Antibiotic resistance is the ability of a microorganism to withstand the effects of an antibiotic. It evolves naturally via natural selection through random mutation. Once, such a gene is generated, bacteria can then transfer the genetic information in a horizontal fashion (between individuals) by plasmid exchange. If a bacterium carries several resistance genes, it is called multiresistant or, informally, a SUPERBUG.

Several studies have demonstrated that patterns of antibiotic usage greatly affect the number of resistant organisms which develop. Overuse of broad-spectrum antibiotics, such as second- and third-generation cephalosporins, greatly hastens the development of methicillin resistance. Other factors contributing towards resistance include incorrect diagnosis, unnecessary prescriptions, improper use of antibiotics by patients, and the use of antibiotics as livestock food additives for growth promotion.

Some examples of resistance bacteria are:

        1. Methicillin-resistant Staphylococcus aureus (MRSA), a strain of bacteria no longer sensitive to different forms of penicillin. It can cause complicated skin infections and pneumonia and requires stronger antibiotics such as vancomycin for treatment. Researchers have recently demonstrated the bacterial protein LexA may play a key role in the acquisition of bacterial mutations. Resistant pathogens Staphylococcus aureus (colloquially known as "Staph aureus" or a Staph infection) is one of the major resistant pathogens. Found on the mucous membranes and the skin of around a third of the population, it is extremely adaptable to antibiotic pressure. It was the first bacterium in which penicillin resistance was found—in 1947, just four years after the drug started being mass-produced. MRSA (methicillin-resistant Staphylococcus aureus) was first detected in Britain in 1961 and is now "quite common" in hospitals. MRSA was responsible for 37% of fatal cases of blood poisoning in the UK in 1999, up from 4% in 1991.
        2. Multidrug-resistant tuberculosis, which is very difficult to treat because it is caused by microorganisms that are resistant to 2 of the first-choice drugs used to treat tuberculosis.
        3. Extended-spectrum β-lactamase bacteria, a group of bacteria that have evolved to produce an enzyme that can interfere with the mechanisms of several newer antibiotics, making these bacteria resistant and difficult to treat.

The first antibiotic was penicillin. Such penicillin-related antibiotics as ampicillin, amoxicillin and benzyl penicilllin are widely used today to treat a variety of infections.

Penicillin is also the first-Generation antibiotic. This means that it has a narrow spectrum of clinical use, but is good for Gram positive bacteria. Some bacteria produce beta-lactamase which inactivates penicillin. Second generation antibiotics is ampicillin and amoxicillin. They have an extended or broad spectrum of clinical use and the Gram negative bacteria is more sensitive to 2nd generation. Third generation (i.e. carbenicillin and ticarcillin) has broader spectrum of use than the second-generation penicillin, and is in use for serious infections. Fourth generation (i.e. mezlocillin sodium, piperacillin) is even more potent. As the lower generation antibiotics are losing their effects, we are left with fewer options to treat the diseases. If this continues, things as common as strep throat or a child’s scratched knee could once again kill, says Dr. Margaret Chan-Director General of WHO in his keynote address of the conference on Combating Antimicrobial Resistance, Denmark