What makes a bug super?

By Siouxsie Wiles 17/02/2011 5


The term ‘superbug’ gets bandied around quite a bit when referring to some bacteria. But what is a ‘superbug’? That font of all knowledge that is Wikipedia defines a superbug as a bacterium that is resistant to several antibiotics. But is it really as simple as that?

I think if you asked a journalist (certainly one that writes for the British ‘newspaper’ the Daily Mail*) to name a superbug, the first that would spring to mind would probably be Staphylococcus aureus. Actually they are much more likely to call it MRSA** but more of that later. Worryingly, it causes more than 1 in 10 of the infections patients catch while in hospital. But it doesn’t stop there and plenty of people get sick without stepping foot in a hospital.

To me S. aureus has three attributes that classify it as a superbug:

Firstly, it has an amazing repertoire of genes that enable it to cause a variety of illnesses, from nasty boils and food poisoning to much more life-threatening diseases such as pneumonia, meningitis, toxic shock syndrome and blood poisoning (bacteraemia).

Secondly, it is naturally very resistant to antibiotics. And everything becomes much more problematic when a person gets a particularly nasty variant called methicillin***-resistant S. aureus, abbreviated to MRSA. There are really only a few options for treating MRSA. First choice is vancomycin, but resistant strains are beginning to emerge. And when they do, we are left with just three or four antibiotics which are either very expensive or have to be given intravenously and are quite toxic at high doses.

Thirdly, S. aureus is very good at lurking around, especially in the hospital environment; on those curtains that are all that pass for privacy, the taps, the bed rail, and so on. It’s preferred home is people though. About a third of healthy people carry S. aureus, usually in the nose but sometimes on the skin or in the intestines. This is one of the main reasons for those alcohol gel dispensers you are supposed to use when you go and visit someone in hospital. Now, picture the scene. Gran has had a nasty fall and ended up in hospital. You have got some flowers and chocolates to cheer her up. The flowers make your nose a little itchy so you give it a scratch. You are in a rush and forget to use the hand gel. You deposit a nice splodge of S. aureus on her water glass when she asks you to get her a drink. She picks up the glass and a nasty infection. Hope you weren’t carrying MRSA…..

So some numbers. A recent study of S. aureus bacteraemia published by the Australia New Zealand Cooperative on Outcomes in Staphylococcal Sepsis (ANZCOSS) identified 1860 episodes of S. aureus bacteraemia in Australia and 134 in New Zealand between June 2007 and May 2008. Remember that’s just bacteraemia. About 1 in 5 of those people died. So next time you see that alcohol gel dispenser? Use it!

*I’m not going to put in a link. No-one should be encouraged to read that rag.

**Or mersa if they are American…

***Or multidrug, take your pick.

UPDATE:

It’s a bit out of date now, but I saw an article in the weekend Herald about a guy who has lost count of the number of artificial shoulder joints he has had to have replaced because of superbug infection, including S. aureus and ESBL’s*. This reminded me of a fourth attribute that helps classify S. aureus as a superbug: the ability to colonise medical devices. When a piece of plastic or metal is inserted into the body, be it an artificial shoulder joint or a catheter, it is rapidly covered in host proteins. S. aureus possesses a huge number of proteins that recognise and bind to these host proteins, allowing them to tether themselves to the artificial joint, etc. The bugs then form an amazing little community called a biofilm (wikipedia has a nice picture of a S. aureus biofilm on a catheter) where they can grow protected from antibiotics and the immune system. Quite a lot of work is going on (including here in Auckland) to try to produce plastics with antimicrobial properties to make medical devices that bacteria won’t be able to colonise.

*Bacteria expressing extended spectrum beta-lactamases. Will post on these next so watch this space.


5 Responses to “What makes a bug super?”

  • There are different strains of MRSA. Undoubtedly there are some strains resistant to vancomycin and vancomycin would be the wrong antibiotic to use for them.

    Locally we have a significant proportion of community acquired MRSA that are sensitive to erythromycin. With these, if the infection is not life threatening, we can use erythromycin tablets rather than intravenous only vancomycin!

  • More alarming than the above three points is the fact that Staph aureus mutated to produce an enzyme called Panton-Valentine Leucocidin (PVL) This enzyme effectively ruptures white cells in vivo and gives the Staph unhindered ability to cause cellulitis and sepsis. The ability is not limited to MRSA, and PVL positive Staphs have a remarkable ability to spread amongst normal healthy and active individuals. Outbreaks have been associated with the military, gyms, day-care centres and nursing homes. It is community acquired. We see about 4-5 a week and they are spectacular!
    As to erythromycin, make the best of it while it lasts, regional isolates will vary in susceptibilty patterns, but the trend is towards increasing resistance and increasingly virulent bacteria. These new ones make the 1950’s H. bug look like a teddy Bear

  • Thanks Jock, I guess for me PVL gets lumped into the first category of virulence genes. I recall hearing at some meeting that S. aureus has over 50 of them (virulence genes, that is). Saying that, PVL is interesting. Jerome Etienne gave a talk on PVL at the International Symposium on Staphylococci and Staphylococcal Infections (ISSSI) held in Bath last year. I can’t find my notes at the moment, but one thing he said stuck with me, and that is that PVL seems to be involved in allowing the bug to attach to the hair follicle and is why it is most associated with skin infections.