# Anaerobic Culture Methods

> Anaerobic culture methods for NEET-PG Microbiology: McIntosh-Fildes jar, GasPak, Robertson cooked meat broth, thioglycollate and anaerobic specimen handling.

- Canonical URL: https://prepelephant.com/topics/neet-pg/microbiology/anaerobic-culture-methods
- Exam / course: NEET-PG · Subject: Microbiology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-14
- Last updated: 2026-10-14
- Reference basis: Ananthanarayan and Paniker's Textbook of Microbiology; Murray's Medical Microbiology; Cowan and Steel's Manual for the Identification of Medical Bacteria
- How to cite: "Anaerobic Culture Methods", PrepElephant, https://prepelephant.com/topics/neet-pg/microbiology/anaerobic-culture-methods

## Direct answer

Anaerobic culture methods grow obligate anaerobes, which are killed by oxygen because they often lack catalase or superoxide dismutase and so cannot detoxify reactive oxygen species. Oxygen is removed or reduced by methods such as the McIntosh-Fildes jar, the GasPak system and reducing media like Robertson cooked meat broth. Specimens must be collected without air exposure and processed promptly to keep sensitive organisms viable.

## What you must remember

- **Obligate anaerobes:** organisms such as Clostridium and Bacteroides that cannot tolerate oxygen, often lacking catalase or superoxide dismutase.
- **McIntosh-Fildes jar:** an airtight jar in which hydrogen and a palladium catalyst remove oxygen as water, shown by methylene blue turning colourless.
- **GasPak system:** a disposable sachet that releases hydrogen and carbon dioxide when water is added, with a catalyst that removes oxygen.
- **Cooked meat broth:** Robertson medium, in which the cooked meat particles consume oxygen and keep the medium reduced.
- **Thioglycollate broth:** a reducing medium with a resazurin indicator, which supports both aerobes and anaerobes at different depths.
- **Specimen collection:** deep specimens are best collected by syringe with air expelled and the needle capped, not by an open swab.
- **Vitamin K and haemin:** needed by Bacteroides, so Brucella blood agar is supplemented with both.
- **Prompt processing:** specimens should reach the laboratory quickly, because prolonged air exposure kills sensitive anaerobes.

## A stepwise anaerobic culture workflow

Anaerobic work succeeds or fails at the bedside and in transport, not only in the incubator. The steps below follow a deep abscess specimen from collection to reporting.

1. **Collect without air:** express air from a syringe, cap it immediately and label it as an anaerobic specimen.
2. **Transport promptly:** send the specimen in an anaerobic transport vial, and plate it as soon as possible, ideally within about 30 minutes.
3. **Inoculate reduced media:** plate onto Brucella blood agar with vitamin K and haemin, and onto a selective blood agar containing kanamycin and vancomycin.
4. **Create the anaerobic environment:** place the plates in an anaerobic jar or chamber with a gas generator and an indicator strip.
5. **Incubate and read:** incubate at 35 to 37 °C for at least 48 hours, and longer for slow-growing species before reporting no growth.
6. **Confirm anaerobiosis:** place a duplicate plate in an aerobic atmosphere, because growth only in the anaerobic environment confirms an obligate anaerobe.

Each step guards against a single failure point, since a specimen exposed to air at step one cannot be rescued by a perfect jar later. The aerobic duplicate in step six is the internal control that shows the anaerobic system is working.

## Where the exam tries to catch you

Anaerobic culture questions usually test the specimen collection, the indicator or the supplements needed.

- **Trap:** using an open swab for a deep abscess — **Correct:** a syringe with air expelled is preferred, because swabs expose the sample to air and surface contamination.
- **Trap:** assuming a blue methylene blue indicator shows that the jar is anaerobic — **Correct:** methylene blue turns blue in the presence of oxygen, so a colourless indicator indicates anaerobiosis.
- **Trap:** using plain nutrient agar for Bacteroides — **Correct:** Bacteroides needs vitamin K and haemin, so Brucella blood agar with these supplements is used.

## Frequently asked questions

### Why are obligate anaerobes killed by oxygen?
Obligate anaerobes are killed by oxygen because they often lack catalase or superoxide dismutase, so toxic reactive oxygen species accumulate inside the cell. Anaerobic culture methods remove oxygen to prevent this damage during growth.

### How does the McIntosh-Fildes jar remove oxygen?
The McIntosh-Fildes jar removes oxygen by supplying hydrogen, which combines with oxygen over a palladium catalyst to form water. Methylene blue indicator turns colourless when the jar is anaerobic, which shows that the oxygen has been removed.

### Why is Robertson cooked meat medium used for anaerobic culture?
Robertson cooked meat medium is used for anaerobic culture because the cooked meat particles consume oxygen and keep the medium reduced. It supports the growth of obligate anaerobes such as Clostridium species within the tube itself.

### Which supplements allow Bacteroides fragilis to grow on blood agar?
Bacteroides fragilis needs vitamin K and haemin, so Brucella blood agar is supplemented with both for its growth. Adding kanamycin and vancomycin to a blood agar base produces a selective medium for this organism.

### Why must anaerobic specimens be collected with air removed?
Anaerobic specimens must be collected with air removed because oxygen exposure kills sensitive obligate anaerobes before they can be cultured. A capped syringe with the air expelled is preferred over an open swab for deep specimens such as abscess aspirates.