Drug-Resistant Pathogenic Bacteria
Abstract
Bacteria are amongst nature’s perfect examples of evolutionary biology. They have a plethora of means in order to adjust, regulate, and assimilate their newly respective environmental conditions. Such mechanisms include the development of filamentous structures (the so-called pili or fimbria), the alternation between sexual and asexual reproduction (depending on whether the respective environmental conditions are stable or potentially dangerous for the existence of the bacteria). Therefore, the following lines intend to analytically present the evolutionary traits that have allowed bacteria to develop antibiotic resistance and, hence to survive. Moreover, this article shall unfold the role of the antibiotics in bacterial infections, while at the same time exhibiting why antibiotics consequently allow for bacterial evolution. Finally, we will conclude by focusing on the potential means in order to overcome this ‘antibiotic’ failure that gradually renders bacteria stronger and evolutionary more alert.
Introduction
Bacteria comprise a large domain of the prokaryotes and have a length of a few micrometers (μms). Furthermore, they are differentiated according to their morphology (shape e.g. cocci), their chemical composition (gram positive, gram negative), biochemical and biomechanical activities, and source of energy. In fact, bacteria are everywhere; most of them in harmless co-existence with our soils, waters, food, or even within our bodies. For example, the Lactobacillus acidophilus bacterium, which resides naturally in our intestines, helps in the digestion of the food (DiPardo, 2014). However, we naturally tend to associate the topic of bacteria with infections, antibiotics, or even death. Bacterial infections are initiated by the primary adhesion of bacterial pathogens to host tissues. The subsequent invention of antibiotics (e.g. penicillin), forced bacteria to follow the path of evolution in order for them to be able to survive. Therefore, bacteria developed a plethora of counteractive measures and means in order to adjust, regulate, and assimilate their newly respective environmental conditions. Furthermore, numerous mutations in their genetic material allowed bacteria to express resistance against traditional antibiotics; alternation between sexual and asexual reproduction, modification of the bacteria’s cellular wall, development of hair-like filamentous structures (fimbria or pili) for the initiation of bacterial adhesion, mutations, are simple examples that illustrate bacterial ‘surviving’ techniques. Hence, it comes as no surprise that bacteria evolve resistance to antibiotics within only a few months. Therefore, the following lines intends to focus and efficiently describe these evolutionary traits that have allowed bacteria to develop antibiotic resistance and, thus to survive and continue to inflict infections on humans.
The role of antibiotics
The technological advances in medicine and pharmaceuticals brought about a new era that propelled the subsequent invention of antimicrobial (antibiotic) drugs. In fact, the use of antibiotics, regardless of how intentional or unintentional it was, had been very common in ancient civilizations such as the Sudanese Nubians or the Egyptians (Aminov, 2010). In both cases, traces of tetracycline were found in their human skeletons, thus demonstrating that these populations enhanced their immunity against infectious diseases that were prevalent at these times. On the other hand, and in the modern era, antibiotics were firstly given a great deal of attention by the public in 1928 with the invention of penicillin by Alexander Fleming. Prior to its introduction, there was no efficient means of fighting infectious diseases such as pneumonia, gonorrhoea or even rheumatic fever (acs.org). Therefore, simple cuts would, ultimately lead to blood poisoning, while at the same time the respective doctors would simply sit and watch, unable to prevent the infectious process. Thus, penicillin offered the opportunity for immunity against diseases such as syphilis or infections caused by bacterial agents such as Staphylococci or Streptococci. Antibiotics manage to interfere with the ability of bacteria to repair their damaged DNA or by rendering their cell wall extremely weak, to the point that it is fragile enough to burst. Nevertheless, and in both cases the ability of reproduction (growth of new cells) is hindered, and this is the great success of antibiotics. However, and despite the fact that antibiotics are considered to be an extremely successful form of chemotherapy, they have, additionally ‘forced’ the same bacteria to follow the path of evolution in order for them to be able to survive. After all, the evolutionary process does not explicitly refer to the human species. In other words, antibiotics have propelled a counteraction that gradually enabled the same bacteria to alter their properties (e.g. by mutations) and, thus to resist the effect and action of antibiotics.
Development of antibiotic resistance – adjustment to the respective environmental conditions
But how exactly can bacteria acquire ‘immunity’ against the antibiotics (or in other words how do they develop their anti-antibiotic action)? It was previously said that this is a profound example of bacterial evolution. Furthermore, one of the main reasons supporting this argument is that bacterial pathogens are allowed to evolve far more rapidly because they have a much shorter generation cycle than the human organism. Bacteria contain many genes that express themselves in a variety of ways, and, in principle, they are freely mixed (Morris, 1988). Once these bacteria are exposed to an antimicrobial agent (or environment), then the large majority will naturally die; however, there will be a small portion of bacteria that will survive due to the fact that they possess some arbitrary genetic recombination (or genetic mutation that allows them to efficiently survive the antimicrobial action). Therefore, the surviving bacteria now possess antibiotic resistance that will be virtually extended to the whole population because these surviving bacteria are the only ones that can reproduce. Thus, it comes as no surprise that bacteria evolve resistance to antibiotics within only a few months. However, in this case evolution can be seen under two different spectra. One that suggests that evolution offered the means to bacteria to assimilate the necessary means in order to survive the antibiotic action (thus, the bacteria gained knowledge). On the other hand, the alternative prism involves that there was no production of new genetic information and that bacteria actually manage to develop antibiotic resistance because some form of genetic information (e.g. the genes that were prone to be affected by the antibiotic action) was lost. In both cases the development and the survival of bacteria is a clear sign of evolution, regardless of how evolution is perceived in each one of them.
On the other hand, and more specifically, the evolution of bacteria in order to adjust to the respective environmental conditions is also proved by the modification of the bacterial cell wall or by enzymatic inactivation of the antibody (Benveniste & Dabies, 1973; Wright, 2005). In the first case the bacteria modify the target side, which is now not recognized by antibiotics, whereas on the latter, an existing cellular enzyme acts upon the antibiotic. The end result is that the antibiotic no longer affects the bacterium (Toddar, 2013).
Development of pili (fimbria)
Bacterial attachment to a host cell comprises the first step for the initiation of an infection. This first step is realized by long, thin filamentous structures, so called pili, which act as a nexus between the host and the bacteria, thus allowing their initial colonization and the subsequent biofilm formation (Andersson et al., 2006; Bullitt & Makowski, 1998; Castelain et al., 2009; Lowy, 2003; Nallapareddy, 2006; Passali et al., 2007). In other words, pili are of upmost significance for the initiation of the bacterial pathogenesis in the host cells because they provide the requisite specific receptor–ligand interaction. The expression of pili is crucial for bacterial adhesion. However, and more importantly, the subsequent understanding of the function of such structures is extremely important not only for the acquisition of a more spherical view with respect to the adhesion progress but also for designing of new antimicrobial drugs.
Nevertheless, why did bacteria develop such structures? The answer lies in evolution and the confrontation of the natural defences of the human organism, such as respiration and urination that can literally flush these bacteria out, by applying a shear force upon them. More specifically, these forces will induce a torque on the bacteria, thus resulting in their detachment from the cells. Therefore, and in order to resist these shear forces, bacteria have developed these hair-like filamentous structures that are utilized in mediating the adhesion; and they do so by bringing the adhesion in direct contact with the host cell. Pili have been found to possess extraordinary elongation properties that allow them to survive the shear forces produced by the urinary flow (Andersson et al., 2006). More specifically, when the urinary flow begins, these pili elongate (as much as 10 times their initial length) in order to sustain their position and contact with the cell (Escherichia coli). Once the flow is over, they begin to retract, thus returning to their initial position, maintaining contact and, thus withstanding the natural defence of urine. Moreover, the same concept applies for the Streptococcus pneumoniae bacteria that colonize the upper respiratory tract, with a high percentage of mortality and morbidity worldwide (Castelain et al., 2009). These piliated bacteria manage to anticipate the natural defences of the human organism (coughing or sneezing for instance) by making use of the elongation–retraction properties of their pili. Finally, it needs to be also mentioned that an additional reason that bacteria have developed pili is that both them and the host cell membranes are negatively charged. Nevertheless, these structures were developed as a response to these forces and exhibit how evolution can allow bacteria to prevail and continue inflicting bacterial infection upon humans.
Sexual and asexual reproduction (parthenogenesis)
An additional example that illustrates in a rather impressive way the evolutionary process that bacteria had to take in order to survive, is the alternation between sexual and asexual reproduction (parthenogenesis) that bacteria are able to exhibit during stable and unstable environmental conditions. First of all, during asexual reproduction, the respective offspring inherits all their chromosomes from their single parent (an unfertilized egg develops into a new individual); hence, they both share an identical DNA (Stearns & Hoekstra, 2005). Asexual reproduction is extremely simple and requires very little energy as compared to the sexual one. Moreover, it is extremely faster and the offspring have no genetic variations. However, this last property is what renders asexual reproduction as very beneficial only when performed under stable environmental conditions. In the case where the prevalent conditions do not favour the survival of the bacteria, then sexual reproduction is promoted in order to effectively deal with the respective environmental changes. In other words, sexual reproduction allows for genetic variations (combination of both the father and the mother DNA) that can differentiate and, hence evolve in order to survive.
A very interesting, yet depictive example of species that would normally reproduce asexually but in times of environmental stress they reproduce sexually, is met in rotifers. Rotifers are opportunistic organisms that are usually found in freshwater environments. It was shown that several species have the ability to reproduce both sexually and asexually, an alternation that purely depends on the environment within which they grow and thrive (BIRKY & GILBERT, 1971). For example, the genus Asplachna can alternate between sexual and asexual reproduction depending on the existence of specific molecules, the so-called inducers. In another article Snell has examined the effect of temperature on Brachionus plicatilis (Muller) with respect to sexual and asexual reproduction (Snell, 1986). His findings suggested that by increasing the temperature at 38º ‘asexual female fecundity reached its highest level, while sexual female fecundity declined 15%’. Furthermore, an additional example includes the Paramecium, a genus of Ciliate protozoa (Russell, 2013). More specifically, and under certain environmental conditions, such as overcrowding, this bacterium will allow two individuals to exchange genetic material (in other words to mate), a process known as conjugation. Nevertheless, it must be noted that the selection of sexual reproduction is selected only in the case that it definitely leads to successful fertilization.
Alternatives to antibiotics
There is a genuine fear that we are slowly, yet steadily heading into an era where there will be no antibiotic protection. Despite the fact that antibiotics manage to kill bacteria, these bugs manage to find ways in order to prevail by genetically transforming into something more solid, more flexible, and more infectious. Therefore, the first thing we, as society, need to do is to stop this extended overuse of antibiotics. Furthermore, we need to take into closer consideration the evolutionary perspective of our health. After all, we should not forget that ‘Most cells in our body are not human but microbial; the ratio of ‘them to us’ is about 10:1 (Blaser, 2006). Finally, it needs to be mentioned that several alternatives have been also suggested such as herbs, plants, diets, or even antibiotic peptides, the latter being tested successfully in laboratories (Fraunhofer-Gesellschaft, 2011).
On the other hand, and in the case of the piliated bacteria, we need to focus on their biomechanical properties which might further enlighten the exact elements that are found on the equation of bacterial infections. The question is not how to invent new massive production antibiotics, but to understand the exact chemistry and properties that will slow down the evolution of bacteria to the minimum. For instance, by measuring the biomechanical properties of these thin structures, we can define their behaviour under stress, and also their dynamic response to the presence of external forces. Streptococcus pneumonia pili have been found to ‘work’ (force – extension response) in accordance with the WLC model (worm-like chin model) (Castelain et al., 2009). Therefore, they behave similarly to continuous semi-flexible biopolymers and we can easily find several intrinsic properties such as the Persistence Length Lp or their Contour Length Lc. On the other hand, Escherichia coli pili have been found to be linearized polymers with an open helix-like structure (Andersson et al., 2006). This model of action allows these pili (and consequently the bacteria themselves) to distribute shear forces that are caused by the natural defences of the human organism, such as urinal flow, amongst various pili. Therefore, this distribution allows them to maintain and prolong their adhesion to the respective (epithelial) human cells.
Conclusions
This article effectively presented the reasons but also the means with which bacteria can successfully overcome and survive the action of antibiotics. Each time scientists discover a very important antibiotic, they also propel the same bacteria to alter their genetic properties in order to survive its action. However, the extended overuse of antibiotics has accelerated the respective bacterial immunity against antibiotics; hence we might have to focus on alternative techniques that can potentially turn out to be extremely helpful in decelerating the respective evolution of bacteria. Whether we want it or not, bacteria will always accompany us in our lives. They were here before us, and they will be here after us. Therefore, it is in our hands to learn from our mistakes, and, more specifically the tremendous overuse of antibiotics, and focus on either a better use of them, or find alternative protection against bacteria. In any case, if this article was to be written in 100 years, the exact same words would be written. The only thing that would change would be the respective titles that would describe each defensive mechanism (evolutionary traits) that bacteria would have followed in order to survive. After all, evolutionary changes are not limited to the human race…but in every living thing. Evolution has made us find antibiotics and the same evolution has allowed these bugs to develop their own survival strategies.
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