Antimicrobial activity of Serratia marcescens endosymbionts of Rhabditis nematodes against selected antibiotic resistant bacteria
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Serratia marcescens is a gram-negative non-sporulating gamma proteobacteria of the family Enterobacteriaceae. It is a symbiont of the Oscheius nematodes. The bacterium has shown capacity to kill insect pests, thus the need to evaluate its efficacy against antibiotic tolerant bacterial pathogens. Serratia marcescens bacteria were isolated from hemolymph of previously nematode-infected Galleria mellonella larvae. Cultures were made by streaking on selective media then incubated at 28°C for 48hrs. Antimicrobial bioassays were then performed on Staphylococcus aureus, Methicillin-resistant S. aureus (MRSA), Escherichia coli, Bacillus cereus and Pseudomonas aeruginosa. Genomic DNA was isolated using the Cetyl Trimethyl Ammonium Bromide method followed by amplification of the 16S rRNA gene using B27F and 1492R primers. The two strains scored 99.26% and 98.22% identity to Serratia marcescens strains B195 and RS, respectively. No significant differences were observed between the two isolates with respect to antimicrobial efficacy towards the test pathogens. In general, the least susceptible pathogen was P. aeruginosa with a mean diameter of 13.00 mm followed by MRSA (14.83 mm), S. aureus (15.92 mm), E. coli (21.09 mm) and B. cereus (26.92 mm). Serratia marcescens should be exploited in the management of antibiotic resistant human pathogens.
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This study appears to be preliminary results only. It could be considered for publication if some further work is carried out. At the moment it really just is one experiment that shows this is worth studying further. I recommend rewriting the introduction to focus on what you are actually looking at, so a focus on AMR and Serratia and shorten the parts on the ecology and background info of the nematodes which is not relevant for what the results relate to. It is very convoluted at the moment and hard for the reader to decipher what is important. The statistical analysis seems very complicated for a simple assay, I am also not sure I understand some of the comparisons- why would you compare different pathogens with each other in terms of their reaction to the extracts? I cannot see what this comparison would tell you. At least some …
This study appears to be preliminary results only. It could be considered for publication if some further work is carried out. At the moment it really just is one experiment that shows this is worth studying further. I recommend rewriting the introduction to focus on what you are actually looking at, so a focus on AMR and Serratia and shorten the parts on the ecology and background info of the nematodes which is not relevant for what the results relate to. It is very convoluted at the moment and hard for the reader to decipher what is important. The statistical analysis seems very complicated for a simple assay, I am also not sure I understand some of the comparisons- why would you compare different pathogens with each other in terms of their reaction to the extracts? I cannot see what this comparison would tell you. At least some further experiments to try and characterise what the extracts are chemically, eg with HPLC or LC-MS would be necessary. Perhaps a preparative HPLC and then testing the different fractions to see which one might be responsible for the inhibition observed. Ideally WGS of the isolated strain as 16S only shows you the taxonomic identity and BGC content can be rather different even if from the same species. Individual Reviewer Comments to Author (Editor's Copy)
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Comments to Author
This manuscript isolates two bacterial strains from Galleria infected with nematodes. These nematodes are known to harbour symbiotic bacteria which potentially produce antibacterial compounds. The two isolated strains are identified as Serratia marcescens and the antimicrobial activity of their secreted supernatants is tested against 5 bacterial strains representing bacteria of pathogenic importance to humans. Major concerns: - The introduction is incredibly long winded and focuses more on the nematodes than on the bacteria this manuscript is studying. Whilst it is all interesting stuff, it is not relevant to the data you are presenting. The introduction should be completely paired down to basically just say interactions between nematodes and symbiotic bacteria exist, these bacterial symbionts are …
Comments to Author
This manuscript isolates two bacterial strains from Galleria infected with nematodes. These nematodes are known to harbour symbiotic bacteria which potentially produce antibacterial compounds. The two isolated strains are identified as Serratia marcescens and the antimicrobial activity of their secreted supernatants is tested against 5 bacterial strains representing bacteria of pathogenic importance to humans. Major concerns: - The introduction is incredibly long winded and focuses more on the nematodes than on the bacteria this manuscript is studying. Whilst it is all interesting stuff, it is not relevant to the data you are presenting. The introduction should be completely paired down to basically just say interactions between nematodes and symbiotic bacteria exist, these bacterial symbionts are suspected producers of antimicrobials (and why they are suspected to be good sources of antimicrobials) which could be useful in the fight against AMR pathogens, hence we isolated bacteria from nematodes and tested their efficacy against strains ABC. - I took a look at the data presented in the thesis which the manuscript is linked to. What is the rationale for only presenting the data for strains SM93B and SM93C, and excluding the antimicrobial potential of the other two strains you isolated (samples A and D in the thesis)? And for dropping the Candida albicans inhibition data? - The statistical analysis on the zones of inhibition seem massively over complicated for such simple data. I'm not convinced of the merit of comparing the significance of differences between zones at the different concentrations (Capital letters down the columns in Table 2). It is also a shame that the cell free media containing suspected antimicrobials wasn't diluted down until the inhibitory effect was lost. - In the discussion you compare the findings of this study to published work which show that S. marcescens produce antibacterial compounds. Without further characterisation of the antibacterial effects of your cell free suspensions, how do you know your results aren't mediated by the same compounds that have already been described? One line in the discussion states "Results of the present study demonstrate the inherent ability of S. marcescens strains to secrete various antimicrobial compounds" but the results really don't support that conclusion. The results show that you isolated two new strains of SM and they too produce something which has antimicrobial properties. To back up that statement you would want to identify what the compounds are, which genes encode that compound, and then look across many published genomes to see how conserved those genes are across the species. Overall, there isn't anything wrong with the data presented in this manuscript, but it requires a significant rewrite to focus on the bacteria and the problem of AMR rather than the nematodes themselves. Additionally, it feels a bit premature to be publishing the data from these experiments without the follow on work to identify and characterise the compounds responsible for the antibacterial effects. Small points: Figure 3 legend- Describe what well "P" is on the plates. I know it is described in the methods, but please add it to the figure legend as well. Figure 3- the arrangement of N, 100, 50, and P are different to that presented in the thesis. Which is the correct orientation and how does this effect the measurements of the zones of clearing? Line 287-"However, after 3 days of incubation, diameter of the inhibition zones reduced"- does this indicate that the compounds produced are bacterial static rather than cidal?
Please rate the manuscript for methodological rigour
Good
Please rate the quality of the presentation and structure of the manuscript
Poor
To what extent are the conclusions supported by the data?
Partially support
Do you have any concerns of possible image manipulation, plagiarism or any other unethical practices?
No
Is there a potential financial or other conflict of interest between yourself and the author(s)?
No
If this manuscript involves human and/or animal work, have the subjects been treated in an ethical manner and the authors complied with the appropriate guidelines?
Yes
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