resiliency under harsh conditions such as chemical or immunological attack, but the extracellular matrix in which the microbes reside adds a layer of protection. Newly published research support that these two phenomena – the gene expression pattern changes and the physical encasement by the matrix – are related.
The matrix is made of many different sorts of polymers, which vary depending on microbial species and growth conditions. In addition to sticky carbohydrate polymers, different species of bacteria have lipids, extracellular DNA, and extracellular appendages such as pili characterized as part of the extracellular matrix makeup. Research recently published in Antimicrobial Agents and Chemotherapy ties the presence of extracellular matrix DNA in Pseudomonas aeruginosa biofilms to antimicrobial resistance of these biofilms – but not in the ways ECM is traditionally thought to aid biofilm resistance.
Because of its sticky characteristics, the extracellular matrix is able to absorb a number of molecules, such as cationic antimicrobial peptides, that would normally damage the microbial membrane. Because of the small pore size in the matrix, it can also exclude large molecules, such as antibodies, via steric hindrance. Research from Dr. Shawn Lewenza’s lab demonstrates that extracellular DNA also contributes to antimicrobial resistance through a decrease in pH in the biofilm core. Dr. Lewenza has previously published on biofilm genetic regulation influenced by extracellular DNA, which is further explored in these studies.
The researchers observed that the acidification by the extracellular DNA is nearly an entire pH step down – or nearly 100x as acidic as the environment outside the environment. This change in pH activates the PhoPQ and PmrAB two-component regulatory systems in the biofilm-encapsulated P. aeruginosa cells. This leads to the modification of the bacterial membrane and LPS and exclusion of the aminoglycoside drugs tested (see schematic).
P. aeruginosa is a common lung infection in patients with Cystic Fibrosis. In addition to the bacterial extracellular DNA, white blood cells contribute extracellular DNA as part of the immune response, and the CF lung can have up to 20 mg/ml extracellular DNA. One way to reduce this accumulation is to treat CF patients with an inhalable DNase that degrades some extracellular DNA. The good news from this research is that the researchers saw a reversal of antibiotic resistance when the in vitro biofilms were neutralized with either L-arginine or sodium bicarbonate, presenting another potential means to treat CF patients. Like all basic research, it needs to be verified and tested in more complex models before potentially translated, but pH neutralization combined with antibiotic and other treatments may ultimately serve as a way to fight these incalcitrant infections.
-- Julie Wolf