Abstract
An antibiotic is an agent that either kills or inhibits the growth of a microorganism. The term antibiotic
was first used in 1942 by Selman Waksman and his collaborators in journal articles to describe any
substance produced by a microorganism that is antagonistic to the growth of other microorganisms in
high dilution. This definition excluded substances that kill bacteria but that are not produced by
microorganisms (such as gastric juices and hydrogen peroxide). It also excluded synthetic antibacterial
compounds such as the sulfonamides. Many antibacterial compounds are relatively small molecules
with a molecular weight of less than 2000 atomic mass units.
With advances in medicinal chemistry, most modern antibacterial are semi synthetic modifications of
various natural compounds. These include, for example, the beta-lactam antibiotics, which include the
penicillin (produced by fungi in the genus Penicillium), the cephalosporin, and the carbapenems.
Compounds that are still isolated from living organisms are the amino glycosides, whereas other
antibacterial—for example, the sulfonamides, the quinolones, and the oxazolidinones—are produced
solely by chemical synthesis.
In accordance with this, many antibacterial compounds are classified on the basis of
chemical/biosynthetic origin into natural, semi synthetic, and synthetic. Another classification system
is based on biological activity; in this classification, antibacterial are divided into two broad groups
according to their biological effect on microorganisms: Bactericidal agents kill bacteria, and
bacteriostatic agents slow down or stall bacterial growth.
What is Antibiotic Resistance?
Antibiotic resistance is a form of drug resistance whereby some (or, less commonly, all) sub-
populations of a microorganism, usually a bacterial species, are able to survive after exposure to one
or more antibiotics; pathogens resistant to multiple antibiotics are considered multidrug resistant(MDR)
or, more colloquially, superbugs.
Antibiotic resistance is a serious and growing phenomenon in contemporary medicine and has
emerged as one of the pre-eminent public health concerns of the 21st century, in particular as it
pertains to pathogenic organisms (the term is especially relevant to organisms that cause disease in
humans). A World Health Organization report released April 30, 2014 states, "this serious threat is no
longer a prediction for the future, it is happening right now in every region of the world and has the
potential to affect anyone, of any age, in any country. Antibiotic resistance–when bacteria change so
antibiotics no longer work in people who need them to treat infections–is now a major threat to public
health."
In the simplest cases, drug-resistant organisms may have acquired resistance to first-line antibiotics,
thereby necessitating the use of second-line agents. Typically, a first-line agent is selected on the
basis of several factors including safety, availability, and cost; a second-line agent is usually broader
in spectrum, has a less favorable risk-benefit profile, and is more expensive or, in dire circumstances,
may be locally unavailable. In the case of some MDR pathogens, resistance to second- and even
third-line antibiotics is, thus, sequentially acquired, a case quintessentially illustrated by
Staphylococcus aureus in some nosocomial settings. Some pathogens, such as Pseudomonas
aeruginosa, also possess a high level of intrinsic resistance.
It may take the form of a spontaneous or induced genetic mutation, or the acquisition of resistance
genes from other bacterial species by horizontal gene transfer via conjugation, transduction, or
transformation. Many antibiotic resistance genes reside on transmissible plasmids, facilitating their
transfer. Exposure to an antibiotic naturally selects for the survival of the organisms with the genes for
resistance. In this way, a gene for antibiotic resistance may readily spread through an ecosystem of
