Because when women and men carry purses or watches, respectively, they gain a whole lot of properties, capacities not shown before. They bring up their outfit colors, make them look younger, offer them more confidence, provide them with resistance to a lot of eminent issues like space for meds, documents, mascara and other paraphernalia, or just kind of 'elevate' their social status. Purses and watches basically make a woman/man feel empowered, stronger, more able, even if that is just a temporary property. The wrong purse/watch though will downgrade the carrier to an image of sloppiness others see as an inappropriate taste for fashion, thus unmasking their incapacity.
These circular rings of DNA (plasmids) confer characteristics to the bacteria by adding properties to their genotype that can, eventually, propagate to their phenotype. But how many types of plasmids are there? Well, this question is so redundant that the best way to answer it is with a blunt straight answer. Many I'd say, but we could reduce all those not to three as initially proposed, but actually to five very important classes. But let me twist the example given before and throw out the pursues and watches once for all. Lets talk about plasmids as military roles, I guess it will help us understand their these different classes a bit better:
Resistance plasmids, also known as R-factors, these are the most common ones in the laboratories worldwide spread. These are plasmids that attribute their carriers different resistances to antibiotics, toxicants, poisons etc! We could call them the shield-division for they really stand against the toughest of the environments and give their bacteria protection to a lot of chemical harm.
Virulence plasmids are potential weapons of mass destruction! It's like a really skinny dude taking a bunch load of steroids, Suddenly they make this crazy weak bacteria perform like a Dwayne Johnson. They can make any bacteria a pathogenic liability by conferring them the means to produce some harmful substances.
Fertility F-plasmids, also known as conjugative plasmids or F-factor - of around 100 kilo-bases of length, they hold their own origin of replication and origin of transfer, so they are empowered to 'decide' when and if conjugation will happen. They are the intelligence gathering, the sexy 007 spies. They result in the expression of sex pili that transfers DNA between bacteria. As you can see they're a very avant-garde bunch in the world of DNA transfer.
Conditional plasmids are a sub-division of the military bacterial paraphernalia. They only act when triggered to by different and specific conditions like temperature, for example. When prompted to act they will then produce whatever is written in their artificial genetic code, whether it is producing a certain protein or just protect against a certain antibiotic, you name it, you're the boss.
Degradative plasmids are the fat guys that really want to be part of the army but have no physic to do it. So they just get hired to work in the kitchen and now and then enable bacteria to digest substances that are not very typical in the bacteria's environment. It's like you don't know how to eat a raw potatoe, these fat guys will slice it for you, cut it in little stripes, fry it and salt it and then serve it to you like a freakin' awesome gourmet dish!
Col plasmids, or shall we 'col' them the snipers. They help bacteria produce proteins (bacteriocins), like for example colicin, that will kill other bacteria, point blank. They are indeed very 'col' assassins. I guess that's enough punning! I'm 'col' if you are too, heheheheh.
Have a wonderful day!
1st image taken from Cell meets plasmid, Deviant Art, [http://velica.deviantart.com/art/Cell-meets-plasmid-151423037].
2nd image source unknown.