Bacteria – Size, Shape And Arrangement

The field of microbiology is, perhaps a little ironically, massive in scope. For living things that are so small, there is so much to learn about them, it can almost be dizzying.

Then again, perhaps that isn’t too much of a surprise.

Considering that bacteria make up the building blocks of almost every other life form on Earth, perhaps it’s no wonder that these tiny microorganisms have so much information in and around them.

And considering that it is believed that pretty much every complex life form on Earth is descended from a common ancestor with bacteria billions of years ago, they’ve certainly had plenty of time to continue evolving and diversifying in that time!

With so much information out there, it can feel like you’re swimming in an ocean of disjointed facts at having no bearing on each other.

Bacteria - Size, Shape And Arrangement

Truly, information overload on a scale this big is the stuff of nightmares of school kids, as well as scientists and researchers alike.

Fortunately, once you start to get a grasp on how all that information is related and connected, you can start to get an idea of the bigger picture and the process of microbial life.

Or, at the very least, you can start to understand where you need to start focusing and researching next.

That is why we have created this guide to Bacterial life and cells. In this article, we are going to outline and explain what exactly bacteria are, the scale and size that they operate on, as well as their main properties, and how they are structured.

Not only that, but we will also explain how bacteria are classified into scientific groups, and explain how they are related and connected, as well as some of the crucial details that make them distinct from one another.

There’s a lot of ground to cover with these little organisms, so let’s get started!

A Brief Introduction To Bacteria

Before we start to explain even the simpler aspects of bacteria, we first need to establish what bacteria are in the first place, and what makes them a useful classification in the first place.

Despite how often bacterial infections are brought and discussed in mainstream news and media, especially in the last few years, even the basics of these biological building blocks often get lost to people.

What Are Bacteria?

In the simplest terms, bacteria are a form of microorganisms, also known as microbes, and are some of the oldest living things on earth.

As we mentioned in the introduction, it is wildly believed that pretty much every other organism on Earth evolved from a common ancestor with bacteria some 2 billion years in the past.

The main distinction between bacteria and other microbial life, such as those that are found in plants and animals, is the simplicity of their cell structure.

Bacteria consist of a single biological cell, in which all the genetic material to live and survive is contained within just a single strand of DNA, a topic that we will discuss in greater detail later in this article.

This simplicity is what the vast majority of bacterial life has in common, which allows scientists to place bacteria in an evolutionary group of their own, known as prokaryotes, also known as prokaryotic cells, organisms that are made of a single cell, and lack the components of more complex life forms, such as a nucleus.

What Size Are Bacterial Cells?

The other thing that distinguishes bacteria from other celled life is their size.

When discussing measurements in microbiology, our normal units of measurement simply don’t accommodate sizes at this tiny scale.

In most cases, when measuring microbes, scientists will measure microorganisms using micrometers (or µm), which is to demonstrate a distance that is one-millionth of a unit or measurement or meter.

The size of a bacterial cell will depend on the exact type and species of the bacteria itself, and these can vary wildly at this scale. Generally speaking, however, bacteria will measure between 0.5 to 10 µm in size. 

Generally speaking, this means that the vast majority of bacteria are smaller than other microbial life such as Eukaryotic cells, the group that includes virtually all multi-celled life forms, including plants, animals, and fungi.

However, this isn’t to say that there aren’t some examples of bacteria that are larger than their more complex relatives and descendants. Spirochetes are a family of bacteria known for their spiraling shape and can reach lengths of up to 500 µm.

The world’s largest known bacteria, Thiomargarita namibiensis, can grow so large that it can be seen by the human eye, reaching sizes of over 750 µm, or 0.75mm!

These are outliers, however, and most species are generally impossible to see without the use of a light or electron microscope.

The Properties & Structure Of A Bacterial Cell

The Properties & Structure Of A Bacterial Cell

So, we now know a little more about what exactly bacteria are, and how small they usually are when compared to other types of microscopic life and organisms.

However, to understand some more of the differences between a bacterium and other organisms, we have to take a look inside the contents of a bacterial cell.

As we mentioned earlier, simplicity is the name of the evolutionary game that bacteria have been playing for 2 billion years of Earth’s history.

Whilst each section can be broken down in more detail, there are effectively 4 main components to a bacterial cell: A cell wall, cytoplasm, flagella, and the Nucleoid.

  • Cell Wall – This is the outermost layer of a bacterial cell, and protects the internal contents of the cell from the outside environment. This also gives bacteria cells their many and varied shapes and arrangements (a topic will return to later on).
  • Cytoplasm – This is where the main functions of the bacterial cell take place. This is where the bacteria will metabolize the energy they use, where cell growth takes place, as well as where replication and reproduction take place. Cytoplasm itself is made from an almost gel-like substance that includes a range of nutrients, waste, enzymes, proteins, and water, amongst other materials that the cells require
  • Flagella – This is the main form of locomotion or movement that most bacterial and microbial life have. Flagella appear as a small, hair-like structure that can be found on the surface of the bacteria’s cell wall, and moved in a spiraling fashion to move the cell forward. A bacterial cell can have multiple flagella, depending on the variety and species it is.
  • Nucleoid – As bacterial cells do not have a true nucleus like more complex organisms such as eukaryotic cells have, DNA is stored in a center of cytoplasm gel in the bacteria. The main distinction between a nucleus and a nucleoid is, that although both carry the genetic material of their respective forms of life, nuclei are surrounded by a membrane from the rest of their organism’s functions, whereas a nucleoid is found in the same cytoplasm that the rest of the cell is.

As you can see, with only a few main functions, the internal structure of bacteria is very different from the eukaryotic cells that evolved from them, lacking most of the extra structures that manage more complex processes, which are called organelles.

They do not carry any mitochondria that most complex life forms have. They do not have any chloroplasts that plant cells have, and, as we just mentioned, they don’t even have nuclei!

How Do Bacteria Reproduce?

As very simple organisms, bacterial cells can reproduce and multiply by using simple binary cell fission.

In this process of reproduction, the DNA of the bacterium will divide into two replicas, with enough genetic material for both the existing cell, as well as for the new cell.

At this point, the cell will then elongate, with a replicate of DNA going to each side, before they eventually separate, creating two new cells.

How quickly bacteria reproduce can vary massively depending on the species and their ideal living conditions. However, in those conditions, bacteria can replicate incredibly quickly, in less than 30 minutes in some species.

This means that a single bacterium of a fast-growing species, in less than 10 hours, can generate more than 16 million new bacteria!

Small wonder that bacterial infections can have such an alarmingly quick effect, or recover quickly, even after they have been neutralized by antibiotics.

What Separates Bacteria From Viruses?

On the subject of infections, viral and bacterial infections are terms that are often used interchangeably to describe infections and diseases in general. However, they share very few qualities in common.

Generally speaking, although both types of microbes can have negative effects on larger organisms, bacteria are generally slightly large than most viruses, as can be observed through both a light-powered microscope and an electron microscope, whereas viruses can only be seen with the latter.

Bacteria are also free-living microbes, that usually do not need to be present inside a body to live and replicate, and are their life forms.

Viruses, meanwhile, are not living organisms, as they are only a collection of smaller molecules, and do require some kind of host body to survive and spread.

What Shapes Do Bacterial Cells Take?

We briefly mentioned in the last section how bacterial organisms can take on a large array of shapes, thanks to the formation of their cell walls.

Generally speaking, bacteria can be separated into three broad groups of shapes: Coccus bacteria (sphere-shaped), Bacillus bacteria (rod-shaped), and Spiral bacteria (twisted-shaped).

All gram-positive bacteria, meaning that they can be identified by appearing purple when subjected to the Gram stain test.


This designation is given to a wide range of bacterial families and species that are generally round in their shape, although some species are oval or somewhat squished by comparison.

The bacteria that are classified as part of the group Cocci can range from totally benign species to many species that can become serious pathogens if given the opportunity, such as pneumococcus, which can lead to bacterial pneumonia.


As we mentioned before, bacillus bacteria are best identified through their long, rod-like appearance.

Bacillus can survive in suboptimal conditions for very long periods, thanks to their ability to transform into endospores.

This usually happens when they cannot detect nutrients to absorb and allows them to survive in conditions that would be completely inhospitable to other organisms, such as temperatures exceeding 400 degrees Celsius, or over 750 degrees Fahrenheit.


These bacteria softly twist into a curved shape that corkscrews as it gets longer.

Many larger species of bacteria are part of this classification, as they can get quite long, although their diameter is very rarely more than a few micrometers wide in diameter.

How Are Bacterial Cells Arranged?

So, we have established that most bacteria exist within one of these three orders. But within these groups of shapes, however, there is a massive variety in the number of arrangements that bacteria can take.

Cocci Arrangements

Cocci Arrangements
  • Cocci bacterial cells can be arranged in a wide variety of formations. Many can exist as single-cell arrangements, whereas others can form pairs that can become chains of bacteria. Pathogens such as Streptococcus Pneumoniae, the cause of bacterial pneumonia, are often observed to form chains with other bacteria of the same species.
  • Cocci are also able to form collections of four cells and are known as tetrads, as the bacterium is joined with three other bacteria to make this arrangement, such as Aerococcus. This bacteria is often found in brines that are made for curing meats. 
  • Similarly, Sarcinae is like tetrads but forms a cube by joining with another tetrad.
  • Lastly, cocci can form masses of grape-like clusters formed by the bacterial cells dividing unevenly. Bacteria like Staphylococcus, which can lead to staph infections, can be found like this.

Bacillus Arrangements

Like the cocci bacterial cells, bacilli cells can be found in a wide variety of shapes and arrangements.

  • Single bacillus cells will appear as a single rod-like formation, whereas diplobacilli will join end-to-end, forming the start of what looks like a small chain.
  • Streptobacilli are similar to diplobacilli and form a chain of multiple organisms that join from end to end. S. moniliformis, a pathogen that can exist in the respiratory tracts of rats, falls into this category and can cause rat-bite fever in the most aggressive cases.
  • Coccobacilli is a type of bacteria that exists between the cocci and bacilli groups, forming a somewhat oval shape, which still has some of the properties that bacillus bacteria have. Haemophilus influenzae, a bacterium that can cause bacterial influenza, falls into this category.
  • Palisades are bacilli bacteria that form chains, but start to curve and bend inward as the bacterial cells start to divide, and appear to join together width ways, rather than end-to-end, forming a fence-like structure.

Spiralla Arrangements

The main component of spiralla bacterial arrangements, as we have mentioned a few times, is the intensity of the curve or twist that these bacteria form when they join together.

  • Vibrios are the simplest formation that you’ll find out there, appearing as a comm when viewed under a microscope, and don’t have a full spiral in them.
  • Spirallas is where we start to see the trademark corkscrew begin to form.
  • Spirochetes are much longer arrangements of spirallas, and appear to move around by rotating similarly to a single cell flagella, though it is tricky to spot actual flagella on these bacteria. Leptospira is a genus of bacteria that form like this and can cause flu-like symptoms.

Final Thoughts

We’ve covered a lot of information in this piece. But hopefully, you are starting to understand the sheer complexity of these tiny little lifeforms, and the depths of information that there is about them.

Jennifer Dawkins