This free antibiotic reference table covers 22 common laboratory antibiotics used in molecular biology, microbiology, and cell biology research. Instantly look up working concentrations, stock preparation protocols, solvent requirements, storage conditions, and mechanism of action — all in one searchable, filterable reference. Designed for graduate researchers, lab technicians, and scientists who need accurate, quick-access data at the bench.
| Antibiotic | Class | Working Conc. | Stock Conc. | Solvent | Storage (Stock) | Target Organisms |
|---|
How to Use the Antibiotic Reference Table
Step-by-Step Instructions
Search: Type an antibiotic name (e.g. Ampicillin), abbreviation (e.g. Kan), class name (e.g. Aminoglycoside), or solvent keyword (e.g. water, DMSO, ethanol) into the search box to filter the table in real time. The search covers all fields including target organisms.
Filter by type or solvent: Use the filter buttons to narrow the table to bacterial antibiotics only, antifungal agents, or to antibiotics grouped by their stock preparation solvent (water-soluble, ethanol-based, or DMSO-based). This is especially useful when planning experiments with solvent-sensitive cell lines.
Click any row to expand the full detail panel below the table. The detail panel shows all concentration parameters, dilution factors, stock and working storage conditions, mechanism of action, step-by-step stock preparation instructions, and important usage notes for that antibiotic.
Understanding the Columns
The Working Concentration column shows the final concentration in media at which the antibiotic is active for selection or inhibition. The Stock Concentration shows the recommended preparation concentration (typically 500× to 1000× the working concentration) to minimise solvent carry-over. The Dilution Factor is the ratio between stock and working concentrations — use this to verify your dilution calculation. Solvent specifies the solvent used to prepare the stock solution before adding to aqueous media. Storage columns show recommended conditions separately for the concentrated stock and the diluted working solution.
Stock Solution Preparation Formula
To calculate how much stock solution to add to achieve a target working concentration, use the C₁V₁ = C₂V₂ equation rearranged as:
V(stock) = [Working conc. × Total media volume] / Stock conc.
// Example: Kanamycin 50 µg/mL in 1 L LB broth
Stock = 50 mg/mL (1000×)
V(stock) = 50 µg/mL × 1000 mL / 50,000 µg/mL = 1.0 mL stock
// Always add antibiotics AFTER autoclaved media cools to ≤55°C
This equation assumes the volumes are expressed in the same units. When in doubt, convert all concentrations to µg/mL before calculating to keep the arithmetic consistent.
When to Use This Reference
This table is most useful in the following laboratory scenarios: preparing selective LB or SOB agar plates for bacterial transformation; formulating liquid broth for plasmid-bearing culture; setting up dual-selection experiments with two antibiotics; verifying correct concentrations when troubleshooting poor transformation efficiency; and preparing antibiotic stocks from powder for a new batch. It is also a useful quick reference when checking whether a new plasmid's resistance marker is compatible with your existing antibiotic stocks.
Common Mistakes to Avoid
Adding antibiotics to hot media: Many researchers add antibiotic stocks to agar immediately after autoclaving. Temperatures above 60°C rapidly degrade β-lactams (ampicillin, carbenicillin) and most other antibiotics. Always allow media to cool to 55°C or below — test with a wrist or thermometer — before adding antibiotics.
Using degraded ampicillin plates: Ampicillin is hydrolysed by β-lactamase secreted by resistant colonies, creating an antibiotic-depleted zone around each colony. Satellite colonies are non-transformants growing in this zone. Use plates within 24–48 hours of pouring, and consider switching to carbenicillin for more reliable selection.
Incorrect stock concentration: A common error is preparing a 10 mg/mL stock when a 100 mg/mL stock is needed, then adding 10× too much volume to media. Always label stock tubes clearly with the concentration in mg/mL and the dilution factor, and verify the expected working concentration before use.
Freeze–thaw cycling: Repeated freeze–thaw cycles degrade antibiotic activity, particularly for tetracyclines and macrolides. Aliquot stocks into single-use volumes before freezing to avoid repeated thawing of the same tube.
Interpreting Your Results
The working concentration values listed here represent empirically established ranges for standard bacterial strains and commonly used plasmid systems. For organisms with naturally elevated MICs, you may need to use concentrations towards the higher end of the listed range. If you observe satellite colonies or poor selection, first verify that your antibiotic stock is fresh and has been stored correctly, then consider increasing the working concentration by 10–25%. Conversely, if you see unexpected growth inhibition in non-resistant cultures, verify that your dilution factor is correct and that solvent carryover (particularly DMSO) is within acceptable limits.
Resistance Markers in Plasmids
Common plasmid resistance markers and their typical selection concentrations: AmpR (Ampicillin 100 µg/mL), KanR (Kanamycin 50 µg/mL), CmR (Chloramphenicol 34 µg/mL), TetR (Tetracycline 10–15 µg/mL), SpecR (Spectinomycin 100 µg/mL), GentR (Gentamicin 10–15 µg/mL), ZeoR (Zeocin 25–100 µg/mL), HygR (Hygromycin B 50–200 µg/mL depending on organism), BlastR (Blasticidin 2–10 µg/mL for mammalian cells). Always confirm the specific resistance marker encoded by your plasmid in the vector map before setting up selection.
Frequently Asked Questions
What is the standard working concentration for ampicillin in E. coli selection?
The standard working concentration for ampicillin in E. coli selection media is 100 µg/mL, using a 100 mg/mL (1000×) stock solution dissolved in ultrapure water. This concentration reliably selects for the AmpR resistance marker carried by many common plasmids. Ampicillin degrades rapidly in liquid media at 37°C — plates older than 24–48 hours may produce satellite colonies around genuine transformants. For more stable selection, carbenicillin at 100 µg/mL is a direct substitute, as it is more resistant to enzymatic hydrolysis. Always filter-sterilize ampicillin stocks; do not autoclave.
Which antibiotics must be dissolved in DMSO rather than water?
Several common laboratory antibiotics are poorly water-soluble and require DMSO as the primary solvent. These include chloramphenicol (can also use 100% ethanol), amphotericin B, nystatin (can also use methanol), and some formulations of cycloheximide. When working with DMSO-based stocks, ensure the final DMSO concentration in your culture medium stays below 0.1–0.5% to avoid cytotoxic effects on sensitive cells. Always filter-sterilize DMSO stocks through a 0.22 µm PVDF or nylon membrane — do not use cellulose acetate filters with DMSO.
How do I calculate the volume of antibiotic stock solution to add to media?
Use the dilution formula: V(stock) = (Working concentration × Total volume) / Stock concentration. For example, to prepare 500 mL of LB broth with 50 µg/mL kanamycin using a 50 mg/mL stock: V(stock) = (50 µg/mL × 500 mL) / 50,000 µg/mL = 0.5 mL. Add this volume to cooled media (≤55°C) and mix gently. For agar plates, pour immediately after mixing to prevent the antibiotic from settling unevenly as the agar solidifies.
Why do some antibiotics require storage at −20°C while others can be kept at 4°C?
Storage temperature depends on each antibiotic's chemical stability and susceptibility to degradation. β-Lactams (ampicillin, carbenicillin) degrade via hydrolysis of the β-lactam ring and must be stored at −20°C. Tetracyclines and macrolides are light-sensitive and heat-labile, also requiring −20°C storage protected from light. More stable antibiotics such as kanamycin sulfate and gentamicin in water can be stored at 4°C for several weeks, but long-term storage at −20°C is still recommended. Aliquot stocks to avoid repeated freeze–thaw cycles regardless of storage temperature.
What is the difference between bacteriostatic and bactericidal antibiotics in lab use?
Bacteriostatic antibiotics inhibit bacterial growth without killing cells, while bactericidal antibiotics actively kill bacteria. In laboratory selection, bactericidal agents (ampicillin, kanamycin, gentamicin) kill non-resistant cells outright, giving cleaner selections in dense cultures. Bacteriostatic agents (chloramphenicol, tetracycline, erythromycin) suppress growth but may allow background colonies in high-density plating. For plasmid maintenance in long-term liquid culture, bactericidal antibiotics are generally preferred. Note that the bacteriostatic/bactericidal distinction can be concentration-dependent — some agents become bactericidal at very high concentrations.