MK Chattopadhyay, Retired Senior Principal Scientist, Centre for Cellular and Molecular Biology (CSIR), talks about the urgent need for new antibiotics which will be effective against resistant organisms
During the past few decades, world-wide emergence of microbial strains, resistant to various therapeutically useful antibiotics, has assumed the role of a major crisis. Discussion in this article is confined into the antibacterial antibiotics for convenience. However, similar type of problems is encountered in the management of infections caused by the other types of microorganisms (fungus, mycoplasma, virus) also.
Achievements of antibiotics
Though there are ample evidences of use of several types of antibacterials by people in the ancient civilisations, the modern era of antibiotics started with the discovery of penicillin in 1928 by Alexander Fleming. The sulfur drugs were introduced into the clinical practice during the 1930s. Penicillin, which was superior to the sulfur drugs in various ways, appeared to be a magic bullet against bacterial infections. The average life-span of people in the developed and developing countries alike was substantially enhanced following introduction of antibiotics in the clinical practice. Discovery of a number of other antibacterial antibiotics during the 1940s allured many people to believe (including some scientists) that bacterial infections of any type could be subdued using antibiotics and hence there was nothing to worry about them. The idea appeared to be justified since a number of life-threatening infectious diseases could be successfully cured, risks associated with surgical operations could be substantially reduced and quality of life was significantly improved by the use of antibiotics.
Failure of the life-saving drugs
The chink in the armor was evidenced within a few years following introduction of the antibiotics in the clinical practice. It was found that a subpopulation of the bacteria, which were susceptible earlier to certain antibiotics, was no longer responding to the same antibiotics. By this time, organisms resistant to all the therapeutically useful antibiotics have been detected. So the existing antibiotics are turning ineffective. Widespread emergence of methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococci (VRE) reveals the dimension of the crisis. Detection of a bacterial enzyme called New Delhi Metallo-ß-lactamase (NDM-1), which inactivates a wide-range of antibacterial drugs, makes it evident that no antibiotic is infallible. The world-wide practice of adding antibiotics to the feed of farm animals and poultry is believed to significantly contribute to the problem. If the situation is left as such, it was estimated by a British survey (2014) that by 2050, 10 million people will die each year of infections caused by the antibiotic-resistant bacteria. Although the prediction is disputed, the gruesome dimension of the crisis is not denied by anybody. In a ministerial conference, held in June 2014, Margaret Chain, the then Director –General of the World Health Organization (WHO), commented “Antibiotic resistance is not a future threat looming on the horizon. It is here, right now, and the consequences are devastating.”
Quite reasonably, an urgent need for new antibiotics, which will be effective against the resistant organisms, is appreciated by everybody.
Genetic basis of antibiotic resistance
Antibiotic resistance emerges in antibiotic-susceptible bacteria by a process called spontaneous mutation. The resistance-conferring genes are copied and transferred to antibiotic-sensitive organisms by the resistant organisms. Horizontal gene transfer is a process by which even unrelated bacteria can disseminate resistance-conferring genes among themselves. Genes that confer antibiotic-resistance most often co-occur with genes that confer resistance to toxic metals (mercury, copper, nickel, zinc) which are present in many articles of our everyday use. Thus antibiotic-resistant bacteria are selected to grow in natural environments polluted with heavy metals even in absence of antibiotics.
The double whammy
Professor Stuart B Levy, a researcher and physician at the Tufts University (US), is untiringly advocating for rational use of antibiotics. According to his ‘Antibiotic Paradox’ theory, the possibility of emergence of bacterial strains resistant to an antibiotic, is substantially increased with the use of that antibiotic (more use, more resistance). While available evidences speak volumes in support of this theory, it is also a fact that antibiotic resistance is an outcome of evolution and emergence of resistance does not require exposure to antibiotics. Antibiotic resistance is found to occur even in bacteria which were never exposed to antibiotics. As a matter of fact antibiotic biosynthetic genes and resistance-conferring genes evolved long before antibiotics were introduced into the clinical practice. So simply by avoiding use of antibiotics, we cannot bypass the problem of resistance.
Problems associated in getting effective antibiotics
Antibiotics are generally extracted from the soil bacteria belonging to the genus Actinomycetes. The drugs belong to different types of chemical categories e.g, the penicillins, the tetracyclines, the aminoglycosides and some others types. So if the same source is tapped again in search of new drugs, we are likely to get a new penicillin-type of antibiotic or a new tetracycline-type of antibiotic or a new aminoglycoside-type of antibiotic. But bacteria have already evolved with the capacity to bypass these categories of antibiotics. So the efficacy of the newly-discovered drugs will be short-lived.
Some new antibiotics from the soil bacteria
However, scientists have not given up search for new drugs from the soil bacteria. Recently, researchers working at the Rutgers University, New Brunswick (Canada), the Italian biotechnological company NAICONS Sri and some other organisations, have been successful in isolating a microorganism from a soil sample, collected from Italy. It produces a new antibiotic pseudouridimycin, which is found to be effective against a wide range of antibiotic-sensitive and antibiotic-resistant bacteria. This new drug has been obtained from an organism, which can be grown in the laboratory. Unfortunately most of the naturally occurring bacteria do not offer such advantage to the scientists. They cannot be cultured in the laboratory. In 2015, a collaborative work involving four institutes in the US and Germany and two pharma companies, led to the isolation of a new bacterium from a soil sample obtained from Maime (UK), using an improved device of growing soil organisms called isolation chip or ichip. The bacterium was observed to produce an antibiotic named teixobactin, which binds to the lipid components of bacterial cell-wall and thus kills the organism. No resistance was found to develop in the target bacteria. Scientists are also trying to get new antibiotics using recombinant-DNA technology and by chemical synthesis of new compounds in a very large scale by a process called Combinatorial Chemistry. It is indicated from the whole genome sequence of some antibiotic- producing organisms belonging to the genus Streptomyces, that it might be possible to get some novel antibiotics from them.
Antibiotics from novel organisms
Scientists are also attempting to use organisms not used earlier for antibiotic production. These new candidates include bacteria obtained from the rain forests, the Arctic and Antarctic regions. They are also tapping marine sediments to get novel antibiotic-producers. A couple of years ago, three research teams from the University of Tehran and two other research institutes of Iran, reported isolation of antibiotic-producing bacteria from the sediments obtained from the Caspian Sea. Antibiotic-producers have also been detected in marine sediments collected in British Columbia, Canada. They are found to occur also in marine sediments collected near the Andaman and Nicobar islands by two groups working at the Andhra University, India. The desperate attempt of the scientists to get new antibiotics is evidenced by the unusual and bizarre nature of some sources tapped by them. Antimicrobial substances with improved spectrum of efficacy are reported to occur in the extract of crushed cockroach, extract of the brain of locust, secretion of catfish, proteins extracted of the white blood cells of alligator, skin of frog, blood of panda, leaf-cutter ants of South America and also some other sources which as such are not believed to provide any life-saving drug.
Antibiotics with new targets
The antibiotics that we are using at present work by various mechanisms. Some of them inhibit synthesis bacterial cell-wall, some inhibit bacterial protein synthesis, some inhibit synthesis of folic acid in bacteria, some others inhibit synthesis of bacterial DNA and RNA. Pathogenic bacteria have already been evolved with capacity to escape these modes of actions. Hence, scientists are looking for compounds which target bacterial molecules not targeted earlier by the existing antibiotics. For example, cytoskeleton is a complex network of filaments, essential for cell division, maintenance of the structure and shape of the cell and also for the protection of the cell from damage. The bacterial cytoskeletal protein FtsZ is essential for cell-division. Some inhibitors of this protein like 3-methoxybenzamide derivatives are found to inhibit the growth of several pathogenic bacteria including the antibiotic-resistant strains. A couple of years back, a group of scientists led by Professor Neil Stokes at the Biota Europe Limited, Oxfordshire (UK), developed such a derivative, which was active against some multidrug-resistant organisms. Some other derivatives are also reported with similar efficacy.
DNA topoisomerase II is a bacterial enzyme that is essential for bacteria but absent in higher eukaryotes. Hence it offers a novel target for the development of new antibiotics.
Menaquinones are chemical substances present in the cytoplasmic membrane of bacteria. They are involved in a number of crucially important biochemical and physiological processes. Hence, inhibitors of menaquinone biosynthesis appear to be useful in controlling bacterial infections. Recently a team of researchers, led by Dr David Alland (Rutgers University, New Jersey Medical School, USA), reported the isolation of a novel biphenylamide (designated as DG 70), which was found to inhibit the growth of both drug-resistant and drug-susceptible form of Mycobacterium tuberculosis, by inhibiting menaquinone biosynthesis. Anti-tubercular drugs generally used in therapeutics do not work against non-dividing cells. But inhibitors of menaquinone biosynthesis are active even against the cells which are not dividing.
A study made in 2008 revealed that out of 167 new antibiotics in the pipeline, only 15 had a new mechanism of action. Hence a lot of impetus is the need of the hour to boost development of antibiotics with novel mechanism of action.
Reversal of resistance
As mentioned earlier, number of therapeutically useful antibiotics is decreasing because of rapid development of antibiotic-resistance in pathogenic bacteria. So while searching for new sources of antibiotics or trying to develop new antibiotics with improved efficacy, scientists are also trying to foil the various mechanisms of antibiotic-resistance in bacteria and restore their sensitivity to the existing antibiotics.
Some resistant bacteria chemically modify antibiotics with the help of some substances produced by them. The modified substances do not possess any antibiotic- activity. For example, penicillin-resistant bacteria produce an enzyme, beta-lactamase, which catalyses conversion of penicillin into penicilloic acid. It has no antibiotic property. In 1974-75, scientists working at the British company Beecham isolated a compound which they named clavulanic acid from a bacterial culture. It was found to bind to the enzyme beta-lactamase and inactivate it. Thus when clavulanic acid was added to a formulation of amoxicillin , a penicillin-group antibiotic, the bacterial enzyme could not alter the antibiotic . So it remained active. The combination called augmentin is now widely used for the clinical management of infections caused by penicillin-resistant bacteria.
Bacteria in natural environments always live in consortium with other organisms. When their number reaches a certain level, they start producing an organic polymer. The process is known as Quorum Sensing. Organisms get embedded into the polymer and form a conglomerate called biofilm. Bacterial biofilm is formed on mast of the ships, within the pipeline meant for water supply, within the air-condition machine and even on the pacemaker and urinary catheters placed inside the body of the patients. Biofim is like a fort which protects bacteria from several types of adverse conditions. Bacteria in biofilm show remarkable resistance to antibiotics. If the process of biofilm formation could be disrupted, they become antibiotic-sensitive. Quorum sensing is essential for biofilm formation. Scientists are trying to interfere with the process of Quorum Sensing. They are trying to inhibit the process by the addition of several types of chemical substances called the Quorum Quenchers (QQs). If the QQs are used as antibiotic supplement, they are likely to inhibit biofilm formation and thus antibiotic-resistance will not be developed in bacteria.
Gram-negative bacteria and also some gram-positive bacteria release nano-sized bag-like structures in the environment called Outer Membrane Vesicles (OMVs). Besides helping bacteria in cell-to cell communication, secretion, acquisition of nutrients and self-defence, OMVs also foster antibiotic-resistance in various ways. Hence, inhibitors of vesicle formation bear the potential to be used as antibiotic adjuncts which counteract antibiotic-resistance.
These approaches are beset with limitations. The beta- lactamase enzyme produced by some bacteria, is not inhibited by clavulanic acid. Some bacteria are found to produce inhibitors of QQs. So the strategies are not infallible.
Problems in the development of new antibiotics
Development of a new antibiotic takes a very long time and huge amount of investment. After it comes to the market, doctors hesitate to prescribe it in a wide scale apprehending emergence of resistant strains. The case of bedaquiline presents a revealing example in this context. It is an anti-tubercular antibiotic with a novel mechanism of action (inhibition of mycobacterial ATP synthase) approved for clinical use by the Food and Drug Administration (US) after 40 years for the treatment of multidrug-resistant TB (MDR-TB). A 18-year-old girl, suffering from MDR-TB, was admitted to a Delhi Hospital. But doctors were reluctant to prescribe bedaquiline to her. Her father approached Delhi High Court and ultimately got approval of the National Institute of Tuberculosis and Respiratory Diseases (Delhi) for the use of drug to treat her daughter. The hesitation of the doctors to prescribe the newly-developed antibiotics, though rational, impacts the market. The investors do not get quick return from the new drugs which they develop incurring a huge expenditure. Antibiotics, as such, are not expected to give a lot of profit within a short time. Unlike the anti-diabetic drugs or cholesterol-lowering drugs, which are prescribed for life-long use, antibiotics are generally recommended for short-term use. Moreover, restricted use of the new antibiotics by the doctors, adversely affects the commercial prospect of the new products. Quite reasonably, the entrepreneurs do not feel encouraged to invest money for the development of new antibiotics. Thus we see that microbial strains resistant to the existing antibiotics are emerging rapidly, leading to a decrease in the number of usable antibiotics. On the other hand, new antibiotics are coming to the market in trickles. While more than 20 classes of new antibiotics were marketed during the middle of the last century, only 2 classes of new antibiotics appeared in the market since the 1960s. The situation has raised an alarm among the visionaries. “There is an urgent need for more investment in research and development for antibiotic-resistant infections including TB,” feels Dr Tedros Adhanom Ghebreyesus, Director-General of WHO, “otherwise we will be forced back to a time when people feared common infections and risked their lives from minor surgery.”
In an interview with a correspondent of Scientific American, the Nobel-laureate US scientist of Indian origin Professor Venkatraman Ramakrishnan opined “They (the governments) have to think of this as something generally good for society, the same reason that governments fund education, roads, police, defence and so on. This is one case where governments need to act”. The importance of his suggestion needs hardly to be over – emphasised.
Judicious use the only means
The untiring endeavour of the scientists to maintain the supply of life-saving drugs is promising no doubt. But we must try our best to keep the existing antibiotics active by restraining their use. We must remember that more is the use, more is the chance of development of resistance. We must not forget that antibiotics can be used, changed, or their course can be stopped only when advised by a medical practitioner. In this country, antibiotics are freely available without any prescription. But the liberty contributes to the crisis and threatens to invite a dooms day when no antibiotic will be available for the management of life-threatening infections. Judicious use of antibiotics is the only way we could keep such a devastating situation at bay. Simultaneously we should practice and promote our age-old practices (personal cleanliness, hand washing) which minimize the chance of infections.
Role of Pharmacists Scientists, medical practitioners and health workers must initiate all out effort to raise awareness among the common people about the crisis, of which most of us remain blissfully ignorant. Pharmacists working in hospitals can assume important role in monitoring the process of hand-washing, sterilisation, preparation of the antibiotic -resistant profile of organisms isolated from the different areas of the hospitals and antibiotic stewardship (suggestion given to the doctors in avoiding unnecessary use of antibiotics and choosing the right antibiotic). They could also assume an important role in counseling the patients regarding the appropriate use of antibiotics. Needless to say, effort made by them will not be paid any importance in the beginning. But they have to resort to untiring canvassing to make the authority convinced of the importance of their endeavor. For this purpose they have to keep themselves abreast of the recent development in the relevant area.
Suggested for further reading
1) Levy SB (2002) The 2000 Garrod Lecture : factors impacting the problem of antibiotic resistance: Journal of Antimicrobial Chemotherapy, 49 : 25-30
2) Sengupta S, Chattopadhyay MK, Grossart H-P (2013) The multifaceted role of antibiotic and antibiotic-resistance in nature: Frontiers in Microbiology March 12;4:47. doi: 10.3389/fmicb.2013.00047. eCollection 2013
3) Chattopadhyay MK (2014) Use of antibiotics as feed additives : a burning question Frontiers in Microbioogy,
July 2;5:334. doi: 10.3389/ fmicb.2014.00334. eCollection 2014.
4) Stokes NR et al (2013) An improved small-molecule Inhibitor of FtsZ with superior in vitro potency, drug-like properties, and in vivo efficacy. Antimicrobial Agents and Chemotherapy; 57 : 317-325.
5) Sukheja P et al (2017) A novel small-molecule inhibitor of the Mycobacterium tuberculosis Demethylmenaquinone Methyltransferase MenG is bactericidal to both growing and nutritionally deprived persister cells. mBio. 2017 Jan-Feb; 8(1): p ii e02022-16.
6) Bhardwaj AK (2013) Bacterial quorum sensing inhibitors: attractive alternatives for control of infectious pathogens showing multiple drug resistance; Recent Patents in Anti-infect Drug Discovery. — 8 (1) : :68-83.