Why Don’t Bacteria Get Bigger or Smaller?

Why Don’t Bacteria Get Bigger or Smaller?


It is common knowledge that bacteria are small, microscopic organisms. However, the true scale of their miniature size cannot be easily comprehended. For instance, most bacteria range from 0.2 – 2.0 micrometres in diameter. In context, a dust mite’s waste matter is about 5 micrometres in size. A strand of human hair is between 100-150 micrometres wide. 

 The average human eye can see objects only larger than 50 micrometres, hence, we can see the specs of dust that shimmer in a ray of light. According to the classification of bacteria, the Mycoplasma genitalium is the smallest bacteria discovered till date, measuring in at 0.2 micrometres. There may be smaller organisms, but not bacteria. Read on to discover why bacteria smaller than 0.2 is unlikely to exist. 

Size Constraints

On the other end of the spectrum is the Thiomargarita namibiensis – the biggest bacteria ever found, measuring in at 0.75 millimetres. This bacteria is almost big enough to be seen with the naked eye. However, no bacteria larger than Thiomargarita or smaller than Mycoplasma have ever been found. Computational biologists have put forth an interesting explanation to this problem. The simulations have predicted that bacteria, smaller than 0.2 micrometres, would not have enough space to contain all the cellular organelles. Furthermore, without these organelles, bacteria cannot survive as they cannot carry out their life processes. 

Overcoming Size Constraints

The simulations also show that bacteria larger than the Thiomargarita namibiensis require huge energy demands. To satisfy these energy demands, the bacteria will require a vast number of ribosomes, which cannot be incorporated within bacteria due to space constraints. In conclusion, the structure of bacteria is limited by size and a variety of other factors. However, millions of years ago, the ancestors of bacteria would have faced a similar problem – they may have evolved special adaptations, eventually leading to other forms of life. This is based purely on speculation though – as evidence of the same does not exist.

Beyond Bacteria

Viruses are smaller than most bacteria, yet they seem to thrive and exist in almost every known organism. However, viruses do not contain all the necessary cellular organelles most living organisms would carry. Hence they fall under a grey area – they are neither living nor non-living. Hence, viruses need to infect their hosts and hijack their cellular machinery for self-replication.

Then, there are entities smaller than viruses called prions. These entities are known to cause diseases such as CJD and Mad Cow disease. These entities are not living organisms – but they have the capability to infect and reproduce just like living organisms. Structurally, these entities are essentially a strand of protein that is folded the “wrong way”. When these misfolded proteins come in contact with other healthy proteins, they tend to deform.

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Kimberly Bernard