accidents in science occasionally lead to great discoveries

8/3/2019 Accidents in Science Occasionally Lead to Great Discoveries

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Accidents in science occasionally lead to great discoveries. We owe the identification ofpenicillin to one such serendipitous mishap. Sir Alexander Fleming discovered the first

therapeutic antibiotic in 1929 when a green mold contaminated one of his bacterial culturedishes. Fleming observed that where the mold had invaded, the bacterial colonies

(Staphylococcus aureus) had disappeared. He realized that not only did this moldwhich was of

Penicillium notationhave antibacterial properties in vitro, but that there was also potential forusing the molds secretions in therapies.

How did the mold get into the dish in the first place? As it turns out, Flemings lab was upstairsfrom the lab of a mycologist, and the mold from the mycologists lab contaminated Fleming's

cultures. Although scientists try hard not to contaminate each others work, their fortunate failureto do so in this instance led to a discovery that saved millions of lives.

Actually, Fleming was not the first person to recognize the antibacterial properties of mold. As

far back as 2,500 years ago, the Chinese were using treatments made of moldy soybean curd totreat infections. The ancient Egyptians rubbed moldy bread on wounds to cure them, and moldy

cheese was used for the same purpose in parts of Europe.

BACKGROUND

These days, youre not going to scrape the mold off cheese to procure its antibiotic effects.Myriad antibiotics are available to treat various illnesses, curing bacterial infections ranging

from strep throat to urinary tract infections.

The structure of the cell wall divides bacteria into two groups, the Gram positive and the Gramnegative. Gram-positive bacteria have a thick layer of peptidoglycan, a sugar and peptide coating

that gives a cell its shape and helps it stay intact. The original antibiotic, penicillin, and its

cousins are used to treat infections caused by bacteria that are Gram positive.

Gram-positive bacteria stain blue-violet in a Gram-staining procedure. Streptococcaland

staphylococcalstrains are Gram positive, and these bacteria are responsible for illnesses such asstrep throat, blood poisoning, pneumonia, and toxic shock syndrome. Other classes of antibiotics,

including streptomycins and tetracycline, effectively destroy both Gram-negative and Gram-positive bacteria, making them able to fight pathogens such as Shigella orSalmonella.

BACTERIAL RESISTANCE AND HEALTH

Many bacterial strains now resist the effects of antibiotics that once could destroy them. Everypopulation of bacteria may have some individuals that are resistant. The proliferation of

antibiotics and careless use of the drugs have given some resistant bacteria the upper hand in thefight against disease.

A patient who is prescribed a 10-day course of antibiotics, but who quits taking them after acouple of days because the symptoms have subsided, leaves behind bacteria that resisted the

antibiotic effect. Growth of these bacteria may have slowed in the presence of the antibiotic, but


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the bacteria are not completely wiped out. Some resistant bacteria may survive an even longercourse of antibiotics if the dosage of the drug is not high enough. Typically, after a complete,

full-strength antibiotic course, so few resistant bacteria remain that the bodys own immunesystem can handle them; however, a short course may leave behind so many resistant bacteria

that they proliferate. These resistant bacteria also have a better chance to flourish because the

other, weaker, bacteria have died. Its the scenario for a medical crisis.

HISTORY OF DEVELOPMENT OF RESISTANCE

Resistant bacteria have always been around and existed long before humans began usingantibiotics therapeutically. What is new in the world of resistance is how quickly new resistant

strains arise. The widespread use and misuse of antibiotics contribute to the problem. For thefirst time in decades, people in the United States are dying of bacterial infections that cannot be

treated.

y Right after we began using penicillin, some Staphylococcus strains were identified asresistant to it.

o Today, 80 percent ofStaphylococcus strains do not respond to penicillin.y In the 1940s and early 1950s, streptomycin, chloramphenicol, and tetracycline were

discovered.o By 1953, a strain ofShigella was found that resisted these antibiotics and

sulfanilamides.o By the 1970s, resistant strains of gonorrhea arose.

y The 1990s saw the development of true superbugs, bacteria that resist all knownantibiotics.

o One antibiotic of last resort is Vancomycin, a powerful antibiotic that attacksbacteria on many fronts.

o Now there are Enterococci strains that resist Vancomycin.y Multi-drug resistant tuberculosis strains have arisen.

o By the 1940s and 1950s, a single antibiotic, such as Streptomycin, no longercured tuberculosis, as it had in the past.

o Tuberculosis is the leading cause of death by infectious disease in the world.WHERE ANTIBIOTICS COME FROM

Below are listed some common antibiotics and their natural sources.

SOURCES OF ANTIBIOTICSSource Examples

molds

penicillium penicillin

cephalosporium cephalosporins


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actinomycetes tetracyclineaminoglycosides (streptomycin)

macrolides (erythromycin)chloramphenicol

ivermectin

rifamycins

bacteria

bacilli Dirt-dwelling organisms that form endospores and create antibiotics,

possibly to deter bacterial competition. These organisms are unaffected bytheir own antibiotics, but can be susceptible to other antibiotics. Produce

polypeptide antibiotics (e.g., polymyxin and bacitracin).

B. cereus Zwittermicin

synthetic

oxazolidinones Linezolid (Zyvox)Treat Gram-positive infections. Bind rRNA to prevent

protein synthesis.

MECHANISMS OF ANTIBIOTIC ACTION

The many modes of antibiotic action are shown schematically in the diagram below.

Some specificexamples

b-Lactam antibiotics


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y Penicillin is a b-lactam antibiotic.y These antibiotics contain a b-lactam ringthree carbons and one nitrogen.y Transpeptidase crosslinks the peptidoglycan net in the cell wall of Gram-positive

bacteria.

y The b-lactam ring mimics a component of the cell wall to which transpeptidase binds.y

Penicillin competitively inhibits the binding of transpeptidase.y The affected bacterium will eventually lyse (rupture) because the unsupported cell wall

cannot withstand its growth.

Disrupters ofnucleicacid synthesis

y RNA polymerase synthesizes RNA according to a DNA template.y The antibiotic rifampin interferes with prokaryotic RNA polymerase and thus, interferes

with transcription.

y Fluoroquinolones inhibit DNA gyrase, a bacterial enzyme that unwinds DNA inpreparation for replication and transcription.

y Both of these disruptions prevent bacteria from dividing to make more bacteria.

Disrupters ofprotein synthesis

y Aminoglycosides inhibit nucleic acid or protein synthesis in bacteria.y They are L-shaped molecules that fit into L-shaped pockets of bacterial ribosomal RNA.y When they insert themselves into rRNA, they disrupt ribosomal structure.y Aminoglycosides dont have this effect on human cells because the L-shaped pocket is

specific to bacteria.

Inhibitors ofmetabolism

y Inhibit synthesis of purine and thymidylate precursors folic acid or tetrahydrofolate.y Sulfonomides inhibit bacteria-specific reaction.

MECHANISM OF ACTION OF SELECTED ANTIBIOTICS

Antibiotic Mechanism


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Inhibitors ofcell wall synthesis

Carbenicillin Inhibits transpeptidation enzymes. Activates lytic enzymes of cell wall.

Pennicillin Inhibits transpeptidase enzymes. Activates lytic enzymes of cell wall.The affected bacterium will eventually lyse because the unsupported cell

wall cannot withstand its growth.

Vancomycin Inhibits transpeptidation in cross-linking peptidoglycans. Interferes with

bacterial cells at many levels, disrupting cell wall synthesis, interferingwith RNA, and damaging the plasma membrane.

Inhibitors ofnucleicacid synthesis

Ciprofloxacin Inhibits DNA gyrase; interferes with DNA replication.

Rifampin Blocks RNA synthesis by binding to and inhibiting RNA polymerase.

Inhibitors ofprotein synthesis

ChloramphenicolB

locks formation of new peptide bonds during protein synthesis bybinding to the 50S subunit of the ribosome.

Erthromycin Binds the 50S subunit and blocks translocation of the new protein on the

ribosome, thus effectively halting synthesis.

Fusidic acid Blocks translocation.

Linezolid Binds rRNA to prevent translation initiation and thus protein synthesis.

Streptomycin Binds the 30S ribosomal subunit of the tuberculosis bacterium andprevents the ribosome from forming the complex necessary to initiate

protein translation. Streptomycin is the first line of chemical defense

againstM

ycobacterium tu

berculosis.

Tetracyclines Binds to the 30S subunit and blocks the addition of amino acids,producing incomplete and probably nonfunctional proteins.

Metabolicinhibitors

Dapsone Interferes with synthesis of folic acid, which is required for the synthesis

of purines and thymidine and for the synthesis of the amino acidsmethionine and gycine.

Sulfonamides Competitively inhibits dihydropteroate synthase, an enzyme that convertsp-aminobenzoic acid (PABA) into folic acid. These drugs can also beincorporated into a compound that resembles dihydrofolate and that in

turn can inhibit another enzyme in the pathway, dihydrofate reductase.

Trimethoprim Inhibits dihydrofolate reductase, blocking tetrahydrofolate synthesis.

MECHANISMS OF RESISTANCE


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Bacteria either have preexisting resistance to drugs, or they develop resistance. Human activityhas contributed greatly to the increase in resistant strains of bacteria. Often, when bacteria

acquire resistance to a certain drug from a particular class (e.g., the penicillins), the bacteria alsoacquire resistance to all other drugs in that class.

Some of the many mechanisms of resistance are indicated schematically in the followingdiagram:

Inherentresistance

The principles of Darwinian evolution act on bacteria with inherent resistance: those bacteria that

resist an antibiotic's effects are better suited to survive in an environment that contains theantibiotic. In the case of inherent resistance and vertical evolution, the genes that confer

resistance are found on bacterial chromosomes and are transferred to the bacterial progeny everytime the cell divides.

y Bacteria may begin life resistant to a particular antibiotic.o Example: Gram-negative bacteria are naturally resistant to penicillins.

y Bacteria may be resistant because eithero they have no mechanism to transport the drug into the cell.o they do not contain or rely on the antibiotics target process or protein.

y Specific examples of bacterial strains with known natural resistance:o tetracycline-resistantProteus mirabilis.o ampicillin-resistant Klebsiella pneumoniae.

Acquired resistance


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Bacteria that dont begin life resistant to a certain antibiotic can acquire that resistance. In thecase of vertical evolution and inherent resistance, mutations occur on chromosomes and are then

selected for an environment where resistance increases fitness. In the case of horizontalevolution, genes pass from a resistant strain to a nonresistant strain, conferring resistance on the

latter. The introduction of an antibiotic alters the

environment and acts as a selective pressure.

Conjugation

Transmission of resistance genes via plasmid

exchange.

y Bacteria have circles of DNA calledplasmids that they can pass to otherbacteria during conjugation.

y Plasmids, the key players in conjugation,are even referred to as resistance transferfactors.

y This type of acquisition allowsresistance to spread among a populationof bacterial cells much faster than simple

mutation and vertical evolution wouldpermit.

Transduction

A virus serves as the agent of transfer between bacterial strains.

Transformation

DNA released from a bacterium is picked up by a new cell.

After the new DNA is introducedwhether via conjugation, transduction, or transformationit

is incorporated into the cell and results in the emergence of a new, resistant genotype.


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SOME EXAMPLES OF RESISTANCE

Type of bacteria Resistance to

Gram-negative bacteria Penicillin and other b-lactam

antibiotics

Proteus mirabilis (rheumatoid arthritis, urinary tract

infections)

Tetracycline

Klebsiella pneumoniae (ankylosing spondylitis, a disease

of the joints)

Ampicillin

Staphylococcus aureus Methicillin

Some mechanisms ofresistance

Enzyme-based resistance

There are a number of ways enzymes have been used by bacteria to confer antibiotic resistance:

y Resist b-lactam antibiotics through modifications in the genetic code for the proteins thatbind penicillin.

y Genes for enzymes that can destroy or disable antibiotics are acquired or arise throughmutation. For example, a b-lactamase enzyme can destroy the b-lactam ring of penicillinsthrough hydrolysis, and without a b-lactam ring, penicillins are ineffective against thebacteria.


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y Prevent aminoglycoside disruption of ribosomes. A bacterial enzyme adds a bulkysubstituent to the aminoglycoside, making it impossible for the drug to fit into the rRNApocket and rendering it harmless.

Ribosomal modifications

The ribosome can be methylated so that an antibiotic cannot bind to it.


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Protein modifications

For antibiotics that target DNA gyrase, the enzyme that unwinds DNA for replication, randommutations in the bacterial DNA may alter the gyrase and make it unrecognizable to antibiotics

while still leaving it functional.

Metabolicresistance

In the case of sulfonamides, which operate by mimicking PABA and competing for an enzyme

that synthesizes folic acid, an increase in the amount of PABA can outcompete the sulfonamideand render it ineffective; or an alteration in the code for the enzyme itself can prevent its

sulfonamide binding.

Effluxingthetoxin

One particularly active way a bacterium may deal with an antibiotic is to pump it out, perhapsusing proteins encoded by acquired genes. For example, a strain ofenterococcalbacteria can

pump out tetracycline. This type of pumping is called an efflux phenomenon.

Note: Bacteria without inherent antibiotic resistance can acquirethrough conjugation,

transduction, or transformationthe genes that encode proteins that confer resistance.

WHAT THE FUTURE HOLDS

We use antibiotics for everything from treating viral infectionsagainst which antibiotics areuselessto promoting the growth of livestock to curing acne. People often demand antibiotics


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from their doctors even in the absence of proof of a bacterial infection. And people often neglectto complete a full course of antibiotics once it has been prescribed.

The more often we use antibiotics, the more likely it is that resistance will develop and spread.

Today, about 30% ofStreptococcus pneumoniae strains are resistant to penicillin, and 30% of

gonorrhea bacteria are resistant to penicillin or tetracycline or both. Salmonella typhimurium isresistant to ampicillin, sulfa drugs, streptomycin, tetracycline, and chloramphenicol. Even theantibiotic of last resort, vancomycin, has become ineffectual against some superstrains.

Researchers are turning now to synthetic antibiotics for help against these superbugs.

The good news may be that resistance can disappear in the same way it developed. Mutationsthat reduce resistance may occur, and if the antibiotic is not present, there is no selective pressure

to maintain resistance. In the absence of selective pressure, the bacteria may eventually lose allof their resistance.

accidents in science occasionally lead to great discoveries

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