Introduction

Currently, pharmaceutical industries are lacking therapeutic alternatives against pathogenic bacteria, due to multiresistance mechanisms developed by these ones against the different commercialized antibiotics¹. In the last decade, several key organizations, as the Infectious Diseases Society of America (IDSA), Centers for Disease Control and Prevention (CDC), the World Health Organization (WHO), and the World Economic Forum (WEF), have made of the antibiotic resistance the focal point of the reports, by conferences to accelerate the restriction and control activity faced with antimicrobials². Bacterial resistance evolution is generating a serious problem in public health³, as a result of the indiscriminate use of antibiotics, the application of non-optimal doses, the irregularity in medication taking referred by the professional in health, these are factors that have influenced in the antimicrobial resistance rate rising⁴; being relevant to generate strategies that contribute to decrease it, including the rational use of antibiotics⁵. That is why it has been necessary constant researches and the perseverant development of new therapeutic alternatives⁶. Approximately five years ago, professor Kim Lewis from Northeastern University in Boston, directs the research that discovered a new natural antibiotic extracted from the ground called teixobactin; this research has been developed together with NovoBiotic Pharmaceuticals, which headquarters are located in Cambridge, Massachusetts and are the owners of the patent⁷. The objective of this review is to describe new antibiotics generated as therapeutic alternatives faced with the bacterial resistance emergence, their obtaining methods and actions mechanisms.

Materials and methods

It was made a bibliographic search on data bases as Science Direct, Redalyc, Google Scholar, NCBI, Pubmed, Pro-quest, Dialnet, Lilacs and Toxnet; they were used the next key words teixobactin, eravacycline, tetracycline, bacterial resistance, lipid II, Eleftheria Terrae, Ichip validated in MeSH. It was established for the search of thirteen combinations, finding a total of 70 articles. They were selected original and revision articles published from 2012 to 2018 available in Spanish, English and Portuguese.

Results

Teixobactin

Teixobactin is a depsipeptide that consists of eleven residues of amino acids (aa), including seven L-aa residues and four D-aa ones, among them it is found an inusual L-alloenduracididine (L-allo-End), which is part of the C-terminal tetrapeptide lactone substructure formed by an ester bond between D-Thr8 and L-Ile11. (Figure 1) ⁽⁸ This is a product of a Gram negative bacterium called Eleftheria terrae, which belongs to the proteobacteria group (betaproteobacteria), related to Aquabacterium gender⁹, from this bacterium, it has been isolated a compound called teixobactin with a molecular weight of 1.242 Daltons, which biosynthetic route is made up of two genes txo1 y txo2¹⁰; the technique used to discovered it, entails a higher impact that i sable to originate a new encouraging era in the discovery of new natural antibiotics¹¹, because of the description of its action mechanism in the organism which possibly avoids the creation and expression of any kind of bacterial resistance¹².

Source: Guo C, Mandalapu D, Ji X, Gao J, Zhang Q. Chemistry and Biology of Teixobactin. Chem - A Eur J. 2018; 24(21):5406-22

Figure 1 Teixobactin Chemical Structure. 

Technology for microorganisms obtaining: IChip Method

The microorganisms are cultivated in a laboratory where it is analyzed its pathogenicity, its capacity of producing antibiotics and its evolution, through the control of growth conditions, taking into account those necessary for an optimal development and proliferation¹³, however; approximately 99% of the bacteria is not able to be cultivated in the laboratory due to extract growth demands¹⁴.

Eleftheria terrae is a microorganism that presents extremophile conditions, faced with this, it was developed a semipermeable plastic membrane system called IChip (isolation chip) (Figure 2), which creates an analogous natural environment of bacteria and fungi¹⁵, allowing the growth in its “natural environment” by simulating their environment in the laboratory. IChip is able to cultivate organisms from soil, sea water, saliva, marshes, and sewage water bioreactors¹⁶.

Source: Losee L. Ling, Tanja Schneider. Kim Lewis. A new antibiotic kills pathogens without detectable resistance. Nature. 2015

Figure 2 I chip 

This multichannel device with 192 compartments is used for an initial microorganisms isolation followed by stimulating the proliferation of bacteria which present a difficult growth in the laboratory¹⁷; from a soil sample collected in the state of Maine (U.S.A.) it was found an ecosystem composed by all kind of microorganisms (bacteria, fungi, protozoans), this consists on a central dish (a), which is home to growing microorganisms in their semipermeable membranes in each side of the dish which are separated from the environment and two lateral support panels (b) the central dish and the lateral panels possesses multiple holes (c), when the dish is immersed in cells suspension in molten agar, the holes capture small volumes which get solidified forming small agar plugs, then the membranes join together and the I chip is placed on the soil where the strain came from¹⁸.

By means of this technique, they were isolated about 10.000 bacterial strains from the soil, for this, they were prepared agar extracts and nutrients in order to identify which presented antibiotic activity, discovering in this way, a gram negative bacterium called Eleftheria terrae, which produced a substance that inhibited Staphylococcus aureus growth¹⁹; after several tests, they observed that this technique also worked successfully for Mycobacterium tuberculosis and Clostridium difficile²⁰.

Teixobactin action mechanism

Teixobactin exerts its bactericide effects joining to two bacterial polymers precursors from the cell wall: lipid II (peptidoglycan) and lipid III (teichoic acid) ⁽²¹. (Figure 3)

Fuente: Losee L. Ling, Tanja Schneider. Kim Lewis. A new antibiotic kills pathogens without detectable resistance. Nature. 2015

Figura 3 Teixobactin action mechanism model 

Peptidoglycan is essential to conform the bacterial cell wall structure, teichoic acid has net negative charge, affixing cations that bring rigidity to the cell wall structure²²; this attraction to those lipids explains the efficacy of the compound faced with Gram positive bacteria²³. Teixobactin Works differently to other antibiotics when attacking not only lipids, but also cell walls²⁴, lysing them quickly, stopping in this way the development of resistance mechanisms to this compound²⁵; literature mentions that when inoculating mice with teixobactin, which were infected with Staphylococcus aureus or Streptococcus pneumoniae, their infection gets reduced without any toxicity²⁶.

Teixobactin is ineffective against Gram negative bacteria, since the cell wall composition in its external membrane contains phospholipids and polysaccharides that block the Access of lipid II²⁷, besides these bacteria lack techoic acids, main components in order to make the antibiotic exerts its bactericide action²⁸.

Lewis determined the activity of teixobactin by means of methicilin resistant Staphylococcus aureus (MRSA) ⁽²⁹, deducting that it had an excellent bactericide activity faced with this microorganism, which is superior to vancomycin in the destruction of populations in exponential phase and kept the bactericide activity against intermediate resistance, as it is shown in the figure 4.

Source: Losee L. Ling, Tanja Schneider. Kim Lewis. A new antibiotic kills pathogens without detectable resistance. Nature. 2015; 517. 45

Figure 4 Destruction of pathogenes depending on the time for teixobactin 

Eravacycline

Eravacycline is a new bacteriostatic antibiotic entirely synthetic belonging to tetracycline family³⁰, the compound has exposed a powerful activity against a wide spectrum of sensitive and multiresistant bacteria³¹, in its activity spectrum they are found Gram positive bacteria as Enterococcus faecalis, Enterococcus faecium, Staphylococcus aureus, Streptococcus anginosus; and Gram negative bacteria as Citrobacter freundii, Enterobacter cloacae, Escherichia coli, Klebsiella oxytoca, Klebsiella pneumoniae and anaerobic bacteria Clostridium perfringens, Bacteroides caccae, Bacteroides fragilis, Bacteroides ovatus, Bacteroides thetaiotaomicron, Bacteroides uniformis, Bacteroides vulgatus, Parabacteroides distasonis³².

Technology for the obtention of Eravacycline

As other members of tetracycline family, it has been demonstrated that eravacycline is a powerful inhibitor based on the bacterial ribosome mechanism³³; it has modifications in the positions C-7 (fluoride) and C-9 2- (pyrrolidine-1-yl) ethanamide in the tetracycline nucleus which were possible by the use of an entirely synthetic route³⁴. (Figure 5)

Source: Magnus Ronn*, Zhijian Zhu, Philip C. Process R&D of Eravacycline: The First Fully Synthetic Fluorocycline in Clinical Development. Organic Process Research & Development 2013 17⁵, 838-845³⁵

Figura 5 Eravacycline chemical structure 

Two additional experiments were made to the third generation tetracycline molecule, with C9 substitutions in ring D and as a result it is obtained the eravacycline molecule, it was shown a wide spectrum activity against sensitive bacterial strains resistant to tetracycline³⁶; the studies with radiolabelled tetracycline indicate that Eravacycline has a merger ten times higher for the ribosome tan for tetracycline, and inhibits in vitro translation to concentrations four times lower than tetracycline³⁷.

Eravacycline action mechanism

As other tetracyclines, they inhibit the elongation phase of the proteins synthesis joining together to the 30S ribosomal subunit of bacteria specifically 16S rRNA and blocking the merger of aminoacil ARNt to the acceptor site³⁸, it means, Mrna joins together with the bacterial 30S ribosomal subunit³⁹.

P-site (peptidyl) of the RNA 50S subunit contains the rising polypeptide chain; in normal conditions, eaminoacil tARN is charged with the next aminoacid (aa) which is added, moves to A-site (acceptor), with complementary bases pairing between the ARNt anticodon sequence and the mRNA codon⁴⁰, eravacycline joins together with 30S subunit, blocking the merger of ARNt with the A-site, therefore it inhibits proteins synthesis⁴¹; this is a bacteriostatic antibiotic, since the interaction between tetracyclines and ribosomes is reversible⁴². (Figure 6)

Tetracyclines generally enter to bacterial Gram negative cells through the porins of the external membrane by means of passive spreading or active transportation, this last one requires TFA and magnesium for the active uptake⁴³, however, it maintains the activity when there is the presence of common resistance specific tetracycline mechanisms acquired, it means, four known mechanisms to confer specific resistance to tetracycline as flow pumps, ribosomal protection proteins (RPP), drugs deterioration, and rRNA mutations; of this mechanisms, the efflux pumps and the RPP are the most common⁴⁴; some Gram negative bacteria species demonstrate an inherent resistance to tetracycline due to specific lipopolysaccaride components in their external membranes. Specific tetracycline efflux bombs can be found in the cell membranes of Gram positive and Gram negative bacteria⁴⁵.

Source: Inhibidores de la síntesis de proteína y antibacterianos diversos, Hilal-Dandan R, Brunton LL. Goodman & Gilman. Manual de farmacología y terapéutica, 2e; 2015

Figure 6 Eravacycline action mechanism model. 

This synthetic process is constantly improving and discovering new tetracyclines with enhanced antimicrobial profiles related to previous generations⁴⁶; Nowadays several candidates are in different phases of development using Eravacycline as main compound, a wide spectrum antibiotic for serious diseases⁴⁷.

Currently they are being proposed some antibiotics finding platforms in order to be studied, which follow directly the old practices and can be improved by means of the application of modern tools of validation⁴⁸, it will be needed the development of new data tools based on biology, which is totally feasible taking into account what is currently known⁴⁹. The establishment of rules implied in the penetration of molecules in the bacterial wrap will allow that high technology platforms be applied effectively⁵⁰.

Conclusions

Thanks to the researches that have been made looking for an effective therapy against bacteria, Scientist have had to look for new alternatives for amplifying the therapeutic arsenal and obtaining new molecules that replace the ones that do not work anymore.

The search of new treatments against pathogenic bacteria proposes new strategies, as the development of methods to cultivate and stimulate the growth of this organisms by means of in situ culture, using specific growth factors in chambers that allow the spreading by using electronic chips and the isolation of the chemical compounds with an antibiotic reach, taking into account that currently this new technique is a source not so explored.

Researchers are making modifications to the action mechanisms, to their chemical and molecular compounds, of the antibiotics previously used, transforming them in synthetic molecules for therapeutic use.

The synthesis of new antibiotics has been tested by the FDA (Food and Drug Administration), a fact that has favored to decrease the adverse effects. There are currently new antimicrobials as omadacycline for bacterial pneumonia and acute skin diseases, arikayce indicated for the treatment of pulmonary diseases caused by Mycobacterium avium complex, plazomicin with activity against Gram negative pathogens, including enterobacteria resistant to carbapenems and and tedizolid which indication is the treatment of complicated sin infections and its soft structures.