Editor's Selection IconA. baumannii does not mess around. As opportunistic pathogens go it’s pretty out there. An aerobic, gram negative, almost entirely antibiotic resistant (largely through passive mechanisms) bacterium that’s developing such a terrible reputation that it has picked up the nickname ‘Iraqibacter’, but that’s mostly because of the high proportion of A. baumannii infections in returned American troops.

The real problem with these bacteria is that it is a jack-of-all-trades. If it gets in your lungs it can cause pneumonia. If it gets in your skin it can cause necrotising fasciitis (flesh eating disease). If it gets in your urinary tract you get a nasty urinary tract infection. Basically, if it’s a moist surface A. baumannii will stick there and develop into an infection. Importantly though it seems that you have to already be a little sick for it to take hold. This in itself presents a problem as doctors and nurses are able to act as asymbiotic transmitters between all the sick people they care for.

"What do you think? Have we made this patient sick enough yet? What does the chart say?". Credit: Seattle Municipal Archives

In fact, transmission by other patients, doctors and nurses is resulting in A. baumnanii being one of the fastest growing sources of hospital-acquired infection with hospital-acquired pneumonia being a particularly bad problem. Many commentators are predicting this bacterium will be ‘the next big thing’ in infectious disease, if it isn't already.

All this is made worse by the observation that antibiotics simply do not work as effectively against A. baumannii. A. baumannii contains a number of drug efflux pumps which allow it to remove its cellular contents when under stress, antibiotic stress for example. It also seems to be able to readily pick up DNA from its environment and occasionally incorporate it into a usable form. This mechanism is thought to be responsible for the increasing prevalence in antibiotic resistance generally and in A. baumannii it is predicted to have resulted in a drop from ~90% of clinical isolates being susceptible to ampicillin-sulbactam in 2003 dropping to only ~40% by 2008.

So why bring this bacterium up? A paper came out (a while ago now but I came up with this idea when the paper emerged in 2010) suggesting a very novel means of controlling this nasty bacterium and it all comes back to its name.

The word "Acinetobacter" comes from some interesting Roman and Greek roots. The Greek for non-motile is apparently ακινητο- which when translated into English would normally come out as akinetic- but original transliterations instead gave us akineto- or acineto-. Bacter just means bacteria shaped, so rod.

Funny story, this non-motile bacteria that is so non-motile that its name literally means non-moving rod is actually quite the little mover. Whilst A. baumannii lacks a flagellum it is a very efficient swarming bacterium that moves via twitching motility.

So back to this paper. A recent discovery has found that light may be able to stop the swarming of the bacterium. Yeah, light. Blue light specifically.

Potent antibiotic, apparently. Credit: Kapungo

The researchers found that swarming was most active in the dark but when exposed to blue light the colonies stopped their swarming activity.

What is happening here?

The researchers went back and found a protein in A. baumannii called BlsA (Blue light sensing protein A) that contained a BLUF (blue light sensing using flavin) domain. They mutated blsA and the light sensitivity disappeared but the real question is what is it doing there in the first place?

At this stage there seems to be no answer, all we know is that blue light seems to really mess with this bacteria. Blue light is able to disrupt the formation of biofilms and pellicles, which are particularly relevant in the development of human disease.

Could it be possible that a bacterium that is so difficult to treat, in part due to its inherent resistance to most antibiotics, could be inhibited by a blue light? Time will tell but as was expertly written at the bottom of the commentary in the Journal of Bacteriology “microbiologists [should] open their incubators and let the sun shine in. The results may be illuminating.”

Mmmm. Punny goodness.

In what I hope will be the first of many collaborative cross-posts between Lab Rat and myself (and perhaps other SciAm bloggers) you can click here to find Lab Rat's in depth look at the blue light inhibition of A. baumannii swarming story. Enjoy!


Mussi, M., Gaddy, J., Cabruja, M., Arivett, B., Viale, A., Rasia, R., & Actis, L. (2010). The Opportunistic Human Pathogen Acinetobacter baumannii Senses and Responds to Light Journal of Bacteriology, 192 (24), 6336-6345 DOI: 10.1128/JB.00917-10

McBride, M. (2010). Shining a Light on an Opportunistic Pathogen Journal of Bacteriology, 192 (24), 6325-6326 DOI: 10.1128/JB.01141-10

Reddy, T., Chopra, T., Marchaim, D., Pogue, J., Alangaden, G., Salimnia, H., Boikov, D., Navon-Venezia, S., Akins, R., Selman, P., Dhar, S., & Kaye, K. (2010). Trends in Antimicrobial Resistance of Acinetobacter baumannii Isolates from a Metropolitan Detroit Health System Antimicrobial Agents and Chemotherapy, 54 (5), 2235-2238 DOI: 10.1128/AAC.01665-09

Dijkshoorn, L., Nemec, A., & Seifert, H. (2007). An increasing threat in hospitals: multidrug-resistant Acinetobacter baumannii Nature Reviews Microbiology, 5 (12), 939-951 DOI: 10.1038/nrmicro1789