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In this article we will discuss about:- 1. Meaning of Drug Resistance 2. Mechanisms of Drug Resistance 3. Origin 4. Transmission 5. Drug-Resistance Encounter.
Contents:
- Meaning of Drug Resistance
- Mechanisms of Drug Resistance
- Origin of Drug Resistance
- Transmission of Drug Resistance
- Drug-Resistance Encounter
1. Meaning of Drug Resistance:
Antimicrobial drug resistance refers to the acquired ability of a microbial pathogen to resist the effects of a therapeutic agent (antimicrobial drug) to which it is normally susceptible. Drug resistance does not involve the host, but is a function of the microbial pathogen present inside the host.
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As we know that antibiotics are produced by microorganisms and the latter, in order to survive, developed resistance mechanisms to neutralize or destroy their own antibiotics. In addition, genes encoding these resistance mechanisms can be transferred to other, usually related microorganisms.
As a result, most antimicrobial drug resistance involves ‘resistance genes’ which get transferred between and among microorganisms by genetic exchange. However, the spread of drug resistance in microbial pathogens has become one of the most serious threats to the successful treatment of microbial disease.
Drug resistance has become an extremely serious public health problem especially because of the massive quantities of antibiotics being prepared and used.
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Following are some examples of drug resistance that were reported in the past:
1. Neisseria gonorrhoeae, the causative agent of gonorrhoea is a good example. Gonorrhoea was first treated successfully with sulfonamides in 1936, but, by 1942, most strains were resistant and physicians turned to penicillin.
Within 16 years a penicillin-resistant strain had emerged in the Far East. A penicillinase- producing gonococcus reached the United States in 1976 and is still spreading in this country.
2. In 1946 almost all strains of Staphylococcus were penicillin sensitive. Today, most hospital strains are resistant to penicillin G, and some are now also resistant to methicillin and/or gentamicin and only can be treated with vancomycin. Some strains of Entefococcus have become resistant to most antibiotics, including vancomycin.
Recently a few cases of vancomycin-resistant S. aureus have been reported in the United States and Japan. At present these strains are only intermediately resistant to vancomycin. If full vancomycin resistance develops and spreads, S. aureus may become untreatable.
3. An epidemic of dysentery caused by Shigella was reported in Guatemala in late 1968 and it affected at least 112,000 persons and resulted in 12,500 deaths. The strains responsible for this devastation carried an R plasmid giving them resistance to chloramphenicol, tetracycline, streptomycin, and sulfonamide.
In 1972 a typhoid epidemic swept through Mexico producing 100,000 infections and 14,000 deaths. It was due to a Salmonella typhi strain with the same multiple-drug-resistance pattern seen in the previous Shigella outbreak.
4. Haemophilus influenzae type b is responsible for many cases of childhood pneumonia and middle ear infections, as well as respiratory infections and meningitis. It is now becoming increasingly resistant to tetracyclines, ampicillin, and chloramphenicol.
The same situation is occurring with Streptococcus pneumoniae. It has been estimated that by sometime in 2004, as much as 40% of S. pneumoniae may be resistant to both penicillin and erythromycin.
2. Mechanisms of Drug Resistance:
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No therapeutic drug (antibiotic) inhibits all microbial pathogens and some microbial pathogens possess natural ability to resist to certain antibiotics. Bacteria become drug resistant using several different resistance mechanisms. A particular type of resistance mechanism is not confined to a single class of drugs. Two bacteria may employ different resistance mechanisms to counter the same antibiotic.
However, bacteria acquire drug resistance using resistance mechanisms such as:
(i) Reduced permeability to antibiotic,
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(ii) Efflux (pumping) antibiotic out of the cell,
(iii) Drug inactivation through chemical modification,
(iv) Target modification, and
(v) Development of a resistant biochemical pathway.
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A summarized account of different mechanisms of drug resistance is given in Table 46.1.
(i) Reduced Permeability to Antibiotic:
Bacteria often develop impermeability and become resistant simply by preventing entrance of the drug. Many gram-negative bacteria are unaffected by penicillin G because the drug fails to penetrate the envelope’s outer membrane. Modifications in penicillin binding proteins also render a cell resistant.
A decrease in permeability in bacterial pathogens can lead to resistance against sulfonamide. Mycobacteria resist many drugs because of the high content of mycolic acids in a complex lipid layer outside their peptidoglycan. This layer is impermeable to most drugs.
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Decrease in permeability also can occur as a result of loss of porin proteins. Escherichia coli produces two types of porins, OmpC and OmpF. Mutations result in deficiency in outer membrane porin OmpF confers low- level resistance to tetracycline as well as to β-lactam antibiotics, chloramphenicol, and quinolones. Narrow spectrum imipenem resistance, which can arise in Pseudomonas aeruginosa, is also an example of reduced permeability.
(ii) Efflux (Pumping Antibiotic Out of the Cell):
Microbial pathogenes possess resistance strategy by which they pump the drug out of the cell after it has entered. Some pathogens have plasma membrane translocases, often called efflux pumps, that expel drugs. Because they are relatively nonspecific and can pump many different drugs, these transport proteins often are called multidrug-resistance pumps.
Efflux systems are present in E. coli, Pseudomonas aeruginosa, Mycobacterium smegmatis, Mycobacterium tuberculosis. Staphylococcus aureus, Streptococcus pneumoniae, and Neisseria gonorrhoeae.
Gram-positive and gram-negative bacteria that become resistant to tetracyclines commonly overproduce related membrane proteins that act as efflux pump. Tetracycline resistant bacterial cell takes up the drug as rapidly as do sensitive ones but differ in being able to pump it out again.
Drug efflux is mediated by the Tet membrane proteins which use an antiport mechanism of transport involving the exchange of a proton for a tetracycline-cation complex. Plasmid-encoded multidrug efflux pump confer resistance to olaquindox in E. coli. Salt-inducible multidrug efflux pump is reported Chromohalobacter sp.
(iii) Drug Inactivation Through Chemical Modification:
Many bacterial pathogens show resistance to drug by inactivating drugs through chemical modification. The best-known example is the hydrolysis of the β- lactam ring of many penicillins by the enzyme penicillinase.
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Drugs also are inactivated by the addition of groups. For example, chloramphenicol contains two hydroxyl groups that can be acetylated in a reaction catalyzed by the enzyme chloramphenicol acyltransferase with acetyl CoA as the donor.
Aminoglycosides can be modified and inactivated in several ways. Acetyltransferases catalyze the acetylation of amino groups. Some aminoglycoside-modifying enzymes catalyze the addition to hydroxyl groups of either phosphates (phosphotransferases) or adenyl groups (adenyltransferases).
(iv) Target Modification:
Since each chemotherapeutic agent acts on a specific target, resistance arises, when the target enzyme or organelle is modified so that it is no longer susceptible to the drug. For example, the affinity of ribosomes for erythromycin and chloramphenicol can be decreased by a change in the 23S rRNA to which they bind.
Enterococci become resistant to vancomycin by changing the terminal D-alaninc-D- alanine in their peptidoglycan lo a D-alanine-D-lactate. This drastically reduces antibiotic binding. Antimetabolite action may be resisted through alteration of susceptible enzymes.
In sulfonamide-resistant bacteria the enzyme that uses p-aminobenzoic acid during folic acid synthesis (the tetrahydropteroic acid synthetase) often has a much lower affinity for sulfonamides. Mycobacterium tuberculosis becomes resistant to the drug rifampin due to mutations that alter the β subunit of its RNA polymerase. Rifampin cannot bind to the mutant RNA polymerase and block the initiation of transcription.
(v) Development of a Resistant Biochemical Pathway:
Resistant bacteria may either use an alternate pathway to bypass the sequence inhibited by the agent or increase the production of the target metabolite. For instance, certain bacteria are resistant to sulfonamides simply because they use preformed folic acid from their surroundings rather than synthesize it themselves. Other strains increase their rate of folic acid production and thus counteract sulfonamide inhibition.
3. Origin of Drug Resistance:
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Origin of drug resistance has genetic basis. Drug resistance can be genetically encoded by the microbial pathogen and the genes responsible for it are present on both the chromosome and plasmids (Table 46.1).
(i) Chromosome-Mediated Drug Resistance:
Spontaneous mutations in the chromosome, although they do not occur very often, will make bacteria drug resistant. Usually such mutations result in a change in the drug receptor and therefore the antibiotic cannot bind and inhibit the pathogen (e.g., the streptomycin receptor protein on bacterial ribosomes).
Many mutants are probably destroyed by natural host resistance mechanisms. However, when a patient is being treated extensively with antibiotics, some resistant mutant may survive and flourish because of their competitive advantage over non-resistant strains.
Transposons are a type of transposable elements in bacterial chromosome that, in addition to genes involved in transposition, carries other genes; often genes conferring antibiotic resistance. Many composite transposons contain genes for antibiotic resistance, and some bear more than one resistance gene.
They are found in both gram-negative and gram-positive bacteria. Some examples and their resistance markers are Tn5 (kanamycin, bleomycin, streptomycin), Tn9 (chloramphenicol), Tn10 (tetracycline), Tn21 (streptomycin, spectinomycin, sulfonamide), Tn551 (erythromycin), and Tn4001 (gentamicin, tobranycin, kanamycin).
(ii) Plasmid-Mediated Drug Resistance:
Plasmid is an extra-chromosomal genetic element that replicates independently of the host chromosome, is not essential for growth, and has no extracellular form. We know that large number of different plasmids occur naturally in bacterial cells.
Among the most widespread and well-studied groups of plasmids are the R plasmids (resistance plasmids), which confer resistance to antibiotics. R plasmids were first discovered in Japan in enteric bacteria that had acquired resistance to a number of antibiotics (multiple resistance) and have since been found throughout the world.
R plasmid resistance is usually due to the presence of genes in it encoding new enzymes that inactivate the antibiotic or genes that encode enzymes that either prevent antibiotic update or actively pump it out of the bacterial cell.
For example, the aminoglycoside antibiotics streptomycin, neomycin, kanamycin, and spectinomycin possess identical chemical structures. Strains carrying R plasmids for these antibiotics can synthesis enzymes that chemically modify the antibiotics either by phosphorylation, acctylation, or adenylalion (Fig. 46.1). The modified antibiotics then lack antibiotic-property.
R plasmid genes encode penicillinase enzyme (β-lactamase) that splits the β-lactam ring in penicillins, and inactivate the antibiotic. Chloramphenicol resistance is due to an R plasmid gene-encoded enzyme that acetylates the antibiotic (Fig. 46.2). Several R plasmids confer multiple antibiotic resistance because a single R plasmid may possess different genes, each encoding a different antibiotic-inactivating enzyme.
4. Transmission of Drug Resistance:
Drug resistance and its spread has become an extremely serious public health problem.
Following are the main factors responsible for development and spread of drug resistance:
(i) Drug Misuse:
Misuse of drugs have resulted in much of the difficulty. It has been estimated that over 50% of the antibiotic prescriptions in hospitals are given without clear evidence of infection or adequate medical indication. Many physicians have administered antibacterial drugs to patients with colds, influenza, viral pneumonia, and other viral diseases.
A recent study showed that over 50% of the patients diagnosed with colds and upper respiratory infections and 66% of those with chest colds (bronchitis) are given antibiotics, even though over 90% of these cases are caused by viruses.
Frequently antibiotics are prescribed without culturing and identifying the pathogen or without determining bacterial sensitivity to the drug. Toxic, broad-spectrum antibiotics are sometimes given in place of narrow-spectrum drugs as a substitute for culture and sensitivity testing, with the consequent risk of dangerous side effects, super-infections, and the selection of drug-resistant mutants.
The situation is made worse by patients not completing their course of medication. When antibiotic treatment is ended too early, drug-resistant mutants may survive. People in many countries usually practice self-administration of antibiotics and thus help increase the prevalence of drug-resistant strains.
(ii) Extensive Drug Treatment:
Extensive drug treatment helps the development and spread of antibiotic- resistant strains. It is because the excess antibiotic destroys normal, susceptible bacteria that would usually compete with drug-resistant strains.
The result may be the emergence of drug resistant pathogens leading to a superinfection. Super-infections are a significant problem because of the existence of multiple-drug-resistant bacteria that often produce drug-resistant respiratory and urinary tract infections. A classic example of a superinfection resulting from antibiotic administration is the disease pseudomembranous enterocolitis caused by Clostridium difficile.
When a patient is given clindamycin, ampicillin, or cephalosporin, many intestinal bacteria are killed, but C. difficile is not. This intestinal inhabitant, which is normally a minor constituent of the population, flourishes in the absence of competition and produces a toxin that stimulates the secretion of a pseudo-membrane by intestinal cells.
If the superinfection is not treated early with vancomycin, the pseudo-membrane must be surgically removed otherwise the patient will die. Fungi, such as the yeast Candida albicans, also produce super-infections when bacterial competition is eliminated by antibiotics.
(iii) Movement of Resistance-Genes:
Resistance genes present on composite transposons can move rapidly between plasmids and through a bacterial population. Often several resistance genes are carried together as gene cassettes in association with a genetic element known as an integron. An integron has an attachment site for site-specific recombination into which genes can be inserted as an integrase gene.
Thus integrons can capture genes and gene cassettes. Gene cassettes are genetic elements that may exist as circular non-replicating DNA when moving from one site to another, but which normally are a linear part of a transposon, plasmid, or bacterial chromosome. Cassettes usually carry one or two genes and a recombination site.
Several cassettes can be integrated sequentially in an integron. Thus integrons also are important in spreading resistance genes. Finally, conjugative transposons, like composite transposons, can carry resistance genes. Since they are capable of moving between bacteria by conjugation, they are also effective in spreading resistance.
(iv) Use of Drugs in Animal Feed:
The use of antibiotics in animal feeds is undoubtedly another contributing factor to increasing drug resistance. The addition of low levels of antibiotics to livestock feeds does raise the efficiency and rate of weight gain in cattle, pigs, and chickens (partially because of infection control in overcrowded animal populations).
However, this also increases the number of drug-resistant bacteria in animal intestinal tracts. There is evidence for the spread of bacteria such as Salmonella from animals to human populations.
In 1983, 18 people in four mid-western States of America were infected with a multiple- drug-resistant strain of Salmonella new-port. Eleven were hospitalized for salmonellosis and one died. All 18 patients had recently been infected by eating hamburger from beef cattle fed sub-therapeutic doses of chlortetracycline for growth promotion.
Resistance to some antibiotics has been traced to the use of specific farmyard antibiotics. Avoparcin resembles vancomycin in structure, and virginiamycin resembles synercid; synercid is a mixture of the antibiotics, streptogramin, quinupristin and dalfopristin that inhibits protein synthesis.
There is good circumstantial evidence that extensive use of these two antibiotics in animal feed has led to an increase in vancomycin and synercid resistance among enterococci. The use of the quinolone antibiotic enrofloxacin in swine herds appears to have promoted ciprofloxacin resistance in pathogenic strains of Salmonella. Elimination of antibiotic food supplements might well slow the spread of drug resistance.
(v) Use of Triclosan:
Triclosan is an antibacterial substance found in products such as soaps, deodorants, mouth-washes, cutting boards, and baby toys. There is increasing evidence that the widespread use of triclosan actually favours an increase in antibiotic resistance.
5. Drug-Resistance Encounter:
Various strategies can be used to encounter the emergence of drug resistance.
Important ones are the following:
(i) Strategic Use of Drugs:
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Following are some specific modes proposed to use drugs to discourage the emergence of drug resistance:
(i) The drug can be used in a high enough concentration. This is considered to destroy susceptible bacterial pathogens and most spontaneous mutants of pathogens that might arise during drug- treatment.
(ii) Two different drugs can be given to the patient simultaneously. This may help in the way that each drug will prevent the development of resistance to the other.
(iii) Chemotherapeutic drugs, particularly broad-spectrum antibiotics, should be used only when definitely necessary. If possible, the pathogen should be identified, drug sensitively tests should be performed, and finally, a proper narrow-spectrum antibiotic should be given to the patient.
(ii) Search for New Antibiotics:
Search for new antibiotics that microbial pathogens have never faced is a major approach. Drug manufacturing companies collect and analyse samples from around the world in a search for completely new antimicrobial drugs. Structure-based or rational drug design is emerging as an important tool in this area.
If the three dimensional structure of a susceptible target molecule such as an enzyme essential to microbial function is known, computer programs can be used to design that precisely fit the target molecule. These drugs might be able to bind to the target and disrupt its function sufficiently to destroy the pathogen.
Pharmaceutical companies are using this approach to attempt to develop drugs for the treatment of AIDS, cancer, and the common cold. At least one company is developing “enhancers”, which are cationic peptides that disrupt bacterial membranes by displacing their magnesium ions.
Antibiotics then penetrate and rapidly exert their effects. Other pharmaceutical companies are developing efflux-pump inhibitors to administer with antibiotics and prevent their expulsion by the resistant pathogen.
There has been some recent progress in developing new antibiotics that are effective against drug-resistant pathogens. Two new drugs are fairly effective against vancomycin-resistant enterococci, and they are synercid and linezolid (zyvox). Synercid, as stated earlier, is a mixture of the streptogramin antibiotics (quinupristin and dalfopristin) that inhibits protein synthesis.
A second drug, linezolid (Zyvox), is the first drug in a new family of antibiotics, the oxazolidinones. It inhibits protein synthesis and is active against both vancomycin-resistant enterococci and methicillin-resistant Staphylococcus aureus.
(iii) Identifying New Targets for drugs:
Recent knowledge coming from the sequencing and analysis of pathogen genomes almost certainly will be useful in identifying new targets for antimicrobial drugs. For convenience, data obtained from genomics studies can be used for research on inhibitors of both aminoacyl- tRNA synthetases and the enzyme that removes the formyl group from the N-terminal methionine during bacterial protein synthesis.
Bacteria must synthesize the fatty acids they require for growth rather than acquiring the acids from their environment. The drug susceptibility of enzymes in the fatty acid synthesis system is being analyzed by screening pathogens for potential targets.
(iv) Phage Therapy:
Phage therapy is emerging as a most interesting approach to overcome the problem of drug resistance. This therapy is based on the idea proposed by d’Herelle in mid of second decade of 20th century. d’Herelle proposed that bacteriophages could be used to treat bacterial disease.
Although many microbiologists did not favour d’Herelle idea due to technical difficulties and the advent of antibiotics, Russian scientists pursued his proposal actively and developed the medical use of bacteriophages.
Currently, Russian physicians use bacteriophages to treat many bacterial infections. Bandages are saturated with phage solutions, phage mixtures are administered orally, and phage preparations are given intravenously to treat Staphylococcus infections. Three American companies are actively conducting research on phage therapy and preparing to carry out clinical trials.
