Article Review
Synergistic effect of human plasma and antimicrobial peptidomimetics and antibiotics on pathogenic bacteria: The Application is a promising tool in enhancing the efficacy of future antibiotics.
This work aims at summarizing and critiquing the Research in Microbiology journal research paper titled ‘Improved In vitro evaluation of novel antimicrobials: potential synergy between human plasma and antibacterial peptidomimetics, AMPs and antibiotics against human pathogenic bacteria’ .
Abstract
Condisering the hurdles of cytotoxicity and adverse side effects posed by existing antibiotics and the ever rising resistance of microorganisms to these compounds, there is a dire need for development of innovative antimicrobial peptidomimetics (AMPs) . Despite several efforts, researchers have gained little success in developing antibiotics with reduced toxicity and side effects and that mirror the properties similar to physiological conditions. In an attempt to find such novel compunds, several antimicrobial peptidomimetics have been screened for their antibacterial effect. Recent literature points to the usefulness of human plasma in enhancing the antibacterial effect of certain AMPs against bacteria such as E. coli . The goal of this particular study was to extend this investigation to a wide range of pathogenic strains. The findings demonstrate a clear enhancement of this potentiation effect and open avenues for employing human plasma in combination with AMP’s as a promising therapeutic strategy against pathogenic bacteria.
The orgininal title is coherent and self explanatory, although it lacks the message of an important message regarding the application of human plasma and its synergistic effect with antimicrobial peptidomeimetics in therapeutic strategies towards a gamut of pathogenic bacteria. The modified title is not only encompassive but also effectively highlights the application of this piece of reasearch in medical therapeutic strategies used to improvize the efficacy of antibacterial antibiotics.
Introduction
Increasing resistance of microorganisms like bacteria and viruses to antibiotics has become a serious challenge in recent years . This has necessitated the invention of innovative antimicrobial compounds to combat infections caused by these organisms. Antimicrobial peptides (AMP’s) are molecules isolated from a gamut of organism that include bacteria and humans . These are cationic compounds that bind to cell membranes and induce cell lysis and death.
Several strategic research methods have been employed to develop novel AMP’s that lack shortcoming such as their adverse side effects of cytotoxicity and hemolysis. Methods such as production of stable artificial forms of AMP’s have been used to decrease the vulnerability of AMP’s to degradation by proteases . Structural alterations may permit optimization of antibacterial effect to toxicity ratio towards human cells and therefore facilitate the therapeutic index . Despite the discovery of a myriad antibacterial compounds, researchers have encountered challenges in catapulting these compounds from pre-clinical to clinical trial phases . Several studies have recognized the need to establish laboratory systems to replicate in vivo milieu in the search for more efficient candidates for clinical trials . Antimicrobial compounds extracted from human platelets preserve their physiological effect in presence of plasma. There is also evidence of conservation of antimicrobial activity of AMP rich in arginine when treated with serum and plasma . These results emphasize the benefits of artificial analogs over natural AMPs (for instance, increased proteolytic activity in vivo), but also highlight the need to involve laboratory investigations replicating physiological milieu.
A recent study shows that the occurrence of human plasma unanticipatedly enhanced the activity of a peptide/b peptoid peptidomimetics to inhibit E. Coli . While this study focused on E. coli, it did not examine whether the effect could be extrapolated to other pathogens. Hence, as an extension of this study, the current study sought out to investigate the antimicrobial influence of peptidomimetics on a wide range of organisms . Since plasma showed to enhance peptidomimetics against all bacteria examined, they hypothesized that this effect may be attributed to the contribution of complement system.
The findings of this study demonstrate that the antibacterial action of two compounds against many Gram-positive and Gram-negative pathogens is facilitated by the presence of human plasma and serum.
All the information necessary to reproduce the date have been mentioned including the growth conditions and manufacturer’s information. References for all the strains utilized have been quoted in the supplementary data table to avoid too much information in the actual paper.
There is adequate background regarding the structure and chemistry of the peptidomimetics 1 and 2 utilized for the experiments to orient the reader in the right direction. The information regarding replicate trials and specific procedural precautions undertaken to prevent errors is meticulously presented. The data pertaining the various cultures and treatment in different conditions is detailed and informative.
Electron microscopy has been employed as a very efficient method to investigate the action of antimicrobial proteins on bacterial cell envelope and the authors have effectively utilized scanning electron microscopy to visualize and record the membrane disrupting effect of the peptidomimetics.
Results
The researchers analyzed MIC and MBC’s of peptidomimetic compound 1 in presence of 25 % plasma using plates of various materials. Heat treatment of plasma and serum to decrease the intrinsic action of complement system. Compared to the untreated plasma, a substantial rise in the MIC was observed for all species in presence of 12.5% heat treated plasma. Addition of 25 % plasma resulted in an approximate 10-fold decrease in MIC of peptidomimetic polymyxin B against P. aeruginosa and E. coli and a 4-fold decrease in S. aureus. The data pertaining MIC and MBC values are clearly presented in a tabular form.
The peptidomimetic 1 showed a significant bactericidal action on E. coli, as seen from the reduced cell counts post incubation for 2-3 hours in a dose-dependent fashion. The graphs showing cell counts at various time points are lucidly represented with appropriate standard deviations (error bars) of the averages, making the data statistically relevant.
Considering that several AMPs and peptidomimetic compounds exert their action on the bacterial cell envelop, the authors anticipated a detrimental effect of human plasma on the envelope, consequently leading to the potentiation with a cumulative effect. In contrast to this expectation, they observed no cell damage of E. coli in presence of 25 % human plasma. In fact, increasing amounts of the peptidomimetic 1 causes variable extents of damage to the cell envelope, in the form of blebbing and pore formation in the apical region of the cells. Images from the scanning electron microscopy showing membrane perturbing effect of the peptidomimetics in presence of plasma are clear and unambiguous. The authors have also used appropriate control treatments with absence of either plasma or the specific compound or presence of plasma alone.
Discussion
On the whole, the study highlights that the activity of peptidomimetics towards a gamut of Gram positive and Gram negative bacteria is enhanced by the occurrence of human plasma. Most of the findings are in line with previous observations of identical studies and provide additional evidence to underscore the effect of human plasma on the potentiation action of peptidomimics on a wide range of bacteria.
While the results of this study have insinuated the involvement of complement proteins, they have eliminated the participation of individual complement proteins C3, Factor H and Factor I, by using serum deficient in these factors. It would, therefore, be worthwhile to conduct future analyses involving pure fractions of complement and coagulation proteins to identify specific contributors to the potentiation action .
Moreover, the researchers speculate a combined action of coagulation factors and complementary proteins on potentiation, from the fact that plasma showed a more pronounced effect relative to serum. These results are in line with earlier studies that have demonstrated a combined action of complement proteins and antimicrobial compounds, for example the action of polymyxin B being enhanced by serum . While this is a logical interpretation, the assumption entails further validation by future experimentation.
A significant finding of this study was a 2-16-fold decrease in the MIC values of thirteen bacteria with human plasma in the environment, without any influence on the cell membrane. These settings mirror appropriate in vivo conditions for clinical trials. They also demonstrated the requirement of minute concentrations of peptidomimics or antibiotics for these trials, which would be much lower than conventionally calculated from MICs detected in laboratory media. This is a critical finding that allows low concentrations of antibiotics and prevents the adverse reactions such as toxicity and other side effects caused by them.
While the data obtained from this research analysis is significant, it entails additional experiments to investigate into the exact mechanism though which human plasma influences the potentiation effect . Identification of the specific complement proteins involved and their exact roles is important to replicate the experiments in vivo and translate them into clinical settings. Although, the researchers have shown that the complement proteins do not independently affect the potentiation action, they need to perform more meticulous analyses with purified proteins to corroborate these findings. A long-term yet significant objective would be to employ the low concentrations of antibiotics for treatment strategies of bacterial infections and obviate the characteristic adverse effects and toxic reactions caused by usage of high doses. The authors have reinforced their assumptions and hypotheses with several relevant articles and publications to emphasize the importance of their findings and investigations.
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