Evaluation of antibiotic and peptide vaccine strategies for mirror bacterial infections

Reviewer #1 (Public review):

Summary:

The manuscript entitled "Evaluation of Antibiotic and Peptide Vaccine Strategies for Mirror Bacterial Infections" addresses a topic that is well established in the literature. The authors investigate the activity of enantiomeric (D-form) antibiotics against bacteria and the immunogenicity of D-form peptides, proposing that D-enantiomers are ineffective both as antibacterial agents and as vaccine candidates. While the subject matter is relevant, the concepts explored are already well known, and the manuscript offers limited novelty.

The authors demonstrate that D-enantiomeric antibiotics lack antibacterial activity compared to their naturally occurring L-forms and that D-form peptides fail to elicit detectable immune responses. These observations are consistent with existing knowledge regarding molecular chirality in biological systems. However, the manuscript relies on a limited experimental dataset while extrapolating the findings broadly, which weakens the strength of the conclusions.

Strengths:

The manuscript introduces the topic of Mirror Bacterial Infections, likely to occur if no regulations or restrictions are placed immediately.

The manuscript addresses a relevant topic and has potential value, particularly in framing discussions around chirality and pathogen interactions. With a more cautious interpretation of the results, the manuscript could better justify its conceptual framework and strengthen its contribution to the field.

Weaknesses:

(1) Several sections of the manuscript are overly descriptive and would benefit from deeper comparative analysis and critical synthesis. In multiple instances, the discussion relies on hypothetical scenarios supported primarily by selective citations rather than robust experimental evidence. The introduction of the term "mirror microbiology" or "mirror bacteria" appears largely conceptual and is used to unify what are essentially two separate lines of investigation, enantioselective antibiotic activity and peptide chirality in immune recognition, without sufficient mechanistic integration.

(2) To the best of this reviewer's understanding, the manuscript does not present substantial novelty. The pronounced differences in biological activity between L- and D-forms of small molecules and peptides are well documented, including their implications for antimicrobial efficacy and immune recognition. While the manuscript is written in clear and accessible language suitable for both specialists and interdisciplinary readers, novelty remains limited.

The manuscript reiterates well-established principles of stereochemistry and biological recognition. Given the extensive existing literature demonstrating that enantiomeric antibiotics are typically inactive due to stereospecific target interactions, the failure of D-form antibiotics is expected and does not constitute a novel finding.

(3) Critical experimental details are lacking, particularly regarding the peptide design. It is unclear whether the peptides were synthesized entirely in the D-configuration or whether only select amino acids were substituted. This distinction is essential for interpreting immunogenicity results and for comparison with prior studies.

(4) The authors conclude that D-form peptides are poorly recognized by the immune system. However, the data presented indicate that neither the L- nor the D-form peptides tested elicited a measurable immune response. Without demonstrating immunogenicity of the corresponding L-form peptides, the conclusion that immune non-recognition is specific to the D-form is not sufficiently supported.

Reviewer #2 (Public review):

This paper by Kleinman et al. tackles an increasingly discussed biosecurity scenario, namely the possibility that "mirror bacteria" could evade key elements of host immunity and therefore demand bespoke medical countermeasures. The authors experimentally probe two such countermeasure concepts: (1) whether existing chiral antibiotics might still work against mirror bacteria (this is tested indirectly by measuring the activity of antibiotic enantiomers against natural-chirality bacteria), and (2) whether D-peptide antigens can be made immunogenic. Briefly, the authors show that enantiomers of four approved antibiotics have little to no activity in MIC assays, argue this implies the parent drugs would likely fail against mirror bacteria, report limited single-dose tolerability data for the enantiomers in mice, and show that selected bacterially derived D-peptides can elicit strong binding antibody titers when conjugated to a carrier protein and given with adjuvant.

Overall, the study is quite interesting but constrained by the fact that D-peptide immunogens and related ideas have been explored for decades, by prior literature showing that D-enantiomeric peptides can themselves be strongly antimicrobial vs conventional bacteria, and by a number of conceptual and experimental limitations outlined below.

(1) A blanket statement indicating that flipping chirality makes antibiotics ineffective cannot be true across all classes. Indeed, there is extensive precedent for "mirror" (D-amino-acid) peptides that retain, or even improve, antimicrobial activity against natural bacteria.

(2) The paper's key claim ("parent antibiotics won't work on mirror bacteria") is based on the observation that the enantiomers of chloramphenicol/linezolid/tedizolid/aztreonam largely lose activity against natural bacteria. This is a reasonable proxy experiment given the absence of mirror organisms, but it remains an inference and should be described as such.

(3) The chiral purity needs to be documented more rigorously. The methods mention structural confirmation by NMR and >95% purity by LC-MS/HPLC for enantiomeric compounds, but this is not the same as demonstrating high enantiomeric excess or excluding low-level contamination by the active parent enantiomer.

(4) The residual activity of ent-aztreonam is quite interesting. The authors report slight activity for ent-aztreonam (MIC of 32-128 µg/mL in a subset), still far weaker than aztreonam but nonzero.

(5) For antibiotics, MIC is a starting point, but further experiments are needed. To justify countermeasure relevance, it would help to include at least one additional pharmacodynamic readout (time-kill kinetics, post-antibiotic effect, inoculum effect, or activity in the presence of human serum).

(6) The acute toxicity study is limited (single-dose, short follow-up, small n, one sex/strain, and no histopathology).

(7) The Discussion leans on human equivalent dosing logic to reassure feasibility. Given the lack of PK, bioavailability, metabolism, and repeat-dose data, these comparisons risk overreach.

(8) The readout is ELISA endpoint binding (IgG; and IgA in BALF for one antigen), which is fine for an initial immunogenicity screen. But the manuscript then drifts toward "vaccine strategy" claims without showing any antibody functionality (opsonophagocytosis, complement deposition, neutralization, blocking adhesion, and so on) or even binding to a more native-like antigen format (e.g., D-peptide displayed on particles; D-protein fragments; or any surrogate that goes beyond plate-bound peptide).

(9) The methods report peptide conjugates containing ~10-200 EU/mL endotoxin. That is not trivial and could materially amplify immunogenicity, and should be discussed.

(10) The authors should report how many technical/biological replicates were performed for MIC determinations and for ELISAs.

Reviewer #3 (Public review):

Summary:

There is a threat of mirror life bacteria, which could possibly evade immunity and cause problems for human/animal hosts. This paper evaluates enantiomeric antibiotics and vaccines as a means to understand how this could be combatted in the future.

Strengths:

It is valuable to collect such information, as it is not always clear how an antibiotic in its enantiomeric form would interact with a bacterium in terms of its MIC or towards toxicity. The paper is scientifically sound with regard to assays and statistical methods.

Weaknesses:

The beginning of the paper could be described as hyperbolic. For a paper that demonstrates that mirror-image molecules have (expected) lower MICs and toxicity, some of the claims in the beginning that they are going to cause a pandemic of evading the immune system seem to be a bit overstated. If they are mirror images, how are these bacteria going to generate virulence factors or mediate pathogenesis mechanisms? It seems like the lack of adaptation would go both ways - supported by the empirical data gathered in this manuscript. There is also the issue of only relatively simple and accessible mirror-image antibiotics being available. This is a limitation that - to their credit - the authors do discuss in the discussion section.