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The Snyder agar test is a colour-change culture test in which a saliva sample is added to an acidic agar containing glucose (dextrose) and the indicator bromocresol green. The medium turns from blue-green to yellow as oral acid-producing bacteria, chiefly lactobacilli, ferment the sugar, and how fast it turns yellow is read as a measure of caries activity. It is a screening aid, not a diagnosis.
Short answer
Snyder test agar (also sold as B.C.G.-Dextrose Agar) is a differential medium. HiMedia’s technical data sheet for its Snyder Test HiVeg Agar describes it as recommended “for the estimation of Lactobacilli, as an indication of caries activity”. A faster and fuller shift to yellow means more acid production from dextrose. Published evidence shows the result is useful alongside other measures but not reliable alone.
Definition and scope
Two things share the name, and searches for “snyder test agar” mean the same thing as “snyder agar test”:
- The medium: the agar itself, a dextrose-containing gel buffered to a low pH and coloured green by bromocresol green.
- The test: the semi-quantitative reading of how quickly a saliva sample turns that medium yellow. Snyder described the colorimetric method; Alban later modified it and, according to the data sheet, reported it to be more accurate than the original.
It sits in the family of caries activity tests, which estimate the acid-producing bacterial load in saliva rather than detect existing decay. For the wider field, see what dental research covers.
How the colour change works
The HiMedia sheet gives the HiVeg formulation per litre as peptone 20 g, dextrose 20 g, sodium chloride 5 g, bromocresol green 0.020 g and agar 20 g, with a final pH of 4.8 ± 0.2 at 25°C. The principle is the rate of acid production from dextrose by oral acidogenic organisms: as acid accumulates, the bromocresol green indicator changes from blue-green to yellow.
A Montgomery College microbiology lab handout explains why the low starting pH matters. It inhibits many bacteria other than oral lactobacilli and streptococci, and lactobacillus fermentation lowers the pH to about 4.4, which the handout describes as the acidity at which dental caries tend to form. Organisms it names include Lactobacillus acidophilus and Streptococcus mutans.
Grading schemes compared
Different sources grade the same colour change on different scales. Reading results correctly means knowing which scale a laboratory used.
| Scheme | What is read | Scale | Source |
|---|---|---|---|
| Snyder (original) | Time for the colour to change | Marked if within 24 hours, moderate within 48 hours, slight within 72 hours | HiMedia data sheet MV106 |
| Alban modification | Proportion of medium turned yellow, examined daily for 4 days | No change; + beginning yellow; ++ half yellow; +++ three-fourths yellow; ++++ total medium yellow | HiMedia data sheet MV106 |
| 2013 comparative study | Colour read at 24, 48 and 72 hours against a white background | 0 inactive, 1 limited, 2 definitive, 3 marked | Ramesh et al., J Pharm Bioallied Sci |
| Teaching-lab convention | Yellow within 24-48 hours | Suggests greater susceptibility to caries | Montgomery College BIOL 210 handout |
The data sheet and the lab handout do not use identical time windows, so a “positive at 48 hours” from one source is not automatically the same grade in another. The data sheet also advises that, for the Alban scheme, at least two specimens collected within 2 to 4 days are needed to establish a reference point.
Lab checklist: controls and quality checks
The items below follow the HiMedia data sheet for the medium.
- Control strains: Lactobacillus acidophilus ATCC 314 and L. casei ATCC 9595 at an inoculum of 50-100 CFU, expecting good to luxuriant growth and a yellow positive reaction.
- Uninoculated control: the Alban modification calls for one alongside the specimen.
- Prepared-medium pH: 4.60-5.00, against the 4.8 ± 0.2 specification.
- Appearance: an emerald green, clear to slightly opalescent gel.
- Peptone source: the sheet describes a vegetable-peptone (HiVeg) formulation, and states that the formula is adjusted and standardised to suit performance parameters.
- Repeat specimens: at least two within 2-4 days for a reference point (Alban).
- Disposal: standard microbiological practice, with contaminated materials sterilised by autoclaving before disposal. See what an autoclave is and aseptic technique.
The sheet also notes that some organisms may show poor growth because of variable nutritional requirements, which is one reason the medium reflects lactobacilli and other acidogens rather than all oral bacteria.
What the evidence says about accuracy
Ramesh and colleagues compared three caries activity tests in 90 children in the mixed dentition stage, 45 caries-free and 45 with DMFT of 3-5 (J Pharm Bioallied Sci, 2013;5 Suppl 1:S63-S68). The Snyder test showed activity (scores 1-3) in 8 of the 45 caries-free children and in 12 of the 45 with DMFT 3-5. Against the Cariostat test, sensitivity was 12.5% in the DMFT 0 group and 40% in the DMFT 3-5 group, with specificity of 81.1% and 80% respectively. The authors reported good correlation between Snyder scores 1, 2 and 3 and lactobacillus count, and stated that a combination of tests was found to be good for assessing caries risk groups.
Where it matters for labs and supply decisions
The practical point for laboratory staff is consistency. The same specimen can be graded differently under the Snyder, Alban or study scoring scales, so the scale, incubation temperature (the data sheet gives 37°C for the Snyder method and 35°C for Alban) and reading times should be recorded with each result. Quality-control strains and an uninoculated control belong in the record. For background on media, see types of culture media and what microbiology covers.
Common misunderstandings
- “Yellow means cavities.” In the 2013 study, 8 of 45 caries-free children still showed Snyder activity, and sensitivity against Cariostat was 12.5% to 40%. The colour reflects acid production, not existing decay.
- “It counts all oral bacteria.” The medium is recommended for estimating lactobacilli; it is a differential medium based on acid production, and the sheet notes some organisms grow poorly on it.
- “All labs grade it the same way.” The grading table above shows four different conventions.
- “A single reading is enough.” The Alban scheme asks for at least two specimens within 2-4 days, and the 2013 study authors found a combination of tests useful.
FAQ
What is the Snyder agar test used for?
It estimates caries activity by measuring how quickly oral acid-producing bacteria, mainly lactobacilli, turn a dextrose agar from blue-green to yellow.
What is Snyder test agar made of?
In the HiMedia HiVeg formulation: peptone, dextrose, sodium chloride, bromocresol green and agar, at a final pH of 4.8 ± 0.2.
How is a Snyder test result graded?
The original scheme grades by time (marked at 24 hours, moderate at 48, slight at 72). The Alban modification grades by the share of medium turned yellow, from no change to ++++.
How accurate is it?
In a 90-child study its sensitivity against Cariostat was 12.5% and 40% in two groups, with specificity near 80%. The authors found a combination of tests useful.
Sources
All sources checked 2026-10-29.
- HiMedia Laboratories, technical data sheet MV106, B.C.G.-Dextrose HiVeg Agar (Snyder Test HiVeg Agar): https://www.himedialabs.com/media/TD/MV106.pdf
- Ramesh K, et al. Comparative evaluation of predictive value of three caries activity tests: Snyder, lactobacillus count and Cariostat. J Pharm Bioallied Sci. 2013;5(Suppl 1):S63-S68: https://pmc.ncbi.nlm.nih.gov/articles/PMC3722709
- Montgomery College BIOL 210, lab handout “Snyder Test: Determining Susceptibility to Dental Caries”: https://info.montgomerycollege.edu/_documents/faculty/slester/biol210/lab-handouts/snyder-test-gm.pdf
