Antibiotics are natural or synthetic compounds that kill bacteria. There are a myriad of different antibiotics that act on different structural or biochemical components of bacteria. Antibiotics have no direct effect on viruses. A few antibiotics are effective, however, against certain protozoan parasites. Metronidazole, for example, is sometimes used to treat giardiasis (beaver fever) or amoebic dysentery.
Background and Scientific Foundations
Prior to the discovery of the first antibiotic, penicillin, in the 1930s, there were few effective ways of combating bacterial infections. Illnesses such as pneumonia or tuberculosis were virtually untreatable. Minor bacterial infections could blossom into life-threatening maladies. In the decades that followed the advent of penicillin, scientists discovered many naturally occurring antibiotics. Still more were synthesized towards specific targets on or in bacteria.
Antibiotics are manufactured by bacteria and various eukaryotic organisms, such as plants. These natural antibiotics protect the organism from attack by other bacteria. The discovery of antibiotic compounds involves screening samples of bacteria for an inhibition in bacterial growth. In commercial settings, such screening has been automated so that thousands of samples can be processed each day. Antibiotics also can be manufactured by tailoring a compound to hone in on selected targets. The advent of molecular sequencing technology and three-dimensional image reconstruction facilitates new antibiotic design.
Classes of common antibiotics
Penicillin is one of the antibiotics in a class known as beta-lactam antibiotics. This class is named for the ring structure that forms part of the antibiotic molecule. Other classes of antibiotics include the tetracyclines, aminoglycosides, rifamycins, quinolones, and sulphonamides. The action of these antibiotics is varied. For example, beta-lactam antibiotics exert their effect by disrupting the manufacture of peptidoglycan, which is the main stress-bearing network in the bacterial cell wall. The disruption can occur by either blocking construction of the subunits of the peptidoglycan or preventing their incorporation into the existing network. In another example, amonglycoside antibiotics can bind to a subunit of the ribosome, which blocks the manufacture of protein or reduces the ability of molecules to move across the cell wall to the inside of the bacterium. Quinolone antibiotics disrupt the function of an enzyme that uncoils the double helix of deoxyribonucleic acid, which is vital if the DNA is to be replicated.
Along with variation in their targets for antibacterial activity, antibiotics also can vary in the range of bacteria they affect. Some antibiotics are classified as narrow-spectrum antibiotics. They are lethal against only a few types (or genera) of bacteria. Other antibiotics are active against many bacteria whose construction can be very different. Such antibiotics are described as having a broad-spectrum of activity.
Issues and Developments
In the decades following the discovery of penicillin, many different antibiotics proved effective in controlling infectious bacteria. Antibiotics quickly became (and to a large extent remain) a vital tool in the physician's arsenal against bacterial infections. By the 1970s, the success of antibiotics led to the generally held view that bacterial infectious diseases would soon be eliminated. However, the subsequent emergence of antibiotic resistant bacteria to many commonly administered antibiotics has challenged this idea.
Antibiotic resistance can develop naturally through evolution. It is also prompted when antibiotics are overused or misused. When an antibiotic is used properly to treat an infection, all of the infectious bacteria are killed or weakened enough that the host's immune response will kill them. Using the wrong antibiotic (an antibiotic to which the bacteria is not sensitive), or stopping antibiotic therapy before the prescribed time period can leave surviving bacteria in the population prone to develop resistance. Heavy use of antibiotics in healthy animals intended for meat is also considered overuse, and can contribute to antibiotic resistance.
While the number and effectiveness of available antibiotics declined, scientists advanced the idea of amplifying natural antibiotics. The human body manufactures these in very small quantities. Researchers have identified genes and combinations of genes containing the coding for thousands of molecules that might prove to be useful antibiotics.
Resistance to an antibiotic also can be overcome by modifying the antibiotic slightly through the addition of a different chemical group. This acts to alter the structure of the antibiotic. Unfortunately, modifications like these tend to reduce susceptibility to the new antibiotic in a relatively short time.
If resistance is controlled by a genetic alteration, any genetic change may be passed on to future generations of bacteria. For example, many newer strains of bacteria that cause tuberculosis have also become resistant to antibiotics routinely used to treat older-strain infections. Physicians also caution that antibiotic over-prescription, especially giving antibiotics for viral conditions over which they have no effect such as a cold or the flu, is decreasing the effectiveness of many common and inexpensive antibiotics.
Developing newer-generation antibiotics
A new way to grow soil-based bacteria was found in 2015 that helped scientists produce new types of antibiotics. The last such fundamental discovery of classes of antibiotics took place in the late 1980s. Growing a wider variety of bacteria that already exists increases the chances of isolating new natural antibiotics. Public health experts heralded the breakthrough as a potential game changer, but other researchers called for tempered expectations. They noted that most of the newer antibiotics have yet to undergo comprehensive human testing, and that most of the antibiotics produced thus far work only against Gram-positive bacteria. Newer-generation antibiotics introduced since 2015 that received FDA approval include ceftazidime-avibactam, ceftolozane-tazobactam, and meropenem-vaboractam. All three of these antibiotics are effective against Gram-negative bacteria.
The United States government considers antibiotic resistance as a significant threat to national health and security, roughly on par with the threat posed by a potential pandemic. The World Health Organization also listed antimicrobial resistance as one of its top ten global challenges in 2019. That same year, the U.S. Biomedical Advanced Research and Development Authority (BARDA) dedicated 1.2 billion dollars toward research for the rapid development of 50 new antibiotic drugs.