One hundred years ago, tuberculosis was the most likely cause of death here in Canada. But since the discovery of streptomycin in the 1950s and its use in combination with other antibiotics, deaths in Canada due to tuberculosis are extremely rare. Don’t get too comfortable with that thought, though; one third of the world’s population has tuberculosis and 500,000 cases of multidrug resistant tuberculosis have been reported. Antibiotic-resistant strains of gonorrhoea, Staphylococcus aureus, enterobacteria, and countless other bacterial species have followed suit. Once touted as miracle medicines for diseases formerly believed to be a death sentence, the antibiotics we have come to know and abuse are quickly becoming obsolete and jeopardizing global public health in their wake, forcing scientists to search for alternative therapies.
Antibiotics as we know them act by killing or inhibiting the growth of bacteria, thus acting as selective pressure on bacteria to mutate in order to survive. Resistance to such antibiotics is an ancient phenomenon. A 2011 study from McMaster University proposed that antibiotic resistance genes could be detected in bacteria frozen for over 30,000 years in Yukon permafrost, millennia before Alexander Fleming’s discovery of penicillin inaugurated the age of modern antibiotics. Combining this age-old characteristic of bacteria with the excessive use of antibiotics, the quick international spread of bacterial mutants and copious animal reservoirs has led to the emergence of hardy mutant bacteria over time. Now more than ever, there exists an urgent need for next-generation antimicrobial therapies that fight bacterial infection by targeting something other than cell viability. One such therapy aims to inhibit a mechanism called quorum sensing (QS) by which bacteria communicate with each other to induce group behaviors, including those that are virulent and pathogenic. Inhibiting QS pathways that regulate virulence instead of viability could prevent bacteria from activating virulent group behaviors without introducing a selective pressure on bacteria to mutate, making QS inhibitors an attractive, “evolution-proof” candidate for next-generation antibiotic development.
QS was first observed in culture flasks of the bioluminescent bacterium Aliivibrio fischeri, which would glow only when the bacteria grew to a high cell density. Bioluminescence was lost when the bacteria were re-inoculated at a low density. It was determined that A. fischeri were secreting a small molecule (later termed an ‘autoinducer’) that was used by fellow bacteria as a barometer of cellular density. When the concentration of the autoinducer reached a certain threshold, certain group behaviours- in the case of A. fischeri, the production of light- would be simultaneously activated by the bacterial population. From this discovery came the idea that interfering with autoinducers or their receptors on bacterial surfaces would allow scientists to silence the QS systems required by bacteria to activate pathogenic group behaviours.
Infections caused by the opportunistic pathogen Pseudomonas aeruginosa provide an example of the applications of QS inhibitors. P. aeruginosa forms biofilms, commonly on the lung, by adhering to one another and the surface they cover. Biofilms give P. aeruginosa strength in numbers by physically shielding them from antibiotics, environmental stressors, and the host immune response. These biofilms are especially dangerous for cystic fibrosis patients, in whom exacerbation of existing lung inflammation by P. aeruginosa biofilms can lead to respiratory failure or death. P. aeruginosa are equipped with two key QS systems utilizing an acylhomoserine lactone (AHL) autoinducer that controls biofilm formation as well as the production of virulence factors. Treatment of P. aeruginosa biofilms in vitro or in a mouse model of P. aeruginosa infection with brominated furanone C-30, an AHL receptor antagonist, reduced production of virulence factors and increased permeation of the biofilm by traditional antibiotics.
Curiously, even though QS inhibitors are thought to sidestep selective pressure, C-30 resistance has still been observed. Some P. aeruginosa mutants can overexpress a membrane pump that effluxes C-30 out of the cell, while others produce virulence factors normally even when treated with C-30. It is probable that bacterial virulence and viability are more interconnected than previously appreciated, given the multipurpose nature of genes within the relatively tiny and limited bacterial genome. A single QS system could also be simultaneously influencing several cellular processes, including one that impacts cell viability. Lessons learned from early inhibitors of P. aeruginosa QS suggest that perhaps a foolproof antibiotic doesn’t exist, but that further investigation into how bacteria cooperate through QS will nonetheless be pivotal in developing the next generation of antibiotics.