Postgate B, API RP-38 and Starkey Media: What Is the Difference?
Modified Postgate B, API RP-38 and Starkey media are all used to culture sulfate-reducing microorganisms, but they are not interchangeable. Differences in nutrients, iron source, reducing chemistry, salinity and preparation can change which organisms grow, how quickly bottles turn black and the final MPN or serial-dilution result.
Direct answer
Modified Postgate B is a widely used lactate-and-sulfate medium in oilfield SRB monitoring; API RP-38 is a historical oilfield injection-water medium associated with an iron-nail indicator system; and Starkey media are older water and environmental sulfate-reducer media that use a different mineral and reducing-agent balance.
All three rely on anaerobic growth and sulfate reduction. Sulfide formed during growth reacts with ferrous iron to create black iron sulfide. The selected formulation can strongly affect recovery. Published comparisons have found differences of two or three orders of magnitude between culture media for the same environmental sample.
No medium detects every sulfate-reducing microorganism. The best medium is the one that has been validated for the sample matrix, system salinity, incubation temperature and monitoring objective.
Operators often ask whether Postgate B, API and Starkey bottles should give the same sulfate-reducing bacteria count. They usually do not. Even when the same produced-water or deposit sample is inoculated at the same time, one medium may turn black quickly while another remains clear.
This does not automatically mean that one product is defective. Culture media are selective environments. They decide which cells recover from sampling stress, which substrates are available, how low the redox potential becomes and whether the sulfide reaction is easy to see.
The media also have different historical purposes. API RP-38 originated in biological analysis of subsurface injection water. Starkey media have a long history in water and environmental microbiology. Postgate media were developed to improve cultivation and enumeration of sulfate reducers and were later adapted for oilfield use.
What do these media actually detect?
The traditional term sulfate-reducing bacteria (SRB) is still widely used in oil and gas. The broader term sulfate-reducing microorganisms (SRM) is often more scientifically accurate because sulfate reduction is a microbial function rather than one taxonomic group.
These traditional media mainly recover organisms that can:
- survive the sampling, transport and inoculation process;
- grow under anaerobic or strongly reducing conditions;
- use the available electron donor, commonly lactate;
- use sulfate or another supported sulfur compound as an electron acceptor;
- tolerate the medium salinity, pH and incubation temperature;
- produce enough sulfide to create a visible reaction during the observation period.
This distinction is important for MIC. A culture bottle may contain lactate as the primary electron donor, while microorganisms at a corroding steel surface may use hydrogen, hydrocarbons, fermentation products or electrons associated with the metal surface. A negative lactate-based bottle therefore does not exclude every sulfate-reducing population or corrosion mechanism.
Postgate B vs API RP-38 vs Starkey: quick comparison
| Comparison point | Modified Postgate B | API RP-38-type medium | Starkey-type medium |
|---|---|---|---|
| Historical focus | Versatile cultivation and enumeration of sulfate reducers; later widely applied in oilfield monitoring | Biological analysis of water-flood and subsurface injection waters | Detection and enumeration in water, deposits, soil and environmental samples |
| Typical electron donor | Lactate | Lactate | Lactate; some research variants use acetate or another donor |
| Sulfate source | Usually sodium sulfate, with additional sulfate-containing salts possible | Typically sodium sulfate with magnesium sulfate | Typically sodium sulfate plus magnesium sulfate |
| Nutrition | Often yeast extract, ammonium and mineral salts; exact modified formulations vary | Yeast extract and mineral salts in the modified liquid formulation | Ammonium and mineral salts; many Starkey A formulations contain little or no yeast extract |
| Reducing chemistry | Commonly ascorbate and thioglycolate or another reducing system | Ascorbic acid in a traditional modified formulation; commercial products may add or alter reductants | Commonly thioglycolate and ascorbic acid |
| Iron indicator | Usually dissolved ferrous iron, allowing black precipitate throughout the liquid | Traditional oilfield products may use an iron nail; modified formulations may include dissolved ferrous ammonium sulfate | Low dissolved ferrous iron in many Starkey A formulations |
| Positive reaction | Black iron-sulfide precipitate or blackening | Blackening around or throughout the iron-containing system | Black precipitate or black colouration |
| Matrix adaptation | Often supplied or prepared at different salinities for oilfield waters | Historically developed for injection waters; compatibility still depends on exact formulation | ASTM-style Starkey B uses sample water as the water source for atypical non-freshwater conditions |
| Standards context | Commonly used in TM0194-based oilfield workflows | Associated with withdrawn historical API RP-38; still used in legacy and commercial programmes | Historically described in ASTM D4412 and water-testing methods |
| Main interpretation risk | Formulations called “Modified Postgate B” are not always identical | Legacy naming may conceal differences between nail-based and dissolved-iron variants | Starkey A, B and modified formulations should not be treated as one fixed recipe |
What is Modified Postgate B medium?
A widely used oilfield sulfate-reducer medium
Postgate developed lactate-yeast-extract-sulfate media for cultivating and enumerating sulfate-reducing bacteria. Modern Modified Postgate B formulations usually provide lactate, sulfate, mineral nutrients, reducing agents and dissolved ferrous iron.
The dissolved iron makes the sulfide response visible as black iron-sulfide precipitation in the liquid. Properly reduced medium is designed to create a low-redox environment that supports anaerobic recovery.
Typical component functions
- Lactate: electron donor and carbon source for organisms able to use it.
- Sulfate: terminal electron acceptor for sulfate reduction.
- Yeast extract and ammonium: growth factors and nitrogen sources.
- Phosphate, magnesium and calcium salts: mineral balance and cellular requirements.
- Ascorbate and thioglycolate: reducing agents that help lower redox potential.
- Ferrous iron: reacts with sulfide to form the black visible indicator.
- Citrate in some formulations: can help control iron availability and precipitation.
Strengths
- Familiar in many oil and gas monitoring programmes.
- Clear black visual reaction when the system performs as intended.
- Compatible with serial dilution and replicated MPN designs.
- Can be manufactured at selected salinities.
- Dissolved iron distributes the indicator through the liquid.
- Well suited to historical trending when the exact method is kept consistent.
Limitations
Modified Postgate B is not a universal growth medium for all sulfate reducers. Lactate-utilizing, mesophilic organisms may be recovered well, while organisms requiring other donors, higher temperatures, unusual nutrients or different salinity can be underrepresented.
The word “modified” is also important. Commercial suppliers and laboratories may alter sodium chloride, buffers, vitamins, reducing agents or trace nutrients. Two bottles carrying the same general name can therefore perform differently.
What is API RP-38 medium?
A historical oilfield injection-water method
API RP-38 was developed for biological analysis of water-flood and subsurface injection waters. The publication is now historical and withdrawn, but the associated medium remains familiar in oilfield microbiology and is still commercially available.
The traditional image of API medium is a clear anaerobic bottle containing an iron nail. Sulfide generated during sulfate reduction reacts with iron, creating black iron sulfide around the nail and in the surrounding medium.
A published modified API RP-38 formulation instead uses dissolved ferrous ammonium sulfate alongside lactate, sulfate, yeast extract, ascorbic acid, magnesium sulfate and phosphate. Commercial “API” media may therefore not all use exactly the same iron configuration.
Why API medium is still used
- operators may have decades of historical API-medium data;
- field personnel recognize the nail and black reaction;
- existing action levels may have been developed around that method;
- service providers may maintain the medium for contract continuity;
- switching media can disrupt long-term trends.
Main caution
A historical action level built with API medium cannot automatically be applied to a Postgate or Starkey result. If a new medium recovers more organisms or blackens more rapidly, the apparent increase may be methodological rather than a deterioration in the asset.
What is Starkey medium?
An established medium family for water and environmental samples
Starkey media have long been used for enriching, detecting and enumerating sulfate-reducing microorganisms. Typical formulations provide lactate, sulfate, ammonium, phosphate, magnesium, calcium, reducing agents and a small quantity of ferrous iron.
Modified Starkey A appears in established water-testing procedures. A representative formulation uses dipotassium phosphate, ammonium chloride, sodium sulfate, calcium chloride, magnesium sulfate, sodium lactate, sodium thioglycolate, ascorbic acid and a low concentration of ferrous sulfate.
Starkey A versus Starkey B
In ASTM D4412 terminology, Medium A is prepared with reagent-grade water. Medium B uses the water being sampled as the water source. The logic behind Medium B is that organisms adapted to an unusual non-freshwater environment may recover better when the ionic background resembles their original water.
This principle is relevant to oilfield microbiology, but using produced water as a medium component also introduces complexity. The water may contain biocide, sulfide, iron, hydrocarbons or inhibitors. Any matrix-matched formulation should therefore be validated rather than improvised.
Strengths
- Long history in water, groundwater, deposits and environmental testing.
- Defined mineral formulation suitable for controlled laboratory use.
- Starkey B concept recognizes matrix adaptation.
- Compatible with dilution and MPN procedures.
- Black sulfide indicator provides a simple visual endpoint.
- Useful when linked to an established ASTM or laboratory method.
Limitations
Starkey is also not one universal recipe. Modified versions differ, and many modern research applications use Starkey formulations for environmental treatment studies rather than oilfield enumeration. Results should always be linked to the exact procedure used.
Why does medium composition change the SRB result?
Electron donor selection
Lactate supports many commonly studied sulfate reducers, but not every field population responds equally. Organisms adapted to hydrogen, acetate, propionate, hydrocarbons or complex fermentation products may recover slowly or not at all.
Nutrients and growth factors
Yeast extract, ammonium, vitamins and trace metals can shorten lag time and help stressed cells recover. A minimal medium may be more selective than a richer modified formulation.
Redox potential
Sulfate reduction requires sufficiently reducing conditions. Oxygen introduced during production, storage or inoculation can delay growth. Reducing agents and anaerobic preparation determine how quickly the medium becomes suitable.
Salinity and total dissolved solids
A low-salinity medium can osmotically stress microorganisms from concentrated produced water. A highly saline formulation can inhibit freshwater populations. The ionic background should be compatible with the monitoring objective.
pH and buffering
Different sulfate reducers have different pH ranges. The buffer system also determines how strongly sample acidity or alkalinity changes the culture environment.
Iron concentration and form
Dissolved ferrous iron can produce rapid visible precipitation throughout a bottle. A solid iron nail creates a different reaction geometry. Too much iron or sample-derived black material can also complicate interpretation.
Incubation temperature and endpoint
The same medium can recover a different community at 25°C, 35°C or a thermophilic temperature. Reading one laboratory at day 7 and another at day 28 adds another source of variation.
For a broader interlaboratory discussion, read Why Do MPN Results Differ Between Laboratories?.
Why do Postgate, API and Starkey bottles turn black?
During dissimilatory sulfate reduction, sulfate is reduced through intermediate steps to sulfide. In an iron-containing medium, sulfide reacts with ferrous iron to form black iron sulfide.
Progressive blackening during incubation is therefore commonly used as a visible positive endpoint. However, blackening is an indicator reaction—not direct identification of an organism and not proof of MIC.
Immediate blackening is different
Produced water and deposits may already contain dissolved sulfide or black iron-sulfide particles. These can blacken or obscure a culture bottle immediately after inoculation, before new microbial growth is possible.
Record the starting appearance, use controls and distinguish an immediate matrix reaction from progressive blackening. Read Why Did My SRB Test Bottle Turn Black? for a full troubleshooting guide.
Can different media produce counts that differ by orders of magnitude?
Yes. Published studies show that culture-medium selection can strongly affect environmental SRB enumeration.
Tanner compared standard API RP-38, Postgate B, modified Baar and an enriched modified API formulation. Counts from pure cultures were less sensitive to the medium choice, but environmental samples depended strongly on the formulation. The enriched modified medium generally produced higher and faster counts.
Zamora and Malaver compared Starkey, Postgate B, API and modified Baar media. In their water and soil samples, modified Baar produced counts that differed by approximately two orders of magnitude in water and three orders in soil compared with the other media at an early read point.
Vester and Ingvorsen similarly demonstrated that an improved natural-medium approach could produce much higher SRB MPN estimates than a conventional synthetic method.
These studies do not establish one universal winner. They show that a medium performing well for one isolate or environment can underperform for another. A supplier claim or one pure-culture validation is therefore not enough to prove recovery from a complex produced-water community.
Which SRB medium should you choose?
| Monitoring situation | Practical starting point | Important qualification |
|---|---|---|
| Existing TM0194-based oilfield programme using Modified Postgate B | Continue the validated Postgate formulation for trend continuity | Do not change supplier, TDS or formulation without a bridging comparison |
| Legacy API RP-38 historical dataset | Maintain API-type medium if historical comparability is essential | Recognize that API RP-38 is historical and supplement the programme where current risk questions require more information |
| Drinking water, groundwater or ASTM D4412-type application | Use the Starkey or API formulation specified by the applicable method | Verify performance under the laboratory’s own conditions and sample matrix |
| High-salinity produced or formation water | Use a validated salinity-compatible formulation | Matrix matching must not introduce biocide, sulfide or inhibition without controls |
| Unknown environmental population | Evaluate two or more candidate media during method development | Use representative samples, replicates, controls and a molecular comparison |
| Investigation of suspected MIC at a surface | Culture can be one supporting method | Include deposits or swabs, qPCR, chemistry, morphology and corrosion evidence |
Questions to ask the laboratory or supplier
- What is the exact medium formulation or product specification?
- Which iron source is used?
- What is the sodium chloride or total dissolved solids level?
- Which electron donor is supplied?
- Which reducing agents are present?
- What is the initial pH and redox specification?
- How is the medium prepared and stored anaerobically?
- Which reference strains and field matrices were used for validation?
- What is the incubation temperature and final endpoint?
- How are immediate blackening and black solids handled?
- Is the result a last-positive dilution or a replicated MPN?
- What detection capability and reporting unit apply?
The related guide MPN Protocols, PBS and Culture Media for Oilfield Microbiology explains how dilution fluid, media selection and system salinity interact.
How can you change SRB media without losing historical trend data?
Do not stop the old method on the same day the new medium is introduced. Run a structured bridging study.
- Select representative samples. Include low, medium and high historical counts, different locations and relevant water chemistries.
- Split one well-mixed parent sample. Minimize time, temperature and oxygen differences.
- Run both media in parallel. Use the same inoculum volumes, dilution range, replicates, temperature and observation schedule where possible.
- Record time-to-positive. A new medium may produce the same final count but respond faster.
- Include controls and blanks. Confirm that differences are biological rather than batch or contamination effects.
- Compare more than point estimates. Evaluate detection limits, all-positive series, all-negative series and confidence intervals.
- Add targeted qPCR. This helps determine whether one medium is missing a relevant target population.
- Reset action levels deliberately. Do not transfer an old numerical threshold automatically to the new method.
Use the MPN Calculator for Oilfield Microbiology when replicated tube designs are used. The article What Does “Less Than the Detection Limit” Mean in an MPN Test? explains why a negative outcome must be linked to the tested volume.
When does qPCR add value alongside SRB culture media?
Culture answers whether organisms recovered and generated a visible response in a selected medium. Targeted qPCR answers whether selected DNA targets are present and how abundant they are relative to the sample unit.
| Question | Postgate/API/Starkey culture | Targeted qPCR |
|---|---|---|
| Did recoverable organisms grow in this medium? | Yes, when a valid progressive positive develops | No; standard DNA qPCR is not a growth test |
| Are selected sulfate-reduction genes present? | Not identified by the black reaction alone | Yes, with a suitable validated functional assay |
| Which taxonomic group is present? | Not determined without isolation and identification | Can be determined for the included targets |
| How quickly is a result available? | Days to weeks; commonly followed up to 28 days | Approximately two hours with the MICBUSTERS on-site workflow |
| Can non-growing or stressed cells be detected? | Usually no, if they cannot recover under the culture conditions | Target DNA may still be detected |
| Does a positive result prove active MIC? | No | No |
qPCR is especially useful when:
- Postgate, API and Starkey counts disagree;
- the culture remains negative despite sulfide, deposits or localized corrosion;
- a result is needed before the final culture endpoint;
- the sample may contain residual biocide;
- methanogenic Archaea or other non-SRB functions matter;
- water and surface-associated samples must be compared;
- a medium change is being validated.
Standard DNA qPCR does not directly establish viability, and target selection must fit the monitoring question. Its value is that it is affected by different limitations than culture. Combining the two can reveal whether a low culture result reflects low target abundance or poor recovery in the selected medium.
For a broader comparison, read Culture Tests, MPN, Bug Bottles and ATP for Oilfield MIC.
Is your SRB result measuring the sample—or the medium?
MICBUSTERS helps oil and gas teams review culture-media selection, sample handling and reporting limits. Our portable qPCR workflow adds rapid, target-specific information for water, filters, deposits, corrosion products, pig debris, biofilms and surface swabs.
Leave your business email address to discuss a fit-for-purpose monitoring programme.
Frequently asked questions
What is the difference between Postgate B, API RP-38 and Starkey media?
They are different anaerobic culture-media families. They vary in nutrients, reducing agents, iron source, mineral balance and historical application. These variables affect which sulfate-reducing microorganisms recover and how the positive reaction appears.
Which medium gives the highest SRB count?
There is no universal answer. Published studies show that the best-performing formulation depends on the environmental population and sample matrix. A richer or better-matched medium can produce results two or three orders of magnitude higher in some samples.
Is Modified Postgate B the standard oilfield SRB medium?
It is widely used in oilfield and TM0194-based programmes, but the exact project requirement should be checked. Formulations sold as Modified Postgate B are not necessarily identical.
Is API RP-38 still a current standard?
API RP-38 is a withdrawn historical publication. API-type medium remains in legacy and commercial use, particularly where operators want continuity with older datasets.
Why is there an iron nail in API medium?
The nail provides iron. Sulfide generated by sulfate-reducing organisms reacts with the iron and forms black iron sulfide. Some modified API formulations use dissolved ferrous iron instead of, or in addition to, solid iron.
What is the difference between Starkey A and Starkey B?
In ASTM D4412 terminology, Starkey A uses reagent-grade water. Starkey B uses the sample water as the water source to improve compatibility with organisms adapted to atypical non-freshwater environments.
Why did my SRB bottle turn black immediately?
The sample may already contain sulfide, iron sulfide or black corrosion solids. Immediate darkening should be distinguished from progressive blackening during incubation using photographs, blanks and controls.
Should culture-medium salinity match produced water?
Major salinity mismatch can reduce recovery. The medium should be sufficiently compatible with the sample, but any adjustment or use of sample water should be validated and documented because inhibitors may also be introduced.
Can I compare an API count with a Postgate B count?
Not as though they were generated by one method. Compare them through a parallel bridging study. Historical action limits may need to be revised if the new medium has different recovery or detection capability.
Can qPCR replace traditional SRB media?
qPCR can provide rapid target-specific information without waiting for growth, but standard DNA qPCR does not directly prove viability. Culture and qPCR are often most useful as complementary measurements.
Sources and further reading
- Postgate JR. Versatile medium for the enumeration of sulfate-reducing bacteria. Applied Microbiology. 1963;11:265–267.
- Tanner RS. Monitoring sulfate-reducing bacteria: comparison of enumeration media. Journal of Microbiological Methods. 1989;10:83–90.
- Zamora AC, Malaver N. Methodological aspects for the culture and quantification of heterotrophic sulfate-reducing bacteria. Revista de la Sociedad Venezolana de Microbiología. 2012;32.
- Vester F, Ingvorsen K. Improved Most-Probable-Number Method To Detect Sulfate-Reducing Bacteria with Natural Media and a Radiotracer. Applied and Environmental Microbiology. 1998;64:1700–1707.
- Standing Committee of Analysts. The Microbiology of Drinking Water, Part 12: Methods for Actinomycetes, Sulphate-Reducing Bacteria and Iron-Precipitating Bacteria. Contains representative modified Starkey A and modified API RP-38 formulations.
- ASTM method summary. ASTM D4412: Sulfate-Reducing Bacteria in Water and Water-Formed Deposits. Describes Starkey A and B use in a five-tube MPN context.
- API. Historical Publications Catalogue. Lists API RP-38 as a historical publication.
- AMPP. TM0194-2014: Field Monitoring of Bacterial Growth in Oil and Gas Systems. Consult the official current publication for normative requirements.
- MICBUSTERS. Why Did My SRB Test Bottle Turn Black?
- MICBUSTERS. Why Do MPN Results Differ Between Laboratories?
- MICBUSTERS. MPN Protocols, PBS and Culture Media for Oilfield Microbiology.
- MICBUSTERS. How Long Should You Incubate SRB and APB Test Bottles?
- MICBUSTERS. MPN Calculator for Oilfield Microbiology.