Bacteria are single celled living organisms and unlike viruses, bacteria do not need living cells to multiply in. They can grow almost anywhere. Most, called "normal flora” are beneficial, helping to maintain a homeostatic balance to the environment. Some internal bacteria help with digestion, even helping to manufacture vitamins. Other bacteria called "pathogens" cause disease. Most bacteria behave in a conventional way but there is an exceptional group called rickettsiae that in some ways behave more like viruses. Rickettsiae, as small as virus particles, need living cells to multiply in. Most are transmitted through the bites of external parasites such as ticks, fleas, lice and mites. These are more prevalent in continental Europe than in the UK.
Most bacteria are susceptible to antibiotics which either prevent bacteria from multiplying or actively kill them. Although they are chemically synthesised, virtually all antibiotics are copies of natural substances produced by microorganisms for their own protection. Pharmaceutical companies constantly search for substances that microbes produce to protect themselves, that also inhibit or kill other pathogenic bacteria.
By using antibiotics unwittingly we have accelerated natural selection amongst bacteria faster than at any time in the history of life. Resistance occurs when an antibiotic kills all the "normal" bacteria, leaving the genetic oddball that otherwise would not have survived, alone and capable of multiplying. This genetic variant replaces the old population. The new population is resistant to that antibiotic. Even more ingenious, bacteria can pass their resistance from one species of bacteria to another. This is why at the London Veterinary Clinic we use antibiotics with restraint, only when there is a clear need for them.