Prof Roger Beuerman
Senior Scientific Director,
Singapore Eye Research Institute
Professor Duke-NUS, SRP Neuroscience
and Behavioral Disorders
Adjunct Professor of Ophthalmology,
Yong Loo Lin School of Medicine, NUS
When the first antibiotic, penicillin, was discovered (some say rediscovered) by Alexander Flemming who received the Nobel Prize in 1928 for the achievement, it was indeed an incredible event as the balance of morbidity from infections was quickly shifted toward recovery rather than death. The name originated from the source, a mold named Penicillium (Figure 1), and indeed natural sources are still an important source of new antibiotics (see BioBoard for a SE Asian fruit- Mangosteen- that is being investigated as a source of new anti-microbials). In healthcare systems world-wide, most patients assume that there will be an antibiotic available to fight whatever infection they might have contacted. However, for a number of reasons some serious issues have been met with in the advancement of new antibiotics which have slowed their development, especially at a time when many bacteria are not affected by some of the existing antibiotics and new forms of common bacteria have emerged that find ways of changing their biological activity so that antibiotics lose their potency.
Causes for Antibiotic Resistance
The list of antibiotics is extensive, but issues complicating their use, such as harmful side-effects, specificity in their action, has resulted in limitations in their use and more importantly, the overuse of antibiotics has led to problems affecting their development. Overuse has become a major factor in the development of what is widely known as “antibiotic resistance” resulting in hesitation by the pharmaceutical industry to put effort into discovering or synthesizing new antibiotics resulting in a diminished pipeline of new drugs (Table 1). The term “antibiotic resistance” is fairly common in both the public press and it has been the subject of numerous news reports as well as many scientific papers (1-4). So what exactly is antibiotic resistance and why is it such an issue?
Antibiotic resistance has a genetic basis and from an evolutionary view, bacteria has always had the potential for resistance, but the widespread use of antibiotics in healthcare, food preservation and for animal use has provided the opportunity for resistance to become operational by selecting for bacteria expressing mutated genes (2). As bacteria are hardy organisms, the mutated genes can be shared between bacteria increasing the spread of resistance. It is important to note that resistance in this sense has been an issue mainly for bacteria, but more recently resistance has been recognized for fungal infections as well (5). There are many different types of antibiotics (Table 1), some have very specific action while others are termed broad spectrum, killing both Gram Positive and Gram negative bacteria. When bacteria develops resistance one or more antibiotics may become ineffective and these bacteria are termed “superbugs” and of course, it is more difficult to overcome the infection. Confounding factors are the fact that patients overuse antibiotics, doctors frequently overprescribe their use and in some countries, antibiotics can be readily purchased over the counter (2). The medical community is actively attempting to limit the use of antibiotics to situation where they are clearly needed. In fact, many patients assume that any type of infection can be killed by antibiotics; however, many infections such as the common cold are viral in nature and antibiotics have no effect on viruses. When antibiotics are tested for their effectiveness on bacteria, a common method used in the pathology and microbiology laboratory is the Minimal Inhibitory Concentration, MIC value which is often expressed in amount per volume, such as ug/ml. Note that the MIC value is for growth inhibition not actual killing. If, as happens, bacteria normally have a MIC value for penicillin of 1-2ug/ml and when resistance occurs this value may increase by more than four times. In the laboratory, we can simulate the selective pressure of the overuse of antibiotics by constantly exposing an organism to an antibiotic and then testing to determine if the MIC value changes. An example of this method is seen in Fig. 2. In this example, a standard strain of Pseudomonas was tested for the ability to develop resistance to two different current antibiotics, gentamicin and norfloxicin. Both of these antibiotics showed signs of developing resistance by the increase in MIC values. However, the push in current new antimicrobial research is to develop drugs to which bacteria do not easily develop resistance. As seen at the bottom of Fig. 2, SERI-B2 which has been designed and synthesized at the Singapore Eye Research Institute as a new antibiotic did not show resistance (resistance is defined as more than a 4-fold increase in MIC). It is easy to extrapolate this laboratory example to the real world where many thousands of patients are taking antibiotics with the ensuing development of resistance.
Emergence of Multi-drug Resistance Bacteria
As bacteria are small organisms they thrive in enormously large numbers and they multiply rapidly. Thus, like all organisms, the expressed genome can vary somewhat through mutations. Billions of bacteria when treated with an antibiotic will usually be killed by the drug; however, a few may express a gene which is mutated either spontaneously or by selection, producing a product that essentially escapes the action of the antibiotic. An often used example is of the beta-lactamases, an enzyme that both Gram positive and Gram negative bacteria can express. Beta-lactam antibiotics include penicillin and cephalosporin (see Table 1) which are inactivated by the presence of a beta-lactamase and it includes a large number of antibiotics developed over the years which have been in the market.
An important structural chemical component of these antibiotics is a beta-lactam ring which is disrupted by the beta-lactamase, rendering the antibiotic ineffective. As the antibiotic interferes with the synthesis of the bacteria cell wall making the organism fragile, formation of resistance overcomes the antibiotic and making infectious bacteria multiply rapidly. To overcome the beta-lactamase resistance, some antibiotics, such as augmentin have been developed that included a specific inhibitor of the beta-lactamase. Resistance carries an economic toll increasing health care costs as well as increased suffering and morbidity for affected patients (6). At present, resistance is very common particularly to Gram–positive, MRSA, Enterococcus and more recently to Gram–negative bacteria such as Pseudomonas sp. and these infections account for a large number of hospital and nursing home associated deaths (7-9).
Although resistance to penicillin was initially seen in the 1940s, issues with increasing resistance moved slowly despite an increasing number of scientific publications noting various aspects of resistance. As bacteria modified the structure of the proteins to which penicillin binds a new bacteria emerged, the methicillin–resistant Staphylococcus aureus or MRSA which has had a major impact on health care costs and mortality. The basic organism, Staphylococcus arueus, is a common pathogen often living on the skin or in the nose (Figure 3).
Serious infections are often associated with patients with weak immune systems, the sick, elderly, or in long term care facilities. The term methicillin resistant is from the antibiotic, methicillin, a beta-lactam antibiotic that was used for treatment of MRSA, but although replaced now, it is still used to define this type of bacteria. The infection caused by MRSA is not always more serious than other infections, but the problems arises due to the resistance as the bacteria cannot be brought under control, prolonging recovery and tissue destruction. As MRSA was primarily a hospital acquired infection, it was not so widely spread, but now there is a community acquired MRSA, Com-MRSA, which is seen in otherwise healthy people who have not been in the hospital. Com-MRSA is often seen as a serious skin infection which is not easy to treat and can spread to other vital organs. On a practical note, the hand-washing campaigns that are frequently seen throughout Southeast Asia are an attempt to diminish the number of skin associated MRSA infections.
Antibiotics Fighting Resistance
The broad range of beta-lactam antibiotics which account for a large majority of antibiotics in the market or that have been developed are ineffective against MRSA, these antibiotics include the penicillin and cephalosporin. The penicillin include a chemical family of methicillin, dicloxacillin, nafcillin, oxacillin, as well as other members. Although, methicillin is resistant to beta-lactamase and is able to bind to penicillin-binding proteins and inhibit the synthesis of an important component of the cell wall of the Gram–positive bacteria, peptidoglycan, it is no longer used due to the fact that other antibiotics have fewer side effects and are easier to administer. However, it is important to notice that the critical need to eliminate this infection has prompted the development of specific antibiotics that deal effectively with MRSA and by extension Gram-positive organisms. However, the antibiotics that have been developed for this purpose are generally used specifically for MRSA and some variants such as VRE, vancomycin resistant Enterococcus.
The first of these “last resort” antibiotics was vancomycin, a glycopeptide which was actually discovered in 1953 (Table 1). It originated from soil bacteria and was found to avoid the development of resistance from Gram-positive bacteria making it the choice for treatment of MRSA. This property led to rapid approval in 1958. Although vancomycin had drawbacks, it has poor absorption when taken orally so the usual route of administration is intra-venous, but an oral version was approved in 1986 for use with C. difficle. A second antibiotic in this category is daptomycin, a lipopeptide from natural sources. It is a soil bacteria which was actually discovered in the 1980s but not approved until 2003. It shows efficacy in treating resistant forms of Gram-positive bacteria by disrupting several aspects of bacteria cell membrane function and leads to a loss of membrane potential, as well as inhibition of protein, DNA and RNA synthesis and finally bacteria cell death. A recent entry into the antibiotic spectrum for serious Gram-positive infections, linezolid is somewhat unique as it is a completely synthetic antibiotic. Linezolid was discovered in the 1990s and its approval for clinical use was granted in 2000. The mechanism of action is not completely understood but it seems to inhibit protein synthesis at the stage of initiation. It has a good safety profile as it is not broken down through the cytochrome P450 mitochondrial pathway which is unlike many antibiotics and antifungals in current use. Resistance to vancomycin and daptomycin has been noted for MRSA as well as another Gram-positive bacteria, Enterococcus (10, 11); however, daptomycin may have more robust activity compared to vancomycin and it may actually be a treatment option in the case of vancomycin resistance.
Developing Antibiotics for the Future
It is important to be aware that bacteria resistance will not go away, bacteria that are now showing resistance will remain and others may still develop as genes can be shared laterally. Even if new types of antibiotics are successfully developed that do not select resistant bacteria, it would not be possible to eliminate all resistant bacteria.
The question is then how the situation stands now. In the early days of developing antibiotics, it was necessary to show their efficacy. Now, there is an emphasis on showing that resistance develops more slowly or not at all. Vancomycin was an early attempt at that concept which was not successful. A recent review has found that 20 antibiotics were approved since 2000 and about 40 new antibiotics are in the current pipeline (12). Of these new antibiotics, 11 were from natural products and 9 are synthetic. However, the antibiotics from natural sources, largely fungi and mold, are of a class that has already shown resistance and similar issues are expected when their usage increases. From the class of synthetic antibiotics, most are from the quinolone class. Other members of that class show toxicity and resistance.
An unique class of antimicrobials is found in nature which function similar to those derived from fungus. They protect the organism from pathogenic invasion and are referred to as host defense peptides or defensins (13). These have not found clinical application as yet but there are a number of potential advantages such as the ability to modulate the host immune response and some may be anti-inflammatory. Reviewing this area of research shows that it is feasible to use some of the chemical features of naturally occurring antimicrobials to create new molecules that are actually much more active against bacteria than the naturally derived molecules.
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