Antibiotic resistance is when a strain of bacteria is not affected by an antibiotic, as such the bacteria in question can colonise tissues and cause disease in the presence of antibiotics. This is becoming an increasing problem in many infections and some strains have developed multiple drug resistance meaning they are resistant to a range of different antibiotic treatments and combinations. An example of such a virus is MRSA (methicillin-restistant Staphylococcus aureus) which is very difficult to treat and is causing increasing problems in places such as hospitals where open wounds are present and many of the patients have weakened immune systems. They are therefore more vulnerable to infection. 
One way in which bacteria can become resistance to the antibiotic properties of a drug involves the exposure of the organism to a non-lethal quantity of the drug which will allow the bacteria to adapt to negate the effect of the drug and providing it survives will provide a level of resistance for future exposures and future generation through vertical gene transfer. Bacteria can also pass these properties to other bacteria through horizontal gene transfer involving the processed of bacterial conjugation, the exchange of genetic material through use of a sex pilus, bacterial transformation the uptake of free DNA from the environment, usually plasmid DNA, or bacterial transduction where the bacteria is injected with foreign DNA through use of a bacteriophage, all can result in antibiotic resistance being passed on. 
- ↑ Chambers H, DeLeo F. Waves of resistance: Staphylococcus aureus in the antibiotic era. Nature Reviews Microbiology [Internet]. 2009 [cited 3 December 2017];7(9):629-641. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2871281/
- ↑ Silva J. Mechanisms of antibiotic resistance. Current Therapeutic Research. 1996;57(13):30-35.
- ↑ Barlow M. What Antimicrobial Resistance Has Taught Us About Horizontal Gene Transfer. Horizontal Gene Transfer [Internet]. 2009 [cited 3 December 2017];1(1):397-411. Available from: https://www.ncbi.nlm.nih.gov/pubmed/19271198