Antibiotic-resistant bacteria

DEFINITION: Ability of some bacteria to resist or entirely withstand the effects of antimicrobial agents.

SIGNIFICANCE: Although most bacteria are benign, a small percentage are pathogenic, or disease-causing. Bacteria rank among the most important of all disease-causing organisms in humans, and bacterial infections are countered by a wide variety of antibiotic and antibacterial agents. Repeated use of such agents results in bacterial resistance, necessitating the development of stronger antibacterial agents. Increasing fears that antibiotic-resistant strains of bacteria may be used as bioweapons add urgency to efforts to develop new antibacterial agents.

Less than 1 percent of all bacteria cause illness in humans. These disease-causing species are notorious for diseases such as cholera, typhus, and syphilis. The most common and some of the most deadly forms of bacterial diseases are respiratory infections, such as tuberculosis, which kill millions of people every year. Countries around the world have used antibiotic drugs to treat bacterial infections for more than fifty years. The initial introduction of antibiotics was markedly successful, but continued and widespread use has resulted in a phenomenon in which microbial adaptation is making targeted bacteria increasingly difficult to control. This bacterial resistance to antibiotics is of special concern, as ever more powerful antibiotics must be developed. The 2025 Global Antibiotic Resistance Surveillance report from the World Health Organization highlighted that as per the 2023 data about one in six bacterial infections across the world were antibiotic resistant.

Antibiotics and Antibacterials

In its broadest definition, an antibacterial is an agent that interferes with the growth and reproduction of bacteria. Although antibiotics and antibacterials both attack bacteria, these terms have evolved over the years to mean two different things. The term “antibacterials” is most commonly applied to agents that are used to disinfect surfaces and eliminate potentially harmful bacteria. The term “antibiotics” is commonly reserved for medicines given to humans or animals to treat infections or diseases.

Bacteria become resistant to antibacterial agents in one of three ways: natural resistance, vertical evolution, and horizontal evolution. Therefore, bacteria exhibit either inherited or acquired resistance to antibacterial agents. Natural resistance occurs when bacteria are inherently resistant to an antibacterial. For example, a gram-negative bacterium has an outer membrane that establishes an impermeability barrier against the antibiotic it is exposed to, so it does not self-destruct.

Acquired resistance occurs when bacteria develop resistance to an antibacterial agent to which the population has been exposed. This may occur through mutation and selection (vertical evolution) or exchange of genes between strains and species (horizontal evolution) of the bacteria exposed to the antibacterial agent.

Vertical evolution represents an example of Darwinian evolution driven by principles of natural selection. Genetic mutations in the bacteria population create new genes or combinations of genes that are resistant to the antibacterial agent. While the nonmutant, sensitive bacteria are killed, bacteria containing the mutated genes survive, and their progeny populate the increasingly resistant colony.

Another form of acquired resistance, horizontal evolution, is the transfer of resistant genes from one bacterium to another in the population. For example, Escherichia coli or Shigella may acquire a gene from a streptomycete that is resistant to the antibiotic streptomycin. Following this transfer, the population contains a mutant E. coli bacterium now resistant to streptomycin. Then, through the process of selection, it donates these genes to further generations, creating a resistant strain.

Transfer of genes in bacteria occurs in one of three ways: conjugation, transduction, or transformation. In conjugation, the gene-containing DNA (deoxyribonucleic acid) crosses a connecting structure, called a pilus, from a donor bacterium to recipient bacteria. In transduction, a virus may transfer genes between bacteria. In transformation, DNA is acquired directly from the environment, having been released from another bacterium. Following transfer, the combination of the newly acquired gene or genes results in a process called genetic recombination that may lead to the emergence of a new genotype. The combination of transfers and genetic recombination promotes rapid spread of antibacterial resistance through a species population and also between strains and other bacterial species.

The combined effects of fast growth rates, high concentrations of cells, genetic processes of mutation and selection, and genetic recombination account for the extraordinary rates of adaptation and evolution observed in bacteria populations. For these reasons, bacterial resistance to antibacterials is a common occurrence and one that promises to be of increasing concern in the future. In fact, high levels of antibiotic-resistant bacteria are found in numerous common infections, including pneumonia and urinary tract infections, and there are an increasing number of cases of antibiotic-resistant tuberculosis. However, a study published in The Lancet in 2022 indicated that researchers were making progress on impairing antibiotic-resistant bacteria that cause human diseases by inhibiting a protein that makes them resistant. If researchers can do this, individuals with bacterial diseases would be able to be successfully treated with antibiotics.

Bacterial Resistance and Forensic Science

The importance of bacteriology in forensic science is recognized in diverse areas, including DNA profiling, toxicology studies, fingerprinting, and the tracing of violence stemming from or potentially relating to murders. Bacteria have been used as weapons and can be the causes of violence, but they may also serve as tools in the investigation of crimes.

The most serious threat posed by bacteria is their possible use in biological warfare, especially in acts of bioterrorism. For example, Bacillus anthracis, which causes anthrax, has become a preferred bacterial strain used by terrorists. Strains of deadly bacteria selected especially for their antibody resistance can pose health threats of enormous proportions at both local and global levels.

Some research has suggested that bacterial infections can lead to criminal behavior. For example, in some children, Streptococcus infections have been linked to hyperactivity, a condition called Pediatric Autoimmune Neuropsychiatric Disorders Associated with Streptococcal Infections. In this case, the body mounts an autoimmune response that affects the brain, causing hyperactivity and affecting emotion, behavior, and movement. These changes have been linked to criminal behavior. Some defense lawyers have used such research findings in attempts to explain their clients’ actions, connecting criminal behavior with infection-caused states of delirium.

In some cases, the bacteria present at the site of a crime can give important clues about the crime itself. For instance, bacteria can reveal how long a person has been dead or the temperature the body was subjected to after death. Heart and spleen blood cultures may be taken at autopsy to identify any possible infections or diseases the deceased may have had.

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