Field of Science

Carnival Time!

The latest Scientia Pro Publica blog carnival is up over at mauka to makai. It's a collection of sciencey-blog posts, with a mix of writing, video-links, and pictures, from a mix of people.There are well-known science bloggers, such as Greg Laden and GrrlScientist, as well as some lesser-known ones including your very own Lab Rat.

I'm quite pleased and proud to feature. After a slightly disappointing end of term result, and not making in past the audition stage of an online writing challenge, it's nice to know that in the Lab Rat incarnation at least I seem to be managing quite well.

Resistance without genetics - persistance in bacterial populations

Most work on bacterial resistance to antibiotics tends to start with genetics. If a bacteria is able to survive a certain antibiotic, it is assumed that it has gained a gene from somewhere (and bacteria can get genes from almost anywhere) which allows it to survive. That makes sense after all...surely two bacteria with exactly the same genetic information should react identically to antibiotics?

No. Not all the time. There is an unfortunate habit of biochemists to get too wrapped up in the genetics to think about epigenetics (control of genetic expression by proteins), especially with 'simpler' organisms like bacteria. Persistence is the property of a totally identical population of bacteria to respond differently to antibiotics, despite having identical genomes they are phenotypically (phenotype = set of observable characteristics) different.

Persistent bacteria aren't only seen in response to antibiotic's either...they can be found for many types of stress, such as heat or starvation. They've been roughly split into two different types:
  • Type I persisters: form in response to stress, usually at stationary phase (i.e after the initial burst of bacterial growth)
  • Type II persisters: are seen forming throughout the bacterial lifecycle
Monitoring the growth rates of individual cells showed that even before treatment with antibiotics, many of the persister cells had reduced growth rates. The mechanism for this is not yet clear, a few genes that may be involved have been identified, but no reason for activating them (or repressing them in non-persisters), has yet been identified.

Although these sound dangerous at first glance, the medical implications of persisters are relatively limited (probably the reason why the mechanism has not been more thoroughly studied). As they have a slower growth rate, and as there are so few of them within an overall susceptible population, they can usually be cleared relatively easily by the immune system. There are, therefore, only three main areas where they are clinically relevant: immunosuppressed patients, pathogens that have adapted to the immune system, and in niches in the body that are less available to the immune system (such as within a biofilm).

The main ecological and evolutionary implications of this are that a colony of identical bacteria can 'survive' antibiotic attack by having a few of its members able to withstand that attack. Although they will grow slower under normal conditions, their ability to withstand stressful conditions means that they can essentially recover the whole colony, without the need for a major genetic change. This is true especially of diseases such as tuberculosis, where very few bacteria are needed to re-start an infection.

One thing that would be really interesting would be to study the behaviour of persisters within biofilms. Along with swarming and quorum sensing, biofilms are an example of pseudo-multicellular bacterial behaviour, circumstances under which having a small population of cells that can regenerate the whole colony would be very useful.


Orit Gefen & Nathalie Q. Balaban (2009). The importance of being persistent: heterogeneity of bacterial populations under antibiotic stress FEMS Microbiology Reviews, 33 (4) : 10.1111/j.1574-6976.2008.00156.x

On the classification of blobs

The first forays into microscopy revealed a whole world of blobs, tiny microscopic organisms that were invisible to the naked eye. These went through a range of different names, from the 'animalcule' denomination given by Anton van Leeuwenheok (which he used to describe everything small he saw under the microscope, including his own sperm) through 'monera' (a more specific name for certain types of blobs, namely those that weren't eukaryotic) to 'prokaryotes', a name that still stands.

Prokeryote means, literally, 'no nucleus', and it's use allows the world of living organisms to be split up into two groups: Prokaryotes and Eukaryotes. Eukaryotes are things with a nucleus, a membrane covered partition to hold DNA, as well as many separate organelles existing within their cells, such as mitochondria, endoplasmic reticulum, the Golgi apparatus...

Prokaryotes are...uh...everything else.

Which means that the label 'prokaryote' was always waiting to fall apart. After all, they may just be blobs but there are a lot of them, and some are very different blobs. Around the 1970's people started noticing that there were a group of the prokaryotes that behaved differently, mainly through studies done by Carl Woese and George E. Fox who created classification tables based on the genetic sequences of ribosomal RNA (the part of the genome most likely to be conserved, this is often used for classification, especially of things in Deep Time). This showed that there was a distinct group of prokaryotes with a mostly separate evolutionary history (more on the mostly later) to the rest of the prokaryotes. They were originally named 'archaebacteria', and together with 'eubacteria' (true-bacteria) were put in the prokaryotes group. They were blobs without a nuclei, and that was where they belonged.

However, things started to get a bit more complicated the more people looked at archaebacteria. They weren't just a group of slightly odd bacteria, they were something else. Something different. Although their metabolic pathways are similar to bacteria, their methods of turning DNA into proteins more resembles eukaryotic processes. Their flagella (tentacle like structures used for movement) have a markedly different structure from bacterial flagella. Like bacteria, they reproduce asexually and (also like bacteria) they can share their DNA around, in fact they can also share there DNA with bacteria, which makes taxonomists tear their hair out. It's very difficult to classify something when it keeps giving its DNA away, and collecting bits from other sources.

It is proposed in the SGM journal (Society for General Microbiology-journal not available on line) that the term 'prokaryote' should be scrapped altogether. As well as being an incorrect label for a large group of organisms it also produces an incorrect evolutionary perspective. The use of the eukaryote/prokaryote terms suggests a very human based linear "One upon a time there were blobs with no nuclei and then they got nuclei and then they were better" sort of story. A more correct view is that of all three superkingdoms; bacteria, archaea and eukaryotes splitting away from each other. Eukaryotes safely packaging their DNA away, allowing a more complex system to build up, yet forfeiting the ability to share bits of DNA. The archaea and bacteria on the other hand, continued to share their genetic material, just became more selective about it as they diverged (hense the 'mostly' seperate history).

Or maybe not. It might be that the archaea/eubacteria formed a very selective group of blobs, which then split further when some developed a nucleus, while the others continued to share their DNA with the bacteria, picking up different metabolic secrets. It's hard to work out; especially given that similarities between the DNA of archaea and bacteria does not necessarily show their relatedness; it might be a gene that has remained conserved in both of them for millions of years, or it might just be one that was exchanged last week.

There a several arguments against removing the 'prokaryote' as a naming system but most of them boil down to the very multicellular-centric argument of: "but they're all just blobs!" The three superkingdoms of archaea, bacteria and eukaryote are a far more accurate, and scientifically and taxonomically correct way of looking at things than the prokaryote/eukaryote model.

My only complaint is that I spent ages in secondary school trying to learn how to spell 'prokaryote'... removing the name means I could have spent that time doing something far more building paper planes and reading 'Redwall'.