Can Facultative Anaerobes Grow Without Oxygen? How and Why

Yes, facultative anaerobes can absolutely grow without oxygen. That is the defining trait of this group: they are flexible enough to survive and multiply whether oxygen is present or not. Without oxygen, they switch to fermentation or anaerobic respiration depending on what alternative electron acceptors are available. Growth is usually slower without oxygen, but it happens, and for some well-known pathogens like Escherichia coli and Staphylococcus aureus, it happens readily enough to cause real food safety problems.

What "facultative anaerobe" actually means

The word "facultative" essentially means optional. A facultative anaerobe can use oxygen when it is available, but it is not dependent on it. That puts it squarely in the middle of the oxygen-requirement spectrum.

Here is how the main categories compare:

The key distinction from obligate anaerobes is that facultative anaerobes are not harmed by oxygen exposure. They just shift their metabolism based on what is available. From a food safety standpoint, this flexibility is what makes them such persistent hazards: reducing oxygen in packaging does not stop them the way it would stop an obligate aerobe.

Yes, they grow without oxygen, here is how

When oxygen disappears, facultative anaerobes do not simply shut down. They activate alternative metabolic pathways, and the specific pathway used depends on what other molecules are available in the environment. In other words, the right electron acceptors can let intestinal bacteria keep respiring and growing even when oxygen is absent intestinal bacteria can grow in the presence of.

Anaerobic respiration (when alternative electron acceptors are present)

If nitrate, nitrite, fumarate, or other electron acceptors are present in the environment, many facultative anaerobes will use them as substitutes for oxygen in their electron transport chain. For example, many facultative anaerobes can grow anaerobically using nitrate as a substitute for oxygen grow without oxygen. E. coli, for example, carries nitrate reductases and fumarate reductase enzymes specifically for this purpose. Nitrate has a high reduction potential, making it a particularly good substitute, and E. coli will preferentially reduce nitrate to nitrite, then continue processing nitrite further. Fumarate respiration (converting fumarate to succinate) is another route, and in E. coli it is switched on by an oxygen-sensing regulatory protein called Fnr when oxygen drops. S. aureus follows the same general logic: it can grow anaerobically using nitrate respiration or fermentation depending on conditions. Obligate anaerobes can grow in the absence of oxygen, but they are distinct from facultative anaerobes that keep growing even when oxygen is present.

Fermentation (when no alternative acceptors exist)

When there is no oxygen and no suitable alternative electron acceptor, facultative anaerobes fall back on fermentation. E. coli, for instance, produces a mixture of end products in this state: ethanol, acetate, lactate, succinate, formate, and CO2, with the exact blend influenced by the oxidation-reduction potential of the environment. S. aureus does the same under purely anaerobic conditions, though its glucose consumption slows noticeably: studies show glucose depletion takes around 8 hours under aerobic conditions versus around 24 hours anaerobically. That slowdown matters for food safety timelines.

How growth changes with and without oxygen

The practical takeaway is that most facultative anaerobes grow faster and more efficiently with oxygen, because aerobic respiration extracts far more energy from the same substrate than fermentation does. But "slower without oxygen" does not mean "stopped. This is closely related to whether they can aerobic bacteria grow in the absence of o2, because many facultative anaerobes keep growing even when oxygen is removed. " Depending on temperature, pH, water activity, and available nutrients, anaerobic growth can still reach dangerous levels.

The metabolic outputs also shift. Under aerobic conditions, carbon is mostly oxidized to CO2 and water. Under anaerobic conditions, you get a range of organic acids, alcohols, and gases instead. This is why vacuum-packaged or modified-atmosphere foods can develop off-odors even when temperature control is good: it is the signature of anaerobic metabolism, not necessarily a sign of safety problems, but it is a useful indicator that something biological is happening.

One thing worth noting: a handful of facultative anaerobes grow at roughly similar rates with or without oxygen under certain substrate and temperature conditions. Do not assume oxygen removal reliably slows all facultative species equally. Growth rate differences are species-specific and condition-dependent.

How to confirm oxygen-free growth in the lab

If you need to verify whether an isolate is a facultative anaerobe, or confirm that growth is actually occurring under anoxic conditions, there are a few reliable approaches used in standard microbiology practice.

Thioglycollate broth with resazurin

Fluid thioglycollate medium is the workhorse for classifying oxygen requirements. The medium contains cystine and sodium thioglycollate, which maintain a low oxidation-reduction potential and create an oxygen gradient from the surface down. Resazurin is included as a colorimetric oxygen indicator: it turns pink or red when oxidized (oxygen present) and becomes colorless when reduced (anoxic). Inoculate the tube by stabbing to the bottom, incubate, and then read growth distribution. A facultative anaerobe will grow throughout the tube, including the anaerobic bottom portion. Compare this pattern against a known obligate aerobe (grows only at the surface) and an obligate anaerobe (grows only at the bottom, away from oxygen).

Anaerobic jars and chambers

For plate-based confirmation, use a GasPak anaerobic jar or a dedicated anaerobic chamber. Plate your organism on appropriate solid media, place it in the jar with a GasPak hydrogen-plus-CO2 generator sachet (which catalytically removes oxygen), and incubate alongside an aerobic control plate in ambient air. If colonies appear on both the aerobic and anaerobic plates, you have a facultative anaerobe or obligate anaerobe on your hands. A candle jar (roughly 2-3% O2, elevated CO2) works for microaerophiles and can serve as a useful intermediate control. Include resazurin-based anaerobic indicators in the jar to confirm oxygen was actually removed before drawing conclusions.

Media supplementation to distinguish respiration from fermentation

If you want to go further and determine whether the organism is using anaerobic respiration or pure fermentation, add defined electron acceptors to the medium. Supplement one set of anaerobic cultures with nitrate (10 mM is a common starting concentration) and another with fumarate, and compare growth rates and end-product profiles (via HPLC or enzymatic assays) against a no-acceptor control. Higher growth yields in the supplemented cultures indicate anaerobic respiration capacity. Detection of ethanol and lactate as dominant products in the unsupplemented cultures points toward fermentation. This approach directly mirrors what researchers have done to characterize E. coli and S. aureus anaerobic physiology.

Interpreting your results

1. Growth in both aerobic and anaerobic conditions: facultative anaerobe (or obligate anaerobe if it fails the aerobic plate).
2. Growth only aerobically: obligate aerobe.
3. Growth only anaerobically: obligate anaerobe.
4. Growth in thioglycollate only near the top: aerobe or microaerophile.
5. Growth throughout thioglycollate tube with heaviest density at the surface: classic facultative anaerobe pattern.

What this means for food safety and storage

This is where the science becomes directly actionable. Many of the pathogens that concern food safety professionals most, E. coli O157:H7, Listeria monocytogenes, S. aureus, Salmonella, are facultative anaerobes. Reducing oxygen through vacuum packaging or modified atmosphere packaging (MAP) does not eliminate the risk from these organisms. That is why facultative anaerobes can still pose risks in low-oxygen foods where some microbes cannot grow it does not stop it. It can slow their growth, but it does not stop it.

Vacuum and modified atmosphere packaging: what the research shows

Studies on beef, chicken, and ready-to-eat meats consistently show that L. monocytogenes and E. coli O157:H7 can survive and grow in vacuum-packaged and low-oxygen MAP products, particularly under temperature abuse. A 75% CO2/25% N2 anaerobic atmosphere suppressed both aerobic plate counts and L. monocytogenes growth on raw chicken in one study, but that extreme CO2 concentration is not typical of commercial packaging. More common MAP formulations (such as 40% CO2/30% O2/30% N2 or simple vacuum bags) show limited suppression of L. monocytogenes compared with aerobic storage, and sometimes vacuum packaging actually produced a shorter lag phase for psychrotrophic bacteria compared with CO2/N2 MAP conditions.

For ground beef, research comparing vacuum bags against low-oxygen CO2/N2 formulations found that certain low-oxygen packaging types were associated with less E. coli O157:H7 growth under temperature abuse, but none of the packaging types eliminated the hazard. The controlling variable is almost always temperature, not oxygen level alone.

Practical guidance for food professionals

• Treat reduced-oxygen packaging as a shelf-life and quality tool, not a pathogen kill step. It changes growth dynamics but does not make products microbiologically safe on its own.
• Temperature control remains the primary critical control point for facultative anaerobes in low-oxygen packaged foods. Keeping products at or below 4°C (39°F) is the most reliable way to slow growth.
• If your operation uses reduced oxygen packaging for temperature-controlled-for-safety (TCS) foods, FDA Food Code and state regulations (such as WAC 246-215-03540 in Washington) require a written HACCP plan that accounts for pathogens relevant to the packaging format, including L. monocytogenes and C. botulinum.
• Do not rely on sensory evaluation alone for safety decisions. Facultative anaerobes can multiply in vacuum-packaged products without obvious odor or color changes, especially at refrigeration temperatures.
• If aerobic plate counts are low in a modified atmosphere product, that does not mean the product is pathogen-free. Obligate aerobes are suppressed, but facultative anaerobes are not.
• For products where nitrate or nitrite is present (cured meats, some brined vegetables), be aware that these compounds can actually serve as electron acceptors supporting anaerobic respiration by organisms like E. coli and S. aureus, rather than simply acting as preservatives.

The broader context: oxygen is one lever among many

Oxygen availability interacts with temperature, pH, water activity, and competing microflora to determine whether facultative anaerobes actually become a problem in a specific food environment. A vacuum-packaged product at 2°C with a water activity of 0.93 and a pH of 5.2 is in a very different risk category than the same product at 10°C with a neutral pH. Oxygen removal shifts the ecological balance (suppressing obligate aerobes and giving facultative species less competition in some cases) but it does not substitute for the other hurdles. Understanding that facultative anaerobes do not need oxygen is the foundation for building a complete picture of where microbial risks actually live in your product or process.