How bacteria build biofilms

Staphylococcus aureus is a very common cause of infection in humans. In its drug-resistant form - MRSA - it can be lethal. And one of the reasons that Staph. is such a challenge is because once it gains a toehold inside the body it can be very hard to get rid of. For instance, if your hip replacement gets infected, often the only remedy is another trip to operating theatre to remove the entire prosthesis. This happens because bugs like Staph. can assemble protective barriers around themselves called biofilms, which shield the bugs inside from the immune system and antibiotic agents. Now two scientists at Rutgers University, Jeff Boyd and Ameya Mashruwala, have discovered the trigger that makes Staph. aureus begin building its biofilm. Chris Smith spoke to Jeff Boyd...

Jeff - If you’ve ever walked in a stream and you slipped on a real slippery rock, a lot of that slippery surface is actually created by microbes that are in biofilms. These organisms that are living in it, they attach themselves to this surface. It allows the water to then flow over these microbes and they can kind of acquire the nutrients that are naturally present in the water but yet, they don’t have to float and fight the current. Under a microscope, it would be a cluster of grapes with maybe some type of a matrix or some type of a goo or a slime that might kind of hold those cells together.

Chris - What was the question about biofilms that you were looking at here?

Jeff - Well, it was kind of a serendipitous discovery. But really, what we’re interested in is how organisms make a shift between aerobic lifestyles and anaerobic lifestyles. Meaning, in the presence of oxygen, or living in the absence of oxygen. What Ameya discovered is just that when these organisms that we study grow in the absence of oxygen, they form very robust biofilms. And then we try to extrapolate that as to what's going on with the physiology of this organism or what it might be doing or having this behaviour.

Chris - Ameya, what did you actually do to study this?

Ameya - I looked to see how this specific bacterium called Staphylococcus aureus – typically, it lives on the human body and it doesn’t cause us infection. But under certain conditions, it can cause infection. People have found that its ability to cause infection is related to its ability to form these complex communities called biofilms. So what I looked at is how Staph. aureus sticks to a surface and forms these multicellular communities called as biofilms.

Chris - So how do they go from being an individual free living bacterium to ‘wanting’ to form this community coated by this biofilm and what is it made of?

Ameya - It turns out, when you remove oxygen, it makes it want to go from living as an individual cell to start living as a community. They start to cooperate. A part of this co-operation is that they start producing a sticky glue-like material called as the matrix. This matrix helps these bacteria stick to each other or it helps them stick to surfaces such as human tissues. What happens is the sticky glue-like material, the matrix the bacteria make, it kind of encases the bacteria almost like a raincoat. This protects the bacteria from components of the human immune system so they can come in contact with the bacteria and kill it. This allows the bacteria to cause infections.

Chris - Jeff, what do they make that bacterial raincoat from?

Jeff - Well, I think this is one of the most interesting findings that Ameya made is that it turns out that Staphylococcus aureus in these conditions is actually making this matrix or this glue out of DNA. When you remove oxygen, a certain portion of the population, these cells are killing themselves. As they do that, they break open and they release DNA. As DNA, it turns out as a very sticky material.

Chris - Ameya, do you understand how they get hold of the DNA – the surviving cells – and then use it?

Ameya - That part is not entirely clear yet. However, what is fairly evident is that it does help them stick together in some manner. One idea is that in addition to DNA, the matrix is also made up of proteins. DNA is negatively charge and a lot of the proteins are positively charged. It’s possible that they help kind of form like an electrostatic net which then helps the cells, the proteins, and the DNA all come together, and make those matrix.

Chris - Jeff, we’ve heard that the bacteria kill themselves but is that choice or is it that some randomly die and then their corpses get assimilated by the survivors or is there actually a predestined, “You're going to die and this is how we’re going to kill you by the rest of the population.”?

Jeff - I think that’s a huge outstanding question. We’ve found systems that are actually able to respond to oxygen. We found that certain cells break open, and we found that DNA is released. Now, how certain cells are chosen to have this behaviour is still a huge mystery to us. We’re not exactly sure what the stimuli might be that say, “Okay, you cell break open and you cell form the biofilm and survive, and take advantage of this lysing cell.” But that is going to be one of the questions that we were going to try to address as we move this project forward.

Chris - Is it just bacterial DNA they’ll assimilate because obviously, one of the mechanisms, one of the virulence factors of organisms – gram positives like Staph. aureus is that they lyse our cells and release various micronutrients – iron and so on – that they can then use? But inevitably, there will be DNA coming out in the process. Could they use that too?

Jeff - It’s an excellent question. I'm going to have to say yes. Other researchers in the field have found that Staph. aureus does produce a series of proteins that have the ability to bind DNA. That DNA that they bind is not specific to the DNA of the organism that's producing these proteins. So I guess I would answer the question by saying, “Yes, Staph. aureus could take advantage of human DNA after it’s lysed to those cells to help facilitate the formation of these multicellular communities.

Chris - Ameya lastly, if you can get a handle on how this works, and you can disrupt it, does this mean we potentially have a new way to combat Staph. aureus?

Ameya - Definitely, that’s our hope. In the forthcoming project that we’re looking at, we’re trying to understand how we can take different molecules and make these biofilms dissolve and let antibiotics treat the infection. So, our hope is that by understanding how this biofilm structure occurs, we should then be able to try and figure out a way to treat these infections.

Chris - Jeff…

Ameya - Yes, I would agree with everything that Ameya said. One thing that he didn’t point out though is that Ameya actually discovered the protein that has the ability to sense the presence or absence of oxygen. Now, knowing the signal for that regulatory system, we might be able to go through – work with some chemists, design some drugs – to inhibit this protein that senses either the presence or the absence of this bacterium breathing.