Author: Winnie Cheung

Hong Kong

infoallglobe.com Published on 11/04/2000

Antibiotics--Penicillins,Vancomycin,Cephalosporins & their derivatives

1)Introduction

Have you ever taken any antibiotics when you are sick? I believe that all of us had this experience especially when you are under bacterial infection. Actually it’s not surprising that bacteria are capable to bring such a significant effects on our health as there are so many bacteria around us, in our home, in our working places, in every tiny space of our planet. They are so small that we cannot observe them with naked eyes which make us difficult to avoid being infected by them.

Antibiotics are natural substances (secondary metabolites) produced by microorganisms that inhibit growth, proliferation, of bacteria or kill them directly. However, antibiotics do not harm viruses. Antibiotics tend to be rather large, complicated, organic molecules and may require as many as 30 separate enzymatic steps to synthesize. .The greatest variety in structure and number of antibiotic is found in the actinomycetes, especially the genus Streptomyces. Another important group is the peptide antibiotics produced by bacteria of the genus bacillus.

In practice, antibiotics typically retard bacterial proliferation by entering the microorganisms and interfering with the production of components needed to form new bacterial cells. For instance, the antibiotic tetracycline binds to ribosomes in order to impairs protein manufacture in bacteria; penicillin and vancomycin impede proper synthesis of the bacterial cell wall.

The majority of antibiotics are natural products that certain bacteria and fungi produce and send outside of their cells. In practice, most commercial antibiotics have been chemically altered in the laboratory to become semi-synthetic which aims to improve their potency or to increase the range of species they affect. In other words, these particular antibiotics are designed to inhibit some process previously identified to be completely unique to bacteria, and necessary for the bacterium to remain alive.

2) Natural Penicillins

(i) Penicillin : the story of an antibiotic

Penicillin, the first antibiotic, was discovered in 1929 by Sir Alexander Fleming who observed inhibition of Staphylococci on agar plate contaminated by a Penicillium mold. Pencillin became generally available for treatment of bacterial infections, especially those caused by Staphylococci and Streptococci. Initially, the antibiotic was effective against all different types of infections caused by these two Gram-positive bacteria. This property is very important as a significant proportion of all human infections are caused by these two bacteria (strep throat, wound infections and scarlet fever).In addition, Penicillin’s selective toxicity (the ability for it to kill the bacterial pathogens only without harming the host that taken it) is another undoubtedly important property for us to deal with a variety of bacterial infection.

(ii) The structure of Penicillin.

Penicillin is not a single compound but a group of closely related compounds, Basic structure of Penicillin is 6-aminopenicillanic acid(6APA) which consists of a thiazolidine ring and aβ-ring which derived from 2 amino acids (cysteine and valine). The 6APA carries a variable acyl moiety in position 6. If the penicillin fermentation is carried out without addition of side-chain precursors, natural penicillins are produced.

(iii) Modes of action of Penicillin

Penicillin have the ability to prevents the cross-linking of small peptide chains in peptidoglycan, the main wall polymer of bacteria. Pre-existing cells are unaffected, however, all newly synthesized cell will grow abnormally since they are now unable to maintain their cell wall rigidity, and thus they are susceptible to osmotic lysis.

The two naturally occurring Penicillin from culture filtrates of Penicillum notatum or P.chrysogenum are Penicillin G and the more acid-resistant penicillin V. These Penicillins are active only against Gram-positive bacteria but not Gram-negative bacteria. Indeed, it’s not surprising as now scientists have shown that Gram-negative bacteria cell wall is protected by an outer membrane that prevents permeation of the penicillin molecule which make them Penicillin-resistant

(iv) Discovery of other antibiotics after the usage of Penicillins

Name	Mode of actions	Target microorganisms
Streptomycin	Binds to protein S12 of the 30S ribosome and cause misreading of the code,so inhibit protein synthesis	Gram +ve and Gram –ve bacteria, Intracellular parasites, tuberculosis bacillus
Chloramphenicol	Binds specifically to the 50S subunit of 70S ribosomes and block the peptidyl transferase
Tetracycline	Binds to ribosomes, thus impairs protein manfacture
Erythromycin	Binds to ribosomes, thus impairs protein manfacture	Gram +ve bacteria
Cephalosporins		Broad range of Gram+ve and Gram-ve bacteria
vancomycin	Preventing the reactions used by microorganisms to link pepetidoglycan precursor together for constructing their cell wall	Broad range of Gram+ve and Gram-ve bacteria

3. A crisis in health care --- the emergence of antibiotic resistance

. The repeated or continued use of antibiotics creates selection pressure favouring the growth of antibiotic-resistant mutants. This is a classical example of Survival Of The Fiffest. Nowadays, we produce such a huge quantities of antibiotics, thus, the selective pressure on bacteria containing plasmids carrying antibiotic resistant gene is intense, particularly in places like hospitals. Whenever antibiotics are used, there is selective pressure for resistance to occur. As a consequence of this evolutionary process, current antibiotics are losing their effectiveness.

Actually, antibiotic resistant is not a recent issue. On the contrary, this problem was recognized soon after the natural penicillins were introduced to control diseases. Indeed, the situation has now become alarming due to the present of pathogenic strains that show multiple resistance to a broad range of antibiotics. One of the most important example concerns is the multiple-resistant strains of Staphylococcus aureus in hospital. Worldwide, many strains of S.aureus are already resistant to virtually all the useful antibiotics including all the natural Penicillins such as Penicillin G and V, aminoglycoside antibiotics such as streptomycin but except vancomycin. These strains of microorganisms are one of the most serious problems encountered by hospitals. It is the major cause of hospital acquired infections. In fact, Vancomycin is sometimes the only available antibiotic left behind that is effective to deal with such microorganisms. Vancomycin, isolated from soil species Streptomyces, target the microorganisms by preventing the reactions used by microorganisms to link peptidoglycan precursor together for constructing their cell wall. Therefore, inhibit peptidoglycan synthesis which in turn inhibit the microorganisms to growth.

However, one fearing news occurred in last year – the emergence of vancomycin-resistant strains. Indeed, with the increased use of vancomycin it is not surprising that resistance to it can now readily be observed. These superbacteria contain plasmids carrying resistant genes to all of the available antibiotics, including vancomycin. This news fearing all the public health experts, making many of S.aureus incurable. This is again an example of evolution in action. Since in the past, many bacteria are resistant to all other antibiotics except vancomycin, so we apply vancomycin to treat all these multiple-resistant bacteria. But, at the time we still believing that vancomycin can treat all kinds of bacterial infections, we ignore the possibility of the emergence of vancomycin-resistant strains due to the overuse of vancomycin. In particularly, since people infected with pathogens are concentrated in hospitals and vancomycin are extensively used to treat all kinds of bacterial infections, the chance are high that antibiotic resistant plasmids will be selected and then passed on to the other bacteria which in turn result in producing superbacteria that resistant to all kind of antibiotocs.

So far, we just mentioned that antibiotic resistance arises significantly. Indeed, besides the one just described about the conjugative transfer of plasmids that contain antibiotic resistant gene, bacteria can still attain these resistant genes through transformation and spontaneous mutation. Based on this, we can imagine the massive chances that provide for bacteria to resist to antibiotics. These certain resistant genes ward off destruction by giving rise to enzymes that degrade antibiotics or that chemically inactivate the drugs. Alternatively, some resistant gene cause bacteria to alter or replace molecules that are normally bound by an antibiotic—changes that essentially eliminate the antibiotics’ targets in bacterial cells. Bacteria might also eliminate entry ports for the antibiotics or, more effectively, may manufacture pumps that export antibiotics before the medicines have a chance to find their intracellular targets. Therefore, there is a presence of a crisis of health care in human if we don’t seek to find out solutions to revert this urgent situation.

Increase of bacteria resistant to 3 antibiotics over time

4) Corrective measures to lessen the health crisis

As we all understand the fearing consequences that could bring to humans by the superbacteria, we have to seek for some reliable methods to improve this threatening situation. Before discussing the different methods to combat these superbacteria that resistant to all antibiotics, we have to understand the reasons and mechanisms by which bacteria resistant to antibiotics.

First, overuse of antibiotics being the most significant contributor to the emergence of antibiotic resistance bacteria. It is because the repeated overuse of antibiotics creates selection pressure favouring the growth of antibiotic-resistant mutants. Detailed way for bacteria to attain this resistant gene and the problems it caused is described in part 3 above.

Second, the ability of many bacteria to provide enzyme(β-lactamase ) to destroy antibiotics’ activities is being another important causes of antibiotic resistance.β-. lactamase cleave the cyclic amide bond of beta-lactam ring of antibiotics thus results in the lose of functions of antibiotics. For instance,

Based on the above basic knowledge of the ways by which bacteria resistant to antibiotics, now we can deeply focus on the methods to improve these bad situation in particular areas :

(i) Minimize inappropriate use of antibiotics

.Notably, many physicians acquiesce to misguided patients who demand antibiotics to treat colds and other viral infections, that cannot be cured by antibiotics. According to the Centre for Disease Control and Prevention, 50 million of the 150 million out patient prescription for antibiotics every year are unneeded. As the less amount of antibiotics use, the less selective pressure for the antibiotic-resistant mutants. Therefore, the prevalence of antibiotic-resistant strains will continue to increase unless physicians and institutions take the necessary steps to control the usage of antibiotics. thus, the minimization of improper use of antibiotics provide a mean to lesson the serious antibiotics resistance problem and in turn lessen the effects that bring from the resistance bacteria on our health.

(ii) Modify the structure of antibiotics --- semi-synthetic antibiotics production

We can determine what an antibiotic looks like, and we can chemically add or remove some things from the original structure, and produce an altered form of the original material. This new, altered substance is called semi-synthetic antibiotic.

As we all know thatβ-lactamase that produced by the antibiotic resistance bacteria can cleave the cyclic amide bond ofβ-lactam ring of antibiotic which in turn can causes the antibiotic to become functionless, so, if we modify the structure of the antibiotics such that they are no longer the substrate ofβ-lactamase,then these modify antibiotics with novel characteristics can be effective again to deal with the previously resistant bacteria and it is a long-sighted solution towards the present problem.

Semi-synthetic Penicillins

In practice, semi-synthetic Penicillins (Penicillins derivatives) can fulfil this requirement. Penicillins derivatives are made from naturally occurring penicillins using in vitro methods ( chemically or enzymatically ). These derivatives have improved characteristics such as acid stability, resistance toβ-lactamase, expanded antimicrobial effectiveness(extended range of activity against Gram-negative bacteria). Therefore, they have come to be extensively used in therapy recently.

By biotechnology, we can synthesize a number of penicillin derivatives by removing the acyl group to leave 6-aminopenicllianic acid and then adding acyl groups that confer new properties. To do so, an enzyme named “Penicillin acylase” is particularly important. The process begin with natural penicillin G. All penicillin have the same basic structure of 6-aminopenicillianic acid (6-APA). They are being cleaved by penicillin acylase to produce 6-APA and from which many other derivatives can be formed by adding appropriate side chain precursors. The resulting derivatives antibiotics can act to combat the antibiotic resistant and have increased stability.

The most commonly Penicillin derivatives(a variety of side chains can be employed)

Antibiotic	Side chain structure	Activies(properties)
Aminopenicllins eg.amplicillin		l Relative broad spectrum of activity l posses less activity than penicillin G but greater activity against a wide range of gram-negative bacilli l still sensitive toβ-lactamase
Extended Spectum Penicillins eg. Carbenicillin		l Broad spectrum of activity l Active against many gram-positive bacteria l Generally less effective than penicillinG and ampicillin
Penicillinase-resistant Penicillins eg.1)cloxacillin		l Significantβ-lactamase resistance l Effective towards bothβ-lactamase positive and negative strains of S. aureus (the one that even resistant to vancomycin) infections
2)methicillin	β-lactamase positive and negative strains of S. aureus (the one that even resistant to vancomycin) infections
2)methicillin

Semi-synthetic Vancomycin

The main target of vancomycin is the D-alanyl-D-alanine terminal depeptides of peptidoglycan precursors, inhibiting bacterial cell wall synthesis. It also damages the bacterial cell membrane and interferes with bacterial RNA synthesis. Since many of the vancomycin resistance mechanisms are observed today, semi-synthetic vancomycin analogues have been produced. These analogues are active against many of the most important species of bacteria that are resistant to vancomycin.

Semi-synthetic Cephalosporins

Cephalosporins areβ-lactam antibiotics containing a dihydrothaizine ring with D-α-aminoadipic acid as acyl moiety. It has low toxicity, broad-spectrum antibiotic comparable to ampicillin. It has about 30% of antibiotic market. However, cephalosporins, like other antibiotics, face the problem of bacterial resistance. To offset this, semi-synthetic cephalosporins such as cephalotin are commerically produced. They are synthesized by chemically splitting to form 7-aminocephalosporanic acid(7-ACA) with subsequent chemical acylation, as well as by modification on the C3 site. These derivatives again have improved characteristics like the pencillin derivatives or vancomycin derivatives.

Semi-synthetic cephalosporins

R₁

R₂

R₃

Cephalotin

To sum up, by modification of natural penicillin,or cephalosporins vancomycin themselves to produce long-acting forms and through the huge development of semi-synthetic derivatives, many compounds are now available with advantages over the parent compound : prolonged action, superior absorption, stability toβ-lactamase and broader spectrum.

(iii) Combination ofβ-lactamase inhibitors with susceptibleβ-lactam antibiotics

Asβ-lactamase production is the predominant cause of resistance toβ-lactams antibiotic in the majority of species, therefore, an attractive approach to the therapy of infections caused by such organisms is to co-administer with the liable antibiotic an agent capable of inhibiting the enzyme. As a result, susceptibleβ-lactam antibiotic can be made more resistant toβ-lactamase by combine them with a specific β-lactamase inhibitors. It has been shown that the antibiotic resistant of some bacteria are greatly reduced by treating them with antibiotic that is combined with inhibitors.

Asβ-lactamase interference with the action of the penicillins (by hydrolyze an esstential bond in the antibiotic molecule) which had led to the development of inhibitors that can be used concurrently. The interaction between clavulanic acid and the acidβ-lactamase of Staph.aureus has been investigated and shown that the rate of inhibition ofβ-lactamases increase when the clavulanic acid concentration increased.

Clavulanic acid are ultimately irreversible inhibitors of manyβ-lactamases. It inhibit many plasmid-mediated and some chromosomal mediatedβ-lactamases because it can penetrate the bacterial cell wall to bind to penicillin-binding protein.

To sum up, we can see that combination ofβ-lactamase inhibitors with the susceptibleβ-lactam antibiotics can provide a mean for us to lessen the significant antibiotic resistance effect that present in bacteria and prevent us safe from fatal antibiotic-resistance bacteria. Besides, we may insert a gene that can encodingβ-lactamase inhibitor (e.g. clavulanic acid)into a cloning vector by DNA recombinant technology. And then introduce this DNA into host cell and in the hope of production of antibiotics from cloned gene that are normally resistant toβ-lactamase of some antibiotic-resistance bacateria.

(5) Conclusions

Antibiotic resistance is of concern to many of us. The problem is serious.This resistance is not local, it is international that it presents not just one hospital or community, but in hospitals and communities worldwide. Therefore, it is not difficult to imagine the significant consequences that antibiotic resistance could bring in the near future if we still ignore this problem.

This existing problem is indeed global and urgent. When we take a look around our world, we can see the importance of this problem. Douglas B.Lowrie and his colleagues work in the Laboratory for Mycobacterial Research in London reported in July 15, 1999,Nature that Mycobacterium tuberculosis continues to kill about 3 million people every year. Effective treatment requires that patients take large doses of antibiotics drug, however, nowadays it’s restricted by the emergence of multidrug-resistance strains of M. tuberculosis. In fact, this phenomenon is not recent. By 1953,during a shigella outbreak in Japan, a strain of the dysentery bacillus was isolated which was multiple-drug resistant. This Mycobacterium tuberculosis was capable of rapid development of resistance had become mainstay in tuberculosis therapy. Therefore, the existing antibiotic-resistance problem is indeed urgent. However, it can still be approached. One of the most important features of antibiotic resistance is the selective pressure for resistant bacteria caused by antibiotics used anywhere in the world, whether it is in animals,or in patients in the hospitals which select resistant bacteria into a common environmental pool. So, the first direct approach to deal with it is to prescribe antibiotics appropriately.

In addition, another direct approach is to discover new antibiotics such as the modified derivatives, although to find structurally new ones has been difficult and expensive. These derivatives may be seen as a new path of antibiotic that current bacteria cannot recognize or they can lessen the antibiotic resistance promoted by certain bacteria. However, appropriate use of new derivatives should be encouraged so that they will remain useful for a long time in the future. Finally, the last approach that we adopt is to design “ resistance-resistant” antibiotic based on the knowledge of the resistance mechanisms. It is done by combiningβ-lactamase inhibitors with susceptibleβ-lactam antibiotics in order to lesson the problem.

In conclusion, as we mentioned above that they are still a number of solutions to deal with the antibiotic resistance, we should be optimistic to carry out all of these approach so as to make all of us have a healthy life.

Reference:

1.Nature, volume 400, 15 July, 1999, Therapy of tuberculosis in mice by DNA vaccination, Amersham Pharmacia Biotech UK Limited.

2.Nature, volume 401, 16 Septmber,1999, Antibiotic resistance found in wild rodents, Amersham Pharmacia Biotech UK Limited.

3.Antimicrobial Resistance: A Crisis in Health Care, Donald L.jungkind,volume 390, Plenum Press, New York, 1995

4.The Beta-lactam Antibiotics: Penicilins and Cephalosporins in Perspective, R,W.Lacey, Selwyn, Sydney, 1980

5. Antibiotic and Chemotherapy, Harold P. Lambert,Churchill Livingstone, 1992.

6. Antibiotic Interaction, J.D. Williams,Academic Presss INC.1979

7 http://www.sciam.com/1998/0398issue/0398levy.html.

8.http://www.fda.gov/fduc/features/795_antibio.html.