Frontiers in Antimicrobial Agents

Antibiotherapy protocols for infectious disease control were first applied in 1940s with penicillin. Although they led to a major decrease in morbidity and mortality rates, they were also responsible for the rapid emergence and dissemination of multidrug- resistant bacteria (MDR). This phenomena led to the prompt development of new antimicrobial compounds, which soon became ineffective due to bacteria ability to develop resistant traits through mutations or resistance genes transfer.

However, the development of new approaches for prevention and control of emerging infections remains one of the major priorities and challenges for Research and Innovation (R&I). The worldwide mortality rate due to infectious diseases keeps exponentially increasing, not only due to MDR bacteria dissemination, but also due to the decline in the development and commercialization of new generations of antibacterial compounds.

Considering the “One Health” concept, new antibacterial strategies are urgent, for human and veterinary medicine. Several R&I approaches are being followed, including phage therapy, antimicrobial peptides and bacteriocins, probiotics, natural compounds, immunomodulation via vaccination and biocides.

Bacteriophages are viral, natural and bacterio-specific entities. As a major part of the biosphere, they were involved in the origin of life. They still play an essential role in evolution and are highly involved in the development of molecular biology. In the issued biotechnology industry, they are promising as a sustainable antibacterial. Felix d’Hérelle, one of the discoverers of bacteriophages, first proposed “(bacterio) phage therapy” in the early 20th century. At the Eliava Institute in Tbilisi, Georgia it was further developed and it is still used in medical practice in all the former Soviet Republics. The Western world, with the advent of antibiotics, almost forgot phage therapy.

The antibiotic resistance crisis placed phage therapy again in the spotlights. The main problem today is the lack of evidence based therapeutic phage studies in accordance to modern standards as well as the lack of an adapted phage therapy regulatory frame. Initiating clinical studies in this context is difficult. Phage therapy is sporadically applied today, although under specific conditions like the Helsinki Declaration and/or specific national regulatory frames (Poland). This impedes clinical application and scientific progress.

However, several groups setup animal and human studies, while the idea of using bacteriophages as antibacterial is already applied in the food industry. In the clinic application seems imminent. Several issues, also from the fundamental scientific perspective, still need to be tackled while practically an adapted regulatory frame is urgently needed.

Most Antimicrobial Peptides (AMPs) are small peptides with 10–40 residues. They are part of the immune system in virtually all multicellular organisms as a defense strategy. They are promising drug candidates for the control or treatment of several diseases or to be used in combination with conventional therapy. There are several AMPs effective against cancer cells and diseases caused by bacteria, fungi and parasites. However, none AMP is currently commercialized due to a number of reasons such as high production costs as compared to conventional antibiotics. The new generation of AMPs, mainly antimicrobial peptidomimetics, has several drug candidates currently in Phase I and II clinical trials. Understanding the AMPs' fully mechanism of action is crucial to develop AMPs into useful antimicrobial agents highlighting their strengths: broad-spectrum antimicrobial activity, low probability to generate resistance, and potential for topical and injected applications.

Probiotics are defined by Food and Agriculture Organization (FAO) and World Health Organization (WHO) as: “live microorganisms that, when administered in adequate amounts, confer a health benefit on the host”. Bifidobacterium and Lactobacillus are the principal probiotic groups; however, there are reports on the probiotic potential of Enterococcus, Bacillus and yeasts. A number of potential benefits of probiotics have been proposed, including: alleviation of lactose intolerance, immunomodulatory activity, hypocholesterolaemic effects, prevention of inflammatory bowel disease and effect on Helicobacter pylori eradication. Currently, most of probiotic strains used commercially were initially isolated from human feces whether adults or children, as well as dairy products. However, recently, it has been shown that non-fermented dairy foods such as vegetable foods are an excellent source of bacteria to find new probiotic strains with a significant probiotic potential. In this review, the history of probiotics, their mechanisms of action and the beneficial health effects are presented.

The relationship between a host and a pathogen is dynamic, and its outcome depends on the virulence of the invader and the relative degree of resistance or susceptibility of the host at that particular occasion. Fortunately, only an infinitesimal minority of microorganisms are able to avoid the host defenses and eventually cause disease. The host has evolved multifaceted strategies for defending itself against invasion, and pathogens have evolved their own strategies of counter-attack for host defenses pointing us to the co-evolution of “defense” and “attack” mechanisms. Many factors determine the outcome of the bacterium-host relationship. In this battle, the development of antibiotics was a game-changing turn. Unfortunately, more and more strains of pathogenic bacteria have become antibiotic resistant. One of this century’s greatest medical challenges is the rapid emergence of multidrug-resistant pathogens. Discovery of new antimicrobial classes and alternative therapeutic options would be especially welcomed in this era. The fact that some bacterial infections might no longer be successfully treated with antibiotics, combined with an increasing population density and mobility, justifies the urgent demands for the development of novel treatments, of which immunotherapies are considered most promising. Immuno modulatory regimens offer an attractive approach as they often have fewer side effects than existing drugs, including less potential for creating resistance in microbial diseases. Nowadays, treating emerging chronic and multidrug-resistant infectious, autoimmune and oncological diseases through reinforcement of the immune system became a major priority.

The emergence of antibiotic resistant strains along with the increasing public health concern regarding infectious diseases and hospital-acquired infections has led to the necessity of finding viable alternatives to antibiotics.

Nature has been a source of compounds with interesting medicinal properties for millennia. For many decades, natural products have been a wealthy source of antimicrobials and more recently academic drug discovery in this research area has been accentuated, though pharmaceutical industries devote fewer resources to antimicrobial drug discovery programs, in comparison with their investment several decades ago.

Though, initially, the developments in drug discovery were aimed towards synthetic chemical libraries, nowadays they are also greatly used in the research of bioactive natural products, in order to keep up with the developments in similar areas. Moreover,

these developments allowed to overcome several problems associated with natural products drug discovery by using molecular techniques (i.e. genome mining) when organisms are not cultivable on the laboratorial environment. In this chapter, various examples of natural products with antimicrobial properties from marine organisms and plants are referred, highlighting their important and promising results against pathogenic bacteria, fungi, viruses and protozoa.

Bacteriocins are antimicrobial peptides produced by numerous bacteria, which may present narrow or broad host range. These ribosomally synthesized antimicrobial peptides are considered a successful strategy in maintaining equilibrium within a bacterial ecosystem. These compounds kill other bacteria by several mechanisms, including the modification of membrane permeability and depolarization of membrane ion gradients, the degradation of nucleic acids or cell walls. In literature, the term bacteriocin is usually restricted to peptides produced by Gram-positive bacteria, while in Gram-negative bacteria, mainly enterobacteria, the toxins are called either colicins (i.e. antibiotic proteins targeting Escherichia coli) or microcins. Many bacteriocins are produced by food-grade lactic acid bacteria, a phenomenon which offers the possibility for preventing the development of specific bacterial species in food. This can be particularly useful in preservation or food safety applications, but also has implications for the development of desirable flora in fermented food. In this sense, bacteriocins can be used to confer a protection and at the same time help processors extend shelf-life after product manufacture.

Acute infections caused by pathogenic microorganisms have been studied extensively for over 100 years. Infections that killed millions of people in the past are now ever more successfully controlled due to the development and use of antimicrobial compounds. Biocides, in particular, have been used in various forms for many centuries to control the dissemination of microorganisms and represent a wider group of substances with a key role in the medical and food industry. This chapter presents the main biocidal classes and mechanisms of action, including their respective spectrums of activity and use in several fields. Factors influencing the effectiveness of biocides are addressed as well as biocide resistance and the link with antibiotic resistance. Several mechanisms of biocide resistance are presented on a molecular level with a particular focus on biofilm resistance and its formation in bacterial populations. This chapter concludes with an introduction to ECHA, the European Chemicals Agency and their recently approved regulatory framework for biocide products in Europe.

The use of antibiotics in animal production has led to the emergence of antibiotic resistant microorganisms with severe consequences in animal and human health. To counterbalance and substitute the use of antibiotics as antimicrobial agents, a comprehensive effort has been directed towards the development of new therapeutic strategies and Veterinary Medicine offers an appealing field for the implementation of such an effort. Probiotics and bacteriophages are two of the alternatives discussed.