(Second of three parts)
Enlightened expressions
The first glimpse on quorum sensing was observed in 1965 by Alexander Tomasz in bacteria that cause pneumonia. Microbiologists were later enlightened on this phenomenon by serendipity because of a squid that glows. Yes, not a firefly but a squid! The ocean not only provided sushi on the table but also a specimen destined to be a luminary!
In 1970, Nealson, Platt and Hastings reported that the glow from the nocturnal Hawaiian bobtail squid is emitted by millions of marine bacterial cells called Vibrio fischeri which are living underneath the skin of the squid. The bacteria live in symbiosis with the squid meaning it’s a “I scratch your back, you scratch mine” kind of relationship. The squid uses the light to avoid predators, attract prey and find a mate. In exchange, the bacteria are provided an environment with a steady supply of nutrients in a specialized compartment of the squid. Within that microcosm, the bacteria are able to multiply in number and emit light. But when they are freely living in the ocean outside the squid, they do not emit light and we will see later on why.
The ability of living organisms to emit light is called biolumiscence. The basic principles of quorum sensing were first observed in more detail in Vibrio fischeri and were confirmed to be responsible for switching bioluminescence on and off. The experiments were carried out this way: Vibrio fisheri cells isolated from the squid were cultured in the laboratory. When only a very, very small number of cells were placed in a flask, the cells did not light up. As the number of cells was increased, a corresponding increase in the intensity of the glow occurred. This simple experiment has demonstrated that the cell density or the number of cells determines the “factor” that triggers bioluminiscence. Their theory was, this factor is a chemical signal which accumulated to a sufficient concentration excreted by enough number of cells, capable of triggering a group response like bioluminescence. How did they confirm the presence of these excreted chemical signals? Well in their next simple experiment, they separated the liquid from where the numerous glowing bacteria were grown. They then added the liquid to the few non-glowing bacteria and then there was light!
Further biochemical tests of the culture liquid have identified the excreted chemical signal as acyl-homoserine lactone or AHL. Now that sounds like a big term. But scientists also call it an autoinducer because enough concentration of it can bind to another protein which triggers the expression of the set of genes for bioluminescence called lux. The expressed enzyme or protein called luciferase catalyzes the chemical reaction to produce light.
In short, when a certain concentration of the excreted AHL is reached by a certain number of cells, that population of cells responds by using that concentration to trigger a population-wide production of luciferase. Vibrio fischeri cells not living symbiotically with the squid but as free cells suspended in the ocean, although constantly excreting AHL at a low level, do not glow because that vast environment dilutes out or diffuses away AHL, unlike in the squid compartment where they are bottled-up accumulating enough concentration to trigger the expression of bioluminescence.
Sickening conversations
Pathogenic or infectious disease-causing bacteria are also very dependent on quorum sensing in order to successfully launch virulence; this term refers to their ability to defeat the defense system of their host. Again, unite and conquer is the name of the game. When the US launched an attack against the Talibans in Afghanistan, they did not send out just one soldier. It was not a one-man act. That would have been totally ridiculous and would not at all achieve the objective. But brainless virulent bacteria are no dimwits. They set out invasions like a fleet of tanks and fighter planes in constant communication with one another to accomplish the mission!
The term quorum sensing was coined by microbiologist EP Greenberg and his colleagues of the University of Iowa. They identified the role of this phenomenon in the virulence of the bacterium Pseudomonas aeruginosa. This bacterium infects immunocompromised patients or those who are suffering from diabetes, AIDS or cancer because it has an impressive armament of toxins that are so fatal that they overcome the immune system. Because they can then thrive so well in a host, they form a compact thin strip of their dense population called biofilm. This can be formed on any surface outside or inside the body like the lungs. Biofilms persist by quorum sensing; together the cells excrete a slimy protective shield against several competing microorganisms and antibiotics.
Pseudomonas aeruginosa also produces pyoverdine which is a siderophore, a molecule which the bacterium sends out to its environment to scavenge and shuttle iron back into itself, like a cargo ship sent out on a mission to bring back iron. It was later found by the research team of Keith Poole of Queen’s University in Australia, that pyoverdine also plays a crucial role in quorum sensing. Genetically engineered pseudomonads lacking a quorum sensing gene exhibited low production of pyoverdine. Further studies showed that increased production of pyoverdine increased the production of toxins. It was later confirmed that pyoverdine indeed is a deadly emissary! It also acts as the autoinducer for triggering the population-wide expression of toxins through quorum sensing.
(To be continued)
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Dr. Ronie J. Calugay obtained his Ph.D. in Life Sciences and Biotechnology at the Department of Biotechnology, Tokyo University of Agriculture and Technology where he studied iron-shuttling molecules called siderophores produced by magnet-producing bacteria. He finished his MS in Microbiology at the University of the Philippines, Diliman. E-mail him at ronix5@yahoo.com.