In short
Zone of inhibition tells you whether something inhibits a microbe. MIC tells you the lowest concentration that inhibits it. MBC tells you the lowest concentration that kills it. For product claims, patents and serious R&D you almost always want MIC and MBC by broth microdilution under CLSI methodology, because those give a number that can be compared. Manna Biotech runs all three in its Hyderabad laboratory for research and development purposes.
The three tests, and what each one actually answers
| Test | What it measures | Output | Best used for |
|---|---|---|---|
| Zone of inhibition (agar diffusion) | Whether the sample inhibits growth on a plate | Diameter in mm | Fast screening; ranking many extracts |
| MIC (minimum inhibitory concentration) | Lowest concentration that stops visible growth | µg/mL or mg/mL | Potency comparison; dose-finding; patent and publication data |
| MBC (minimum bactericidal concentration) | Lowest concentration that kills 99.9% of the inoculum | µg/mL or mg/mL | Distinguishing killing from mere inhibition; formulation claims |
Why MIC and MBC matter more than a plate photograph
A zone photograph is persuasive to the eye but weak as evidence: zone size depends on how the compound diffuses through agar, not only on how potent it is. Two extracts with identical potency can produce very different zones. Broth microdilution removes that variable and yields a concentration, a number that can be compared against a control, a competitor, or a known drug.
That is why patent examiners, journal reviewers and regulatory reviewers weight MIC and MBC data far more heavily. If the result has to persuade someone, get the number.
What CLSI methodology means, and why it matters
CLSI, the Clinical and Laboratory Standards Institute, publishes the reference methods most laboratories follow for antimicrobial testing, notably M07 for broth dilution and M100 for interpretation. Working to CLSI methodology means the inoculum, medium, incubation and reading are standardised, so a result from one competent laboratory can be compared with another.
When you commission testing, ask which method the laboratory follows. A result generated by an ad-hoc protocol may be scientifically fine but is much harder to defend or compare.
Who needs this testing, and for what
- Pharma and generic formulators: potency of a new formulation versus the reference product; comparative data for Section 3(d) patent objections.
- Herbal, AYUSH and nutraceutical companies: antibacterial or antifungal activity of an extract or finished product to support an R&D claim or a publication.
- Probiotic and fermentation companies: antagonistic activity of a strain against pathogens; strain characterisation.
- Aquaculture farms and feed producers: activity of a treatment or additive against relevant pathogens; susceptibility of isolates.
- Academic researchers and PhD scholars: the MIC/MBC dataset a reviewer will expect in an antimicrobial paper.
Genotype and phenotype together: the stronger evidence
Antibiotic susceptibility testing tells you how a microbe behaves. AMR genomics tells you which resistance genes it carries. They do not always agree: an isolate can carry a resistance gene that is not expressed, or show resistance through a mechanism no single gene predicts. Reporting both, cross-confirmed, is markedly stronger than either alone, and it is one of the things a laboratory that does both molecular biology and classical microbiology can offer that a single-discipline laboratory cannot.
What to send, and what you get back
- The sample (extract, formulation, compound or isolate) with what it is and how it was prepared.
- The organisms you care about, or let the laboratory recommend a standard panel.
- What the result is for (internal R&D, a publication, a patent response) because that changes the design.
- Back: a technical report with methods, controls, tabulated MIC/MBC or zone data, and a plain statement of what the data shows.
Frequently asked questions
Is Manna Biotech NABL accredited for antimicrobial testing?+
No. Manna Biotech is not NABL/ISO 17025 accredited and is not an FSSAI-notified laboratory. Its in-house antimicrobial testing is for research, product development, patent evidence and internal quality use. Where a statutory or accredited certificate is required, that is arranged through partner accredited laboratories, and the partner issues that report.
How many organisms should I test against?+
It depends on the claim. A screening study might use two or three representative Gram-positive and Gram-negative strains plus a fungus; a claim-supporting study uses the organisms relevant to the intended use. The laboratory can recommend a panel once it knows the purpose.
How long does MIC/MBC testing take?+
Bacterial MIC/MBC by broth microdilution is typically reported within one to two weeks including repeats; fungal work and biofilm assays take somewhat longer. Timelines are confirmed at scoping.
Can you also identify my isolate?+
Yes. Microbial identification by culture, biochemical characterisation and 16S/18S rRNA sequencing, and whole-genome confirmation where needed, are all in-house.