A new discovery reveals that a variety of bacteria can sense light.  Exposing a type of disease causing bacterium known as Brucella bacterium to light increases its capability to infect humans as well as livestock.  This study shows for the first time that light has a very significant role in bacterial virulence or infection.
The Brucella bacteria have been monitored for years with the aid of modern technology which employs fluorescent microscopy and yet no one knew that it could sense light. Using fluorescent microscopy the scientist discovered that bacterial sensors are closely associated with phototropins.  Fluorescent microscopy shows that phototropins are the light receptors which have the capability to detect blue light.  They likewise direct plants to grow towards the source of light as observed in fluorescent microscopy.  A closer look using fluorescent microscopy revealed that many bacteria have signaling proteins which possess the same absorbing domain found in higher plants.  One such bacterium is Brucella which is known to be a severe pathogen of cattle responsible for causing abortion of calves.  Another species is a vicious pathogen of humans.
Although the researchers are unsure how the light benefits Brucella they suspect that it aids the bacteria to overcome the human or animal host’s disease-fighting defenses and allows it to reproduce fast.  They likewise suspect that the light signals the bacteria once it is outside of host like an infected aborted cow fetus lying in the field.  Under this circumstance, the increased virulence would assist the bacteria to survive as well as infect a new host.
The bacterial sensors were noted to have a protein sequence known as a LOV protein domain when studied with fluorescent microscopy, just like in plant phototropins. It detects light and look like some molecular packages which detect oxygen or voltage.  Approximately 100 different bacteria possess the DNA code for LOV domain proteins as examined with fluorescent microscopy.  However, it would not necessarily mean that the proteins themselves sense light.
The scientists were able to identify the function of LOV domain proteins in four species of bacteria with the help of fluorescent microscopy.  These are the human pathogen known as Brucella melitensis, the cattle pathogen referred to as Brucella abortus, the well studied plant pathogen called Pseudomonas syringae, and the bacteria common in sea water known as Erythrobacter litoralis.
What the scientists did was to insert the genes for the four LOV domains proteins into the Escherichia coli since it is a lab friendly bacterium.  The bacteria were grown in a darkened lab and observed using fluorescent microscopy.  What followed was that the LOV domain proteins were purified using dim red light.  To test if the LOV domain proteins show the photochemical reactions expected, the bacterium was flashed with a strobe light.  A change in color would be a sign that the LOV domain protein had tightened its hold on a molecular group called chromopore which gives color to organic molecules, and changed its shape in ways that activate the rest of the protein.  In reaction to the strobe light, each of the purified protein showed a color change as expected of a LOV domain protein.
The scientists likewise tested whether each of the LOV domain proteins functioned in a two component system with the aid of fluorescent microscopy.  This is the primary mechanism utilized by bacteria to sense to react to a wide variety of environmental stimuli.  Histidine kinase was used since it responds to activation of the sensing domain.  This addition of the phosphate groups to the response regulator is referred to as phosphorylation cascades.
What they did next was to test whether light played a significant role in living Brucella abortus by incubating the bacteria with the cultured animal cells both in dark and light environments and monitored through fluorescent microscopy. The virulence of the bacteria was measured.  The result revealed that the bacteria’s virulence in the dark environment dropped to less than ten percent of the normal virulence.  This decrease shows the maximum virulence of Brucella abortus relies on light.  Brucella abortus then responds to light by triggering a phosphorylation which causes the bacterium to reproduce, thus increasing its virulence as noted through fluorescent microscopy.