Four vendors sell a ready-made antibacterial screening library, and they split cleanly into two kinds of product. Enamine (30,000 compounds) and Life Chemicals (over 18,000) sell diversity selected for the target class, which is what you buy when you want new chemistry. MedChemExpress (1,994) and TargetMol (1,600) sell compounds with already validated antibacterial activity, which is what you buy for repurposing, combination work or a fast positive control set. Picking the wrong kind is the expensive mistake, and it happens because the catalog pages all use the word library for both.
What follows is a comparison of the four on the things that change a decision: how the set was selected, what it contains, the Gram-negative question, and what you should get in writing before a purchase order goes out. Sizes and selection methods come from each vendor's own catalog page.
The four antibacterial screening libraries, side by side
| Library | Compounds | Type | Selection method | Best for |
|---|---|---|---|---|
| Enamine Antibacterial Library | 30,000 | Diversity, target class focused | Knowledge-based: privileged cores, motifs and natural product-like scaffolds, refined on molecular parameters | A genuine hit-finding campaign, with hit resupply and analogs from a large stock behind it |
| Life Chemicals Antibacterial Libraries | 18,000+ | Diversity, two non-overlapping sets | Ligand-based 2D fingerprint similarity (Tanimoto above 0.80) plus structure-based docking against bacterial targets | Teams that want the selection logic itself documented, and a target-directed subset |
| MedChemExpress Anti-bacterial Compound Library | 1,994 | Bioactive, validated actives | Curated collection of compounds with already validated antibacterial activity | Repurposing, combination screening, and a ready set of positive controls |
| TargetMol Anti-bacterial Compound Library | 1,600 | Bioactive, validated actives | Antibiotics and antibacterial bioactives with unique structures, NMR and HPLC or LCMS validated | Smaller repurposing screens where compound purity documentation matters |
Where each one wins is worth stating plainly. Enamine wins on scale and on what happens after a hit, because resupply and close analogs from an existing stock are the difference between a hit and a series. Life Chemicals wins on transparency of selection: the similarity threshold, the activity cutoff and the target list are all published, which makes the set auditable in a way a knowledge-based pick is not. MedChemExpress wins on breadth of validated actives for the money and on format flexibility. TargetMol wins on being the smallest sensible entry point with per-compound analytical data attached.
Diversity set or bioactive set: the question to settle first
A bioactive library of 1,600 to 2,000 known antibacterials will produce hits. That is the point and also the trap, because the hits are mostly compounds whose activity was already in the literature. That is exactly what you want for repurposing, for combination screening against a resistant isolate, or for building a control set that proves your assay can see what it should see. It is not what you want if the program needs composition of matter nobody has filed on.
A diversity set of 18,000 to 30,000 is the other bet: lower hit rate, but hits that can become a series. The practical cost is that the deck is far larger than most plate budgets, which is where the next section starts.
The Gram-negative problem no library solves by itself
A general antibacterial deck screened against a Gram-negative panel typically returns very little, and the reason is not potency. Compounds fail to accumulate: the outer membrane admits small, polar molecules through porins, and RND efflux pumps such as AcrAB-TolC remove much of what does get in. A compound can inhibit the isolated enzyme beautifully and never reach it in a whole cell. The mechanics of that are covered in more detail in the reference on why antibiotics fail on Gram-negative bacteria, and it is the single biggest reason antibacterial hit rates look bad next to other therapeutic areas.
Two vendors respond to this directly. Enamine offers a dedicated Gram-negative sub-library of 6,400 compounds across 80 plates, selected for accumulation-friendly properties rather than for potency alone. Life Chemicals offers a targeted set of about 2,700 compounds picked by docking against specific targets in pneumonia-causing organisms, with 1,673 of those against metallo-beta-lactamase. If your program is Gram-negative, buying the sub-library instead of the full deck is usually the better first purchase, and it is a smaller order to defend internally.
What none of them publish, and what to ask for instead
Not one of the four lists a price. Every purchase is a quote cycle, which is normal in this market and still costs a small team weeks. When the quote arrives, the line items that actually move the total are rarely the ones on the first page:
- Format and volume. 1536-well Echo LDV at 150 nL, 384-well at 10 or 50 µL, 96-well at 50 µL, powder. The cheapest unit price often comes in a format your liquid handler cannot use.
- Concentration. 10 mM in DMSO is the default, but compounds with poor solubility ship at 2 mM or as mg/mL, which quietly splits your plate map across more plates than you budgeted.
- Resupply and analogs. What a hit costs to re-order in gram quantities, and how many close analogs exist in stock. This is worth more than a discount on the deck itself.
- Exclusivity and publication terms. Whether anyone else is screening the same set against the same organisms, and what you may publish.
- Shipping, stability and storage. DMSO plates are hygroscopic. Freeze-thaw cycles cost you potency, and the failure shows up as an assay problem months later.
Budget for the campaign rather than the catalog. The library is usually the smaller number next to plates, media, strains, analyst time and the repeat runs a real campaign needs. If the computational side of the cascade runs in your own cloud account, that bill deserves the same scrutiny, and keeping read-only eyes on cloud and software spend tends to find more money than negotiating the compound order. The four ways this market bills, and what each model hides, are broken down in the antibacterial screening software pricing write-up.
How many of these compounds can you actually plate?
This is the number that decides whether the library was a good purchase. Broth microdilution needs a doubling dilution series per compound per strain. A 6 strain panel at ten concentrations is about 60 wells per compound, so a 384-well plate carries roughly 6 compounds once controls have their share. A 30,000 compound deck is therefore in the region of 5,000 plates of full-panel MIC work, and an 18,000 compound deck about 3,000.
Almost nobody runs that. What happens instead is that the deck gets cut, and the only question is by what. Physicochemical filters cut too bluntly and throw away the accumulation-friendly polar chemistry that Gram-negative work needs. A chemist's intuition cuts well and leaves no written reason behind an exclusion, which becomes a problem the moment a reviewer or an investor asks why those 900 compounds never got a plate. The third option is to rank the whole file computationally and plate the top of the ranking, which is what compound library screening software is for: every compound comes back with a predicted MIC band per strain, an S, I or R call and the mechanism expected to limit it, at about $0.15 per compound on the Program plan.
None of that replaces the assay. It changes which compounds reach it, which is the only lever most teams have on a fixed plate budget. How the assay side is built, and why its hit rates behave the way they do, is covered in the high throughput screening reference.
How many compounds are typically screened in the discovery phase?
For antibacterials, anywhere from a focused set of around 2,000 bioactive compounds to whole-deck campaigns of hundreds of thousands. The commercial antibacterial libraries land between 1,600 and 30,000, which is a deliberate middle: large enough for novel chemistry, small enough that a mid-sized team can contemplate it. The binding number is almost never the library size, it is the assay capacity, and that is usually one or two orders of magnitude smaller.
Is virtual screening better than high throughput screening?
They answer different questions and belong in sequence rather than in competition. High throughput screening measures, and nothing computational replaces a measurement, but it spends plates, compound and analyst time. Virtual screening ranks: cheap, imperfect, and the right tool for deciding what goes on the plate. Teams that treat prediction as the filter and the assay as the confirmation get more out of the same budget than teams that argue about which one is correct.
Which antibacterial compound library should you buy?
If the program needs new chemistry and has resupply ambitions, Enamine's 30,000 compound set, or its 6,400 compound Gram-negative sub-library if the panel is Gram-negative. If you want the selection criteria documented and auditable, Life Chemicals. If the goal is repurposing, combination work or a control set, MedChemExpress at 1,994 compounds or TargetMol at 1,600, choosing between them on format and on the analytical documentation you need.
Whichever arrives, the plate budget will not have grown to match it. Deciding what gets screened first is the part that determines whether the purchase pays back, and it is worth doing with something more defensible than a sort by molecular weight. Run one structure through the panel above to see what the ranking returns per compound, then send the file.