An Introduction to Good & Bad Microorganisms in Food

Food Science is at the heart of this blog, however, in most cases we’ll be applying food science, assuming you’ve got a basic understanding of the food science concepts. Realizing that not everyone knows these basics, we’ve got a series of posts dedicated on teaching those fundamental concepts.

This post will give you a first introduction to the world of food microbiology. We’ll be introducing micro organisms to you (the centerpiece of food microbiology research), how they grow, what types there are  and will introduce you to the most common good and bad micro organisms. Yes, micro organisms aren’t necessarily a bad thing, in plenty instances you actually need them (think: brewing beer, making yoghurt, baking bread, making sauerkraut). In other cases though we try to prevent them from landing in our food (e.g. Salmonella). Time to get started!

What are microorganisms?

Microorganisms are very small living organisms. A single microorganism can only be seen with a microscope. Only when very large numbers of microorganisms are present will you be able to see them (think of moulds growing on stale bread).

In food there are three main types of microorganisms: bacteria, moulds and yeasts (and virusses, but we won’t focus on those here). Let’s introduce them one by one.

example bacteria structure
Simplified visualization of a bacteria cell


Bacteria consist of only one simple cell and have very simple shapes such as rods or little ball shapes. There are a lot of different bacteria and they can be grouped in a lot of different ways. One method is to group them based on their oxygen needs. Bacteria that grow without any oxygen are called anaerobic. Those that need oxygen are aerobic. Some bacteria need a little oxygen, but cannot stand large quantities: microaerophilic. There are also bacteria who don’t need oxygen, but can handle it, others don’t need it and cannot handle it either.

Bacteria can also be grouped by their family names. However, for this purpose there’s no need for that, I prefer using a split based on their behaviour and presence. It makes it far easier to apply.

Bacteria multiply by splitting themselves in two.


1.2 Moulds

Moulds have far more complex shapes than bacteria. They aren’t simple rods, instead, they’ll often have hairy structures, or shapes like a plant. They can form these structures since they don’t exist of just one cell but are multi-cellular. Of course, they’re small, generally about 5-10 micron only.

Moulds need oxygen to grow. There are only a few moulds which can grow a little when there isn’t any oxygen. However, not all moulds might need as much oxygen in the air as we do. Whereas most of the bacteria are very sensitive to their environment, such as pH or water activity. Moulds are a lot less sensitive to these variables, they are able to withstand pretty harsh situations.

Moulds (and yeasts from the next section) both belong to the group of fungi.

1.3 Yeasts

Yeasts are very similar to moulds, they also belong to the fungi but consist of just one cell instead of several. This means their structures are a lot less complex. It is often just a ball or ellipse form.

Just like moulds yeasts need oxygen to grow. However, more of them are able to grow under anaerobic (thus without oxygen) as well.

Yeasts can handle a wide range of pH-values. Specific yeasts can also tolerate very high sugar concentrations, this is often a challenging environment due to the lack of available water.

One of the most famous yeasts is Saccharomyces cerevisiae, which is used when making bread.

2. The bad stuff – Pathogenic microorganisms

The reason food microbiology is such an important topic when it comes to food safety is due to the bad stuff: the pathogenic microorganisms. Pathogenic microorganisms are microorganisms than can mkae you sick. If too high concentrations of such a microorganism are present on a food you’re eating, it might infect you.

It is very important to note that not all microorganisms are bad for you. On the contrary, recent research shows more and more that you need enough of the good bacteria as well, which is why we’ll zoom in on those in the next chapter.

There are a lot of different types of pathogenic microorganisms. We will discuss just a few to give you a feel for the types of things you should take into account. These are five of the most prevalent food infection causes and a lot more common than you might think.

Please notethis information is provided for educational purposes only. In no way can I be held responsible for any actions based upon this content. If you are looking for food safety guidelines/regulations, please refer to a local expert.

2.1 Salmonella

This is the main reason you shouldn’t eat raw chicken. Salmonella can often be found in chicken meats and can grow both with an without oxygen, but do not grow at temperatures above 7C. Fortunately, Salmonella cannot withstand heat well, heating them to 70C for about a minute will kill the bacteria.

Salmonella can cause both a direct disease, within only a few hours or days, or a more longer term infection, which takes more than a week to show. The first type generally lasts a few days and will cause nausea, fevers, diarrhea, etc. The second is more severe though and may often have to be treated. Which disease occurs depends on the type of Salmonella, there are several different species

2.2 Listeria monocytogenes

Listeria grow and live in the ground and can be found on all sorts of raw products, such as dairy or meats. Listeria monocytogenes is a quite strong and hardy bacteria, it can still grow in your refrigerator and it can withstand high salt contents (which a lot of bacteria can’t). Only when the pH is as low as 4,5 does it stop growing. Just like Salmonella, Listeria can be killed by a heat treatment of around 70C.

Once Listeria has entered the body it can cause quite an intense infection, especially for those with a weakened immune system. Also, Listeria monocytogenes is well known for pregnant women, it can cause infections of the unborn, even leading to miscarriages.

2.3 E. coli  – EHEC/STEC

E. coli  is a family member of Salmonella, both belong to the Enterobacteriaceae. E. coli actually is a regular bacteria in our own guts, where it has several beneficial functions. However, a few specific types of E. coli can be very harmful and cause disease.

In animals E. coli is also present commonly. It is part of the natural habitat. Therefore it is often hard to fully eliminate. Nevertheless, proper control is essential and you want to eliminate the harmful. I won’t go into detail of the different types, the most important are: EIEC, ETEC, EHEC and STEC.

Again, proper heating of your products will kill of these bacteria.

2.4 Staphylococcus aureus

S. aureus is also a bacteria that is very common in and on the human body. Contamination of food with this bacterium can often occur simply from the person handling the food. This bacterium doesn’t grow at temperatures below 6,5C, thus in your fridge. S. aureus causes disease through the Enterotoxins it produces. Whereas S. aureus can be killed by heat treatment, this is a lot harder when it comes to Enterotoxins. These can often resist 100C for up to an hour.

2.5 Clostridium botulinum

This anaerobic bacteria produces one of the most toxins we know of in food. Even a very small dose of this toxin is sufficient to initiate a very severe disease which in many cases will be deadly. Even more tricky is the fact that these bacteria can form so called spores. These spores can be highly heat resistant and can withstand harsh situations, such as low pH-values and high salt and sugar concentrations.

Therefore, all products with a risk of C. botulinum contamination should be sterilized (heat treatment at high temperatures for a prolonged duration) or have a pH value lower than 4,5.

2.6 Cross contamination

There is a lot more to learn about these micro-organisms, but I’ve tried to stay clear from too many disease discussions and focus on the influence of the foods on their growth.

One topic that has to be mentioned first though is cross-contamination. Micro-organisms can easily go from one food product to another, simply through touch or when you use the same cutting board or knive for a product.

Foods that have a high risk of contamination (e.g. chicken) should be kept away from food you’re planning to serve/eat raw.

3. The good stuff – Useful microorganisms

Luckily, it’s not only bad stuff when it comes to food microbiology. We also need micro-organisms in quite a lot of cases when making food. We will discuss a few of the most important and common examples.

3.1 Yeasts and bread

Yeasts are probably the most well known positive micro-organisms. Loads of us use yeast regularly when baking bread or other baked goods. Yeast you can buy in the stores are often in the form of dry powders. Just realize, there’s life in these powders, but during storage they have been put ‘on hold’.

During the rising process of bread the yeast consumes sugar and transforms this, amongst others, in ethanol and carbon dioxide. The carbon dioxide is a gas and will cause expansion of the dough. The ethanol produced will evaporate again during the baking process.

Without yeast it wouldn’t be as easy to make the well known fluffy breads we commonly eat nowadays.

3.2 Lactic acid and pickles

Lactic acid bacteria are very common in a lot of foods, especially when foods start getting bad. Souring of milk as well as sour meat are caused by lactic acid bacteria. They are anaerobic and form lactic acid (hence the name) when growing and digesting their ‘food’. There are a lot of different types of lactic acid bacteria, some will be more prevalent on meat, others on meat.

Proper pickles are made using lactic acid bacteria. They will be immersed in a liquid with spices, herbs, and salt and left in this liquid for up to several weeks at a comfortable temperature for the bacteria (18-20C). Some bacteria might be added at the start of the process, but otherwise the process will proceed by itself. The pH of the pickles will sink during this process due to the acids produced by the bacteria.

3.3 Others

There are a lot of other products for which micro-organisms play an important role. We won’t discuss all, but let’s name a few: cocoa, coffee, beer and sauerkraut.

4. Further reading

When it comes to food disease and prevention there’s a well known book, made by the FDA outlining all the most important food pathogens. It’s appropriately called the ‘bag bug book‘ and can be downloaded online. It’s one of the sources I’ve used for this course.

For those speaking German, I would also recommend ‘Lebensmittelmikrobiologie‘, a nice concise book discussing a wide variety of topics. Also a source I’ve used for this course, but with a lot more detail.

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