Are resistant organisms stronger (more virulent)?

No. This comes down to a core evolutionary question.

We've all heard of "superbugs", which are resistant strains of common organisms, usually bacteria. For instance, there's:

• MRSA (methicillin-resistant Staphylococcus aureus) - the bane of many a hospital. (Methicillin is an antibiotic; being resistant to it implies that most other antibiotics in that class won't work.
• MDR TB and XDR TB (multi-drug-resistant tuberculosis, and extremely drug-resistant tuberculosis) - worst in South Africa, this new strain is resistant to most or all of the first-line agents, and in that case of XDR, even some second-line agents.

So how do these strains come about? Simple, by evolution. The genes that create organisms vary randomly, in this case usually due to copying errors that arise when the DNA in a cell is copied prior to it splitting in two. Other sources of DNA mutations are mutagens, for instance certain chemicals, sunlight, radiation, etc. Most of these mutations are harmful to the organism and carriers of such mutations are more likely to die out than other non-mutated competitors.

However, some of these mutations are beneficial (this must be true, by definition, since the mutations are random). Thus, organisms carrying these mutations will be more likely to survive than their competitors. Over time therefore, the population will come to be filled with the 'mutated' type. This is, very basically, how evolution works.

So what sort of mutations could prove beneficial when it comes to dealing with humans? Well, ones that decrease the efficacy of antibiotics would certainly be among them. In this way, it is utterly inevitable that the longer we use an antibiotic, the more likely we are to get resistant strains evolving.

That's a vitally important point, and it isn't often appreciated. With this in mind, however, there are still ways of slowing the evolution of resistance down. For instance, only giving antibiotics when appropriate is important (if your general practitioner gives you antibiotics for every sore throat, shout at him). Also, patients should complete the full course of any antibiotics. There are other measures too; hopefully I'll get round to the topic in another post.

But to return to the question: are resistant organisms stronger? No, not at all. Their mutations have simply caused them to fight off antibiotics better, not to now become more virulent. For instance, a bacterium that used to only cause skin infections won't suddenly give you meningitis once it becomes resistant! In fact, in many cases, diverting expensive cellular resources into antibiotic-fighting measures makes the bacterium 'weaker' - although, since we use antibiotics, the cell would still have an evolutionary advantage over its rivals in enough cases.

Of course, the practical effect of being resistant may to make the bug look stronger, since it's resistance to antibiotics does have the effect of making it more likely to do you damage. However, when compared to the non-resistance strain in the absence of antibiotics, it isn't necessarily any more virulent.