Can someone explain to me how bacteria becomes resistant...

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to antibiotics?

God changes them to have a fighting chance.

On a more serious note: Evolution. Bacteria reproduce much faster than humans or animals, so they evolve more rapidly. Random mutations may make specific bacteria resistent to antibiotics. If this happens incidentally, it's not really an issue, but if antibiotics are widely used, all strains that didn't develop this mutation will die out, while the strains that became resistent continue to thrive.

Exactly as the other repliers have said. I felt I would mention that this effect is not only seen in bacteria in practice but bacteria just reproduce so fast. Anyhoo, apparently rabbits have a tendency to breed like rabbits and love doing it in Australia where they are an invasive species. The government there has been attempting biological tactics to combat the problem and the rabbits eventually develop resistance.
http://en.wikipedia.org/wiki/Rabbits_in ... l_measures

It's pretty much the exact same process as any other kind of evolution. I'll try to explain:

Evolution occurs when selectional pressures occur in the environment in which a group of organisms exist (by selectional pressures I mean factors which will cause some individuals to become less likely to reproduce). In any population of any organism mutations will occur which will may occasionally confer a protection against a particular selectional pressure. If said pressure is particularly powerful then the individuals who do not carry this mutation will find it *very* difficult to reproduce (perhaps by virtue of being dead). They will be out-competed and eventually wiped completely in favour of the individuals which do carry the mutation which confers the selectional advantage.

So, let's apply this to those bacteria which are pathogens of humans. We needn't make reference to any specific real life antibiotic or bacteria. The environment in which the population of pathogenic bacteria exists is a sub-set of the human population. Because antibiotics are frequently very effective at killing bacteria, the existence of antibiotics represents a strong selectional pressure on bacteria. Like I mentioned earlier though, in ANY population of organisms mutations occur. Bacteria reproduce really really quickly which means that a lot of mutations occur. Lets say that a particular antibiotic effects the synthesis of a protein which a bacterium needs to produce its cell wall. Our antibiotic will kill pretty much all of our example bacteria because the cell wall is vital to the function of the bacterium. However, if a subset of our population of bacteria have a mutation which, for example, means that they synthesise the important protein in a different way which is unaffected by the antibiotic then they have a powerful selectional advantage and as discussed in the first paragraph they are likely to eventually out-compete the bacteria which do not possess the protection-conferring mutation.

In real life things aren't quite so simple as this. It's not all that likely that a mutation which confers such powerful protection against a given antibiotic is going to happen all at once, straight away, BOOM! This is the reason why there is so much emphasis on finishing a course of antibiotics. If you have a bacterial infection then the the population of bacteria which comprise your infection may contain a subset that have a mutation which confers them partial resistance to the antibiotic. Their compatriots with no resistance will be rapidly destroyed when you start taking the drug, perhaps within just a couple of days. The ones with partial resistance will eventually succumb if you complete the course of the drug. However, if the course of antibiotics is NOT completed a new population of bacteria may have just been created which now ALL have a partial resistance to the antibiotic in question - all the one with no resistance are dead! This is a strong point from which said population of bacteria may eventually become fully resistant to our example antibiotic.

Again it's not all necessarily as simple as I am making it out. There are other factors which can come into play e.g. horizontal transfer of genes between different bacteria populations. Yes, bacteria essentially swap genes with one another sometimes. This is another means by which resistance to antibiotics can be conferred. That is the reason why there is a great deal of concern about the extensive use of antibiotics in animal farming. We may be creating populations of bacteria which are related to human pathogens and have strong resistance to some of our most useful antibiotics. A mutation or an incident of horizontal gene transfer may occur which either causes the animal pathogens to be able to readily infect humans or which transfers the ability to resist a given antibiotic from an animal pathogen to a human pathogen. It really is a matter of great concern, the worst case scenario if things continue as they are and we do not discover some new antibiotics is very worrying.

what besides renewed/updated phage therapy can be a useful workaround?

While bacteria multiply into the billions and trillions, small random mutations happen in some of them. Most mutations are insignificant (maybe instructing the bacteria to grow two more tentacles or something), and some mutations can mean instant death for the bacteria in question. Sometimes, the mutations can cause it to survive for longer than other bacteria. Soon, the winning mutations then become the most prevalent ones.

If a particular bacteria happens to get a mutation that actually enables it to survive antibiotics (which is pretty plausible given the sheer amount of bacterias and mutations happening), that bacteria outlives every other bacteria without the mutation and get to multiply freely.

can someone explain to me the barriers in phage therapy being rolled out across the board?

It sounds like it could have the potential to eradicate illnesses like menningitis and MRSA.

evolution.

Lots and lots of mutations!