October 21, 2012 | 1
I’m on holiday at the moment, so this post is adapted from the archives. It was originally posted at my old blog over on Field of Science.
There are lots of things I enjoy about studying bacteria. I love their biochemistry and the secret inner workings of their metabolic pathways. I love that everything they do they manage within the confines of a single cell, and I love that you can go into a bacterial cell fairly easily and just mess around with the genes until they make what you want.
But what I’m really enjoying exploring at the moment is more ecological bacteriology; how bacteria interact with their environment, how they respond to changes to stresses and, most importantly, to other bacteria. I’ve covered how natural throat bacteria can help destroy dangerous pathogens such as MRSA Staph aureus so in this post I’m going to look at almost the opposite; how some bacteria can give each other a helping hand in order to infect humans.
Campylobacter jejuni is a bacteria that I feel a special affinity for because I’ve worked with it, back in my first ever summer project. Unfortunately it’s not a very nice bacteria and can lead to bad stomach illnesses with some rare but quite threatening complications. It’s found in chicken meat and cheese as it is perfectly capible of surviving happily in animals without causing them any diseases.
One of the problems with working with Campylobacter jejuni (henceforth shortened to C. jejuni) is that it’s very fussy about the amount of oxygen it’s in. C. jejuni is microaerophilic, which means it needs oxygen, but only small amounts,. If you give it too much all the cells will die. This problem was solved in my old lab by using tightly sealed containers and special packs of … stuff … which were put inside the containers to create the right conditions. This raises an important question; if the bacteria have to be coddled and protected just to culture on a plate in the lab then how on earth do they survive and grow on the surface of chicken meat?
A recent study (reference below) found that the C. jejuni were being aided by surrounding bacteria. The picture below shows both C. jejuni and a bacteria called Pseudomonas putida in close interaction, with long fibre-like structures connecting them. No one seems to be really sure what the fibre-like structures are, they may be being used for chemical communication, or they may just be keeping the bacteria in close physical contact.
Both bacteria were identified as being in close contact, as well as being seen together under the microscope. Further experiments were done to show that the P. putida was required for C. jejuni survival – a range of different C. jejuni strains were grown in both the presence and absence of the supporting P. putida to see how long they could survive in completely aerobic conditions. The results are almost hilarious, without the help of the P. putida bacteria the Campylobacter just die, really quickly:
Figure A (top) shows the C. jejuni with P. putida grown as well, Figure B (beneath) shows the C. jejuni grown alone. The graphs show the count of living cells over time, and it’s clear that without the help of P. putida the C. jejuni dies much quicker. Interestingly it was found that the interaction between particular strains of both C. jejuni and P. putida was fairly specific as well. As you can see in the graph above, only three of the C. jejuni strains have survived past 50 hours with the help of this particular P. putida, although they surive longer than with no help at all.
As P. putida are aerobic, the most likely explanation for how they are helping is that they create a microenvironment of decreased oxygen within their immediate surroundings. This is the kind of environment that it is thought C. jejuni will naturally migrate to. It’s interesting to consider that this might be less of a mutual helping relationship and more of a seriously exploitative one, with the C. jejuni swarming as quickly as possible towards the environment created by the P. putida and then wrapping them all up in a sticky mesh to stop them moving away.
This special relationship is not applicable for all C. jejuni, in other environments such as in humans and in chicken poo the C. jejuni exist fine on their own, but in the highly aerobic environment of the meat surface they rely on other bacteria to survive. The implications for treatment of bacteria are intreguing (especially for antibiotic resistant strains of C. jejuni) but this is another reminder that despite laboratory conditions bacteria do not just exist in isolation. They inhabit a whole tiny world, with challenges of its own, surrounded by other bacteria that change their environment both for better and for worse.
Reference: Hilbert F, Scherwitzel M, Paulsen P, & Szostak MP (2010). Survival of Campylobacter jejuni under conditions of atmospheric oxygen tension with the support of Pseudomonas spp. Applied and environmental microbiology, 76 (17), 5911-7 PMID: 20639377