FOOD MICROBIOLOGY: OXYGEN AND GROWTH
AEROBIC BACTERIA
Need oxygen to grow. Live on open surfaces such as fresh meat. Example: Pseudomonas.
ANAEROBIC BACTERIA
Grow without oxygen; strict anaerobes are killed by it. Live in sealed cans and vacuum packs. Example: Clostridium botulinum.
Figure 1: Oxygen decides where each group grows: aerobes on open surfaces, anaerobes inside sealed packs and cans.
Introduction
Aerobic and anaerobic bacteria differ in their need for oxygen, and that need decides where each group grows in food. Aerobes need oxygen, anaerobes grow without it, and facultative anaerobes grow with or without it.
Quick comparison
Quick comparison: Aerobic and Anaerobic Bacteria
| Aspect | Aerobic Bacteria | Anaerobic Bacteria |
|---|---|---|
| Oxygen need | Need O₂ to grow | Grow without O₂; obligate anaerobes are killed by O₂ |
| Energy route | Respiration using O₂ | Fermentation or anaerobic respiration without O₂ |
| Protective enzymes | Have superoxide dismutase, catalase and peroxidase to remove oxygen radicals | Obligate anaerobes lack these enzymes, which is one reason oxygen kills them |
| Where they grow | Open surfaces, fresh meat, salads, open tanks | Inside of sealed cans, vacuum packs, deep in foods, gut |
| Food examples | Pseudomonas spoils meat surfaces; Acetobacter makes vinegar in air | Clostridium botulinum grows in sealed cans; Clostridium perfringens in bulk-cooked meat |
| Effect of packaging | Slowed by low O₂ and high CO₂ in modified atmosphere packs | Favoured when O₂ is removed, so vacuum packing needs chilling and short shelf life |
| Redox (Eh) | Suit foods with positive Eh, such as fresh meat surfaces | Suit foods with negative Eh, such as the inside of a can or vacuum pack |
| Cultivation | Plates or shallow broth in air; shaking adds oxygen | Candle jar, GasPak system, anaerobic chamber, or pre-reduced media |
| Middle group | Facultative anaerobes grow with or without O₂, such as Escherichia coli and yeasts | Aerotolerant anaerobes do not use O₂ but tolerate it, such as many lactic acid bacteria |
Scientific explanation
Definition
Aerobic bacteria need oxygen to grow. Anaerobic bacteria grow without oxygen. Between them sit facultative anaerobes, which grow with or without oxygen, and aerotolerant anaerobes, which do not use oxygen but tolerate it. Normal air is about 21 percent O₂ and 0.04 percent CO₂, with the rest mostly N₂.
Working principle
Oxygen is useful for energy but its by-products are toxic: superoxide radicals, hydrogen peroxide (H₂O₂) and hydroxyl radicals damage proteins and DNA. Aerobes protect themselves with enzymes: superoxide dismutase turns superoxide into H₂O₂ and O₂, and catalase and peroxidase turn H₂O₂ into water and O₂. Obligate anaerobes lack these enzymes, so oxygen kills them. The oxidation reduction potential (Eh) of a food shows which group it suits: fresh meat surfaces have a positive Eh and suit aerobes such as Pseudomonas, while the inside of a sealed can or vacuum pack has a negative Eh and suits anaerobes such as Clostridium.
Apparatus
Aerobic culture needs plates or shallow broth in air, with shaking to add oxygen. Anaerobic culture needs a candle jar, a GasPak system with a hydrogen sachet and palladium catalyst, or an anaerobic chamber filled with nitrogen, hydrogen and CO₂, with an indicator strip showing when oxygen is gone.
Measurement procedure
For aerobes, spread or pour plates are incubated in air at the right temperature. For anaerobes, plates are placed in a sealed jar or chamber with oxygen removed, using pre-reduced media, and incubated the same way. The catalase test uses the same chemistry: a drop of H₂O₂ on a colony bubbles with O₂ when catalase is present.
Factors affecting readings
Cutting, mincing and cooking change Eh, and packaging that removes O₂ shifts growth toward anaerobes. Raising CO₂ and lowering O₂ slows aerobes and moulds, which is the basis of modified atmosphere packaging, but gas packaging slows growth and does not sterilize, so warm storage still lets anaerobes and facultative microbes grow.
Formulas and calculations
This topic has no standard formula. Growth of each group is measured by plate count, CFU/g = colonies counted divided by (volume plated in ml x dilution factor).
Industrial applications
- Aerobes: vinegar making by Acetobacter in air; spoilage control of fresh meat, salads and open foods; aerobic plate counts for hygiene checks.
- Anaerobes: canned food safety, where Clostridium botulinum spores are the target; vacuum-packed meat, where lactic acid bacteria and Clostridium are watched.
- Both: modified atmosphere and vacuum packaging design, where the gas mix must match the organisms it is meant to hold back.
Advantages and limitations
Aerobic: advantages are simple cultivation in air and fast growth that is easy to count; limitations are that they spoil open foods fast and are only slowed, not killed, by low-oxygen packaging.
Anaerobic: advantages are that their growth can be stopped by keeping oxygen in the food; limitations are that removing oxygen invites them, their spores survive cooking, and Clostridium botulinum toxin in sealed food is a severe hazard.
FAQ
Can one bacterium be both aerobic and anaerobic?
Yes, facultative anaerobes such as Escherichia coli grow with or without oxygen, and they are the most common food organisms.
Why is vacuum packing not enough to keep food safe?
Removing O₂ suits anaerobes, so vacuum packing must be paired with chilling and a short shelf life, and gas packaging slows growth but does not sterilize.
Why do anaerobes die in air?
Oxygen forms superoxide radicals and H₂O₂ that damage proteins and DNA, and obligate anaerobes lack the enzymes superoxide dismutase and catalase that remove them.
What is Eh, and why does it matter?
Eh, the oxidation reduction potential, measures whether a food tends to give up or accept electrons, which decides how much free O₂ is available and whether aerobes or anaerobes will grow.
References and further reading
FTN subjects used: Food Microbiology (Units 1, 2, 3, 4, 5 and 8) and General Microbiology (Units 1, 2, 3, 4, 5, 6 and 10), from the notes at ~/workspace/food-tech-notes/notes/.
Books used: Adams, M. R. and Moss, M. O., Food Microbiology, Royal Society of Chemistry, 2007 (book on disk at food-tech-notes/books/Food-Microbiology.pdf; Google Books page: https://books.google.com/books?id=BZNKI088iooC). Ray, B. and Bhunia, A., Fundamental Food Microbiology, 5th edition, as cited in the FTN notes. Essentials of Industrial Microbiology (book on disk at food-tech-notes/books/Essentials-of-Industrial-Microbiology.pdf).
Google Books pages used: Food Microbiology by Adams and Moss, https://books.google.com/books?id=BZNKI088iooC (contents and chapter list read on 2026-11-11).
