How does natural selection produce bacteria that are resistant to antibiotics?

Antibiotic resistance is a natural phenomenon. Resistant bacteria are found on and in humans, in our environment, on farms, in our homes and on our animals. They are all around us because resistance happens naturally as bacteria defend themselves against attack; resistant bacteria millions of years old have been found in the ice caps and in the frozen remains of woolly mammoths!

Antibiotic use and antibiotic resistance

When an antibiotic is used, bacteria that can resist that antibiotic have a greater chance of survival than those that are ‘susceptible’, and those that are not killed quickly multiply. Some resistance occurs without human action, as bacteria can produce and use antibiotics against other bacteria, leading to a low-level of natural selection for resistance to antibiotics. However, the current higher-levels of antibiotic-resistant bacteria are attributed to the overuse and abuse of antibiotics.

Some bacteria are naturally resistant to certain types of antibiotics. Some mutate to either produce enzymes that ‘deactivate’ antibiotics while other mutations change or close the target area on the bacteria that the antibiotic would normally attack. Some even create mechanisms to push the antibiotic back out of the cell when it attacks. Bacteria can acquire antibiotic resistance genes from other bacteria in several ways. They can transfer genetic material through a simple ‘mating’ process, or through plasmids that ‘reprogramme’ other bacteria to be resistant to antibiotics. They can also pick up stray DNA in their environment or can be infected by viruses.

The spread of antibiotic resistance

Antibiotic resistance spreads as bacteria themselves move from place to place via human contact, for example, through coughing, or contact with unwashed hands, as well as animal contact, contaminated materials and in water, food and the wind. You will find resistant bacteria in the same places you find bacteria – it’s just some of them are resistant. For more on this subject visit our Expert FAQs.

Approaches to reducing antibiotic resistance include undoing current damage, reversing it, and slowing evolution down. Undoing damage will often entail new antibiotics that overcome current resistance. But the basic arms race still churns, costly in terms of new drugs and even lives.

It is possible in some cases to reverse evolution -- to encourage the re-emergence of drug-susceptible bacteria by removing the selective pressure of drugs. Resistance mechanisms sometimes generate an energetic cost, like extra proteins that the bacteria must make for drug detoxification. In the drug's absence, non-resistant bacteria replicate faster, and eventually take over. Unfortunately, few opportunities arise to take advantage of this tradeoff, because many bacteria have evolved low-cost antibiotic resistance.To slow evolution, we can reduce selective pressure by avoiding antibiotics when they are not useful, (e.g., viral infections), or substituting other anti-bacterial strategies instead of chemical control. In agriculture, integrated pest management employs physical reduction of insect pests and thereby reduces reliance on insecticides. Hospitals also use this strategy, except they call it hand-washing.Another method is to change selective pressures periodically by changing the antibiotic used. This alters the trajectory of evolution and can delay the day when full resistance to one antibiotic evolves.A third way to slow evolution is pyramiding, the use of multiple drugs to deliver a strong killing dose. Evolution only occurs in a variable population -- when some bacteria are able to survive an antibiotic dose, but others are not. In the presence of a drug overkill, there is no variation -- all bacteria die -- and evolution slows dramatically. This is what makes triple-drug therapy effective against the HIV virus.In the face of rapid bacterial evolution, all drug strategies are temporary. But by studiously engineering the evolutionary process, we can extend the life of powerful drugs, slow the arms race, and reduce the social and economic costs of disease.(Boldface added.)
Learn how deadly microbes outpace medical breakthroughs.
Are we in the midst of one? And if so, did we trigger it? Antibiotics are chemicals produced by microbes that either kill (bactericidal) or inhibit the growth (bacteriostatic) of bacteria

• Antibiotics are commonly used by man as a treatment for bacterial infections (not effective against viral infections)

In a bacterial colony, over many generations, a small proportion of bacteria may develop antibiotic resistance via gene mutation

• When treated with antibiotics, the resistant bacteria will survive and reproduce by binary fission (asexual reproduction)
• The antibiotic resistant bacteria will flourish in the absence of competition from other strains of bacteria (killed by antibiotic)
• Antibiotic resistant bacteria may also confer resistance to susceptible strains by transferring plasmids via bacterial conjugation
• The introduction of antibiotic (selection pressure) has caused the antibiotic resistance gene to become more frequent (evolution)

An example of antibiotic resistance in bacteria can be seen in the evolution of Staphylococcus aureus (Golden staph)

• Golden staph can cause infections to the skin (lesions and boils) as well as more serious infections (pneumonia, meningitis)
• Historically, these infections were treated using the antibiotic methicillin 
• Bacterial strains developed that were resistant to this antibiotic (methicillin-resistant Staphylococcus aureus – or MRSA)
• These strains proliferated while susceptible strains died out (methicillin-sensitive Staphylococcus aureus – or MSSA)
• MRSA infections are now especially present in hospitals and nursing homes, where the use of methicillin was most common
• Medical practitioners now prescribe alternate antibiotic agents to treat infections caused by Staphylococcus aureus

  Is antibiotic resistance caused by natural selection?

  Antibiotic resistance appearance and spread have been classically considered the result of a process of natural selection, directed by the use of antibiotics.

  What caused the evolution of antibiotic resistant bacteria?

  The development of generations of antibiotic-resistant microbes and their distribution in microbial populations throughout the biosphere are the results of many years of unremitting selection pressure from human applications of antibiotics, via underuse, overuse, and misuse.

  How are bacteria becoming resistant to antibiotics?

  There are two main ways that bacterial cells can acquire antibiotic resistance. One is through mutations that occur in the DNA of the cell during replication. The other way that bacteria acquire resistance is through horizontal gene transfer.

  How does natural selection produce bacteria?

  Mutations in the DNA of bacteria can produce new characteristics. A random mutation might cause some bacteria to become resistant to certain antibiotics , such as penicillin. Antibiotics usually kill bacteria, but in this case the mutation means the bacteria cannot be destroyed by the antibiotic.