Why I love chocolate agar

Chocolate agar, mmm. Sounds good, until you find out that it is made from boiled up blood plus seaweed extract. It is the colour of chocolate though. I love it because not only am I a (chocolate-loving) microbiologist, but I have devoted the last five years of my life to a bug called Haemophilus influenzae.
Haemophilus (which does not cause flu: a topic for another post!),  does not much care for growing outside its normal environment, the nose, throat and ear of a person. Chocolate agar is its preferred laboratory habitat, so I spend a lot of time with chocolate plates. Very unappetising, actually, as the Haemophilus on them smells exactly like snotty tissues. This  is not a coincidence; if you've ever had sinusitis, chances are that Haemophilus was involved.

Haemophilus influenzae on chocolate agar

Agar is a topic dear to my heart. I first became fascinated with it when, as a very junior microbiologist, struggling to get to grips with the complexities of the diagnostic lab, my consultant suggested that microbiology and gardening had a lot in common. Plants only grow on the right soils, with the right conditions of light, shade, water etc, bacteria only grow on the right kind of agar, with the right amount of oxygen, or lack of it. Growth characteristics can help you identify bacteria just as well as plants: they only look typical in their usual habitat. It is one of the amazing things about microbiology that the most modern technology (think DNA fingerprinting) co-exists happily with some very old techniques: pretty much the first thing a microbiologist learns is how to "Gram stain"  a bug; that is make dyes stick to it so it is clearly visible under a microscope. We do it in exactly the same way that Hans Gram did it back in 1884. Agar is like that too; first added to culture broths to set them in 1881 (because the lab worker knew it would set jam!) and used ever since.
Agar comes in all sorts of "flavours", which individually suit some bugs better than others. Blood agar, made from sheep or horse blood, is the workhorse of clinical labs. Some bugs won't grow on it (like my Haemophilus), some grow much too well, so other mixtures are needed to separate what   important germs are found in a sample. A typical faecal specimen might get put onto three or more different plates to try and sort out the bugs: a blue or pink one for ordinary gut organisms, black (with charcoal) for Clostridium difficile,  a different black one for Campylobacter, pink for typhoid and another pink one for E. coli 0157 and so on. The clue to the patient's diagnosis comes not only from whether anything's growing, but what it looks like and what colour it goes. Some agar has indicators (like pH paper) which show when a colony (a group of germs, all growing together) uses a sugar in the agar or not. Although around for over a hundred years, some agar is very "modern", developed to look for specific germs, like MRSA agar, which shows up  pink colonies if a patient's sample contains MRSA, or "Brilliance" agar which has specific colours for different germs containing ESBLs (like the New Delhi Klebsiella which hit the headlines at the end of last year).
Agar is not only incredibly useful, in fact vital to the diagnosis of infection in hospitals across the world every day, it is also a tribute to scientific ingenuity and creative endeavour. Just look at these amazing pictures, created by growing germs on agar plates:  http://www.microbialart.com/