Arxiu d'etiquetes: viruses

Basic Microbiology (I): invisible world

The 7 September 1674 Anton van Leeuwenhoek said having watched a few tiny animals in a drop of water. What you referred to the concept of tiny animals? In many of our articles we refer to these organisms. Read on to start your journey into the fascinating world of the invisible. 


“They are imperceptible to the naked eye and abounded in such a way that the water seemed to be alive.” From a simple sample of water, Anton Leeuwenhoek concluded that there were tiny living organisms that were impossible to observe with the naked eye. With the help of a rudimentary microscope, he described the first microorganisms.

A world microscopic drawings of Leeuwenhoek over what he described as tiny animals. (Photo: Miguel Vicente, Madrimasd).

The concept of microorganism refers to a heterogeneous group of organisms that can only be displayed with the help of microscopes, since they have sizes lower than the limits of vision of humans (approximately 0.1 mm). They may be prokaryotic (bacteria), eukaryotic (Protozoa, algae, fungi…) and even entities acellular, as it would be the case of the virus. These organisms are measured by submultiples of the metro, more specifically in micrometers (μm, thousandth of a millimeter) and nanometers (nm, millionth of a millimeter).

The submultiples of the metro table (photo: Science Park).

This small size has its advantages: a high surface to volume ratio. This factor has an important biological effect. For example, the smaller cells tend to grow and multiply more quickly due to a rapid exchange of nutrients. Be reduced in size on the other hand, favors a more rapid evolution already that to multiply more quickly significantly increases the frequency of mutations (remember that mutations are the raw material of evolution). In addition, microorganisms more quickly adapt to the environment.

Let’s look at the different sizes that can be found in this large group of microorganisms. In the image below we can see a simple comparison between the various organisms and cells.

Different microorganisms and cells size scale. (Photo: Isabel Etayo).


This group of prokaryotes is characterized by a size that includes more than 700 μm and 0.2 μm. It should be noted that this group presents varied morphologies and therefore some are measured by diameter (spherical bacteria or coconuts) or by thickness and height (elongated bacteria or bacilli). A prokaryote’s average size is between 0.5 μm and 4 μm. The bacterium Escherichia coli is usually of approximately of 2 μm x 1 μm. In a small space, as the diameter of the point that there is at the end of this sentence would fit some 500 E. coli.

Size comparative diagram of different bacteria. (Photo: University of Granada).

The largest known bacterium is Thiomargarita namibiensis. This prokaryote was found in Namibia in 1999. Its size is 750 μm in diameter (0.75 mm), so they are almost visible to the naked eye. These microorganisms usually present as large as some nutrient storage mechanism, in this case sulfur. Another great example is that of Epulopiscium fishelsoni with a size of 600 μm. On the right side of the picture below we can see the comparison of the latter with  E. coli.

A. Picture of Thiomargarita namibiensis, of about 750 micrometers. B. comparison between Epulopiscium fishelsoni and E. coli. (Photos: Science Policy)

Having a microscopic size isn’t all advantages, it is obvious that there should be a lower limit. Sizes less than 0.15 μm in a bacterium would be almost impossible. Mycoplasma pneumoniae is the smallest bacterium, with a diameter of 0.2 μm. This is a bacterium without a cell wall which can be purchased in many different ways. Following the example of the final point, at 1 mm diameter would fit 5000 bacteria size of Mycoplasma pneumoniae.


In general, viruses have sizes much smaller than bacteria. They usually have sizes ranging from 20 to 300 nm. So the virus can be up to one hundred times smaller than a bacterium like E. coli. 

Comparison of sizes of different virus and E. coli. (Photo: diversidad microbiana)

The largest known virus is the Mimivirus. This presents 600 nm in diameter (larger than Mycoplasma pneumoniae). In the image below, you can see the comparison between the size of these giant virus and Rickettsia conorii (bacteria that causes human Boutonneuse Fever).

Comparison between Mimivirus and Rickettsia conorii. (Photo: byte Size Biology)

The Polio virus is one of the smallest viruses that are known, with a size of 20 nm (0.02 μm). If we could observe how many polio virus would fit on the point of the end of the sentence, would find some 50000 polio viral particles.


In Protozoa, the size remains varied. The average size is usually 250 μm in length. Even so, small protozoa as bacteria can be found (between 2 and 3 μm, like for example the Leishmania or Babesia) or large protozoa visible to the naked eye (from 16 mm in the case of Porospora gigantea). In the case of Leishmania can be seen as almost a hundred of bodies (thin arrow) can live inside a macrophage of a 30 μm (coarse black arrow).

Leishmania inside a macrophage (black arrow). The bar represents about 20 micrometers. (Photo: Thatawan Pothirat).

Microscopic fungi, such as yeasts, include sizes 6-20 μm. The best-known yeast is Saccharomyces cerevisiae with a size of oscillates between the 6 and 12 μm depending on its stage of ripeness. In the image below we can see an example very clear.

Size of the cells of Saccharomyces cerevisiae. (Photo: Easy notes).


“No view has reached my eye more pleasurable than this of so many living creatures within a small drop of water”. Anton Leeuwenhoek, in 1974, discovered an incredible invisible world.


  • Brock, Biología de los microorganismos. Editorial Pearson.
  • Ignacio López-Goñi. Virus y Pandemias. Editorial Naukas.
  • Cover Photo: Escuela y Ciencia.


Biological warfare: silent weapons

Today, the panic and terror that a biological attack is an important concern for the most powerful countries. What are biological weapons? What organisms are used for their production? Is there a possibility of a bioterrorist attack? This article is the answer to these and more questions.


The military use of microorganisms and substances derived from its activity is known as biological warfare. They are generally used to create disease, panic, terror and death in the population. If this objective is carried out clandestinely, we are talking about bioterrorism. Biological weapons are considered weapons of mass destruction, difficult to control and silent weapons (the detection time is extended due to the periods of incubation). 

These military methods have been used since ancient times. The Mongols threw the corpses infected with the plague against the walls of the cities enemy; in the war of 1867 in Paraguay, the soldiers threw corpses with cholera to wells to infect their enemies and decrease its troops. In the 20th century, with the increase of knowledge in Bacteriology and Virology, many countries began to produce biological weapons, as for example the creation of Fort Detrick in USA facilities and  Biopreparat laboratory in Russia. In 1972 was held the Convention on the prohibition of the production and distribution of biological weapons, was the arrival at an agreement to not produce this type of weaponry. However, in 2001, several civilian Americans suffered a biological attack with anthrax. The result was 5 deaths and an increase in insecurity against a biological attack. 

There are many types of biological weapons according to its objective (humans, plants or animals) or the biological component present (bacteria, viruses, toxins). In this article will analyze different biological agents most likely to use. The following table shows some examples, although the list could increase if we consider genetically modified agents.

Examples of bioagents used in biowarfare. Autor Duraipandian Thavaselvam



This type of weapons is composed of bacteria or its spores (forms of resistance). Most of these organisms are easily found in nature and some of them are genetically modified to acquire a greater capacity for infection, increased resistance to the conditions of the environment, etc.

The body that most feared is Bacillus anthracis, popularly known as anthrax. This form of Bacillus bacterium forms resistant spores that can remain infectious in the environment for years. It may infect via three routes: skin lesions, through ingestion of spores or by breathing them. The latter is the most dangerous case and most often used in biological warfare. Even so, this bacteria cannot be transmitted between people, which would be used in a certain range, like for example one person. 

Photomicrograph of Bacillus anthracis. The red circle designates the endospores. Public photo of CDC


A good biological weapon would be one that could spread through the air and spread person to person. Yersinia pestis is a bacterium that meets this profile. It is responsible for the known as the black death that took the lives of more than 50 million people in the 14th century. There are three types of plague: bubonic (the most common and transmitted by the bite of a flea), septicemic and pneumonic (the most virulent and the most interesting in terms of its use as a biological weapon). Currently, Yersinia pestis is controlled by antibiotics but a laboratory created a resistant strain through genetic modification. This fact would imply that current medicine would not serve to neutralize it.

Electron photomicrograph of scanning (SEM) of Yersinia pestis (Public Photo from CDC)


Bacteria of the genus Brucella (cause of human brucellosis), Francisella tularensis (cause of tularemia), vibrio chloreae (cause of cholera disease) and other microorganisms are considered possible bacteriological weapons. These last are classified at lower levels of endangerment due to factors such as previous immunization of the population. Note that some experts believe that there are many possible bacteria, that genetically modified, they could become dangerous biological weapons.


Viruses are infectious particles that can only multiply inside of other cells. So they have different specific mechanisms to enter and infect a cell and replicate inside it. These features make them the perfect biological weapon. Many of them are natural pathogens of man. They need small infective doses to produce the disease and can transmit different forms of person to person.

Smallpox is the best-known virus used as a possible biological weapon. Humans are the only natural carriers of this virus. This is very contagious and has a large virulence (capacity of infection). Currently, it is considered eradicated, but two samples were kept in laboratories in the United States and Russia. Theoretically, they should be removed at the end of 1993, but no one said that this had happened. The smallpox virus as a biological weapon would be very dangerous since currently population is not immunized and his could generate a new pandemic.

Microphotography with electron microscope of smallpox virus (public photo of the CDC)


Another very important virus in recent years is the Ebola virus. It produces hemorrhagic fevers with a high mortality rate. His diagnosis is difficult and its biological cycle is virtually unknown. These characteristics make this virus a perfect candidate as a biological weapon. Even so, note that it is a fragile virus that only survives a few hours in the middle. It has great difficulty of dispersion is not transmitted by air. There is no cure, but it can be treated in early stages by a serum with antibodies against the virus. Other virus hemorrhagic fevers as the Marburg viruses of the family Araviridae and others are also good candidates for biological agents for war. 

Microphotography with electron microscope of ebola virus (public photo of the CDC)



There are a large number of bacterial toxins that could be used as a biological weapon. They have high mortality rates, are very toxic and are easily produced, as it would be the case of the toxin of Clostridium  botulinum. These toxins produce botulism. Another interesting toxin is ricin (extracted from the shrub Ricinus communis) which has already been used as a biological weapon, has no antidote, and according to the CDC is one of the most powerful poisons that are known.

Using genetic modification has been achieved that bacteria such as Escherichia coli (that do not produce these toxins) can generate them. By inserting special genes in non-pathogenic bacteria, is becoming easier to produce large amounts of toxins.


DO NOT PANIC! Current Nations have extensive prevention and Biodefense programs. The research and knowledge of these microorganisms are the solutions to a possible biological attack.