Looking at Microbes

Microbe Size

Microbes are often very small, even in comparison to microscopic cells from animals.

Learning Objectives

Recall the size of microbes in comparison to human cells and viruses

Key Takeaways

Key Points

  • Microbes are generally described as being microscopic in size. Therefore, they are smaller than a human eye can see.
  • The size of microbes can be hard to imagine because they are so small. In comparison to animal cells, microbes tend to be smaller. They are about 1/10th the size of a typical human cell.
  • Microbes are generally measured in the scale of one millionth of a meter, which is known as a micrometer.

Key Terms

  • protozoan: Any of the diverse group of eukaryotes, of the phylum Protozoa, that are primarily unicellular, existing singly or aggregating into colonies, are usually nonphotosynthetic, and are often classified further into phyla according to their capacity for and means of motility, as by pseudopods, flagella, or cilia.
  • macroscopic: Visible to the unassisted eye.
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A Microbe versus Animal Cell: The large spheres are tick cells. The purple bars and dots are the bacterium Rickettsia rickettsii, which is the causative agent of Rocky Mountain spotted fever. Rickettsia rickettsii is a small bacterium that grows inside the cells of its hosts. These bacteria range in size from 0.2 x 0.5 micrometers to 0.3 x 2.0 micrometers.

Microbiology is the study of microbes. The name of the field is driven by the tool that largely determines if something is a microbe. Basically, microbiology is the study of organisms that one needs to use a microscope to visualize. Of course, there are exceptions. There are types of microscopes that visualize to the atomic level, which is significantly smaller than microbes. Alternatively, there are single cell organisms, such as some types of green algae and some protozoans that are generally studied by microbiologists. These are macroscopic or view-able without a microscope.

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Ventricaria ventricosa: Ventricaria ventricosa is one of the largest known unicellular organisms. They about the size of a ping pong ball.

The size of microbes can be hard to imagine because they are so small in comparison to what most people see day to day. Even in comparison to animal cells, microbes tend to be smaller. They are about 1/10th the size of a typical human cell. So, a microbe such as a bacteria cell would be the size of a cat or small dog in comparison to a human-sized animal-cell. Viruses are about 1/10th the size of other microbes such as bacteria. Therefore, if a bacteria is the size of cat, then a virus would be about the size of a mouse.

To put a numerical value on microbial size, most measurements of microbes are done with the unit of measure of micrometer, which is one millionth of a meter (one 2,500th of an inch). In relation to something more tangible a period or “. ” is about 0.5 millimeters or 0.5/1,000th of a meter. A typical microbe would be about 1/500th of a period.

Of course there are exceptions. Some unicellular organisms studied by microbiologists are macroscopic. This includes Valonia ventricosa, which can be up to 5 cm in length. It is a member of the Chlorophyta phylum which are a sub-group of green algae. Many types of green algae are not microscopic, but they are often studied by microbiologists.

Units of Measurement for Microbes

Measuring microbes presents challenges because they are very small, requiring indirect measures of microbes to understand them better.

Learning Objectives

Recognize the methods used to measure microbial growth

Key Takeaways

Key Points

  • Understanding microbiological life means quantifying it. Since microbes are so small, this is a challenge.
  • The size of a microbe is usually measured in micrometers, or one millionth of a meter.
  • There are many aspects of microbes that can be measured in addition to size, including metric like genome size and growth rates.

Key Terms

  • flow cytometry: A technique used to sort and classify cells by using fluorescent markers on their surface.
  • genome: The complete genetic information (either DNA or, in some viruses, RNA) of an organism, typically expressed in the number of basepairs.

Microbes are broadly defined as organisms that are microscopic. As a result, measuring them can be very difficult. The units used to describe objects on a microscopic length scale are most commonly the Micrometer (oi) – one millionth of 1 meter and smaller units. Most microbes are around 1 micrometer in size. Viruses are typically 1/10th that size. Animal cells are typically around 10 micrometers in size. However, length is not the only measurement that pertains to microbes. Microbes have genomes and these are typically smaller than the genomes of macroscopic organisms such as humans. DNA is measured in base pairs of DNA. For example, the human genome is about 3.4 billion base pairs while the common intestinal bacteria Escherichia coli is 4.6 million base pairs. Additionally, microbes are usually not weighed individually, but can be as an aggregate for various experiments. An estimate of the weight of an individual microbe can be made by estimating the number of microbes. This is especially important for biomass studies where the units of measurement are in units like picog, 10-12 of a kilogram (Kg), nanogram 10-9 of a Kg, and microgram, 10-6 of a Kg (a kilogram is a little over 2 pounds).

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Bacterial Growth Curve: This chart shows the logarithmic growth of bacteria. Note the Y-axis scale is logarithmic meaning that the number represents doubling. The phases of growth are labelled on top.

Microbial growth is an important measure in understanding microbes. Microbial growth is the division of one microbe into two daughter cells in a process called binary fission. As a result, “local doubling” of the microbial population occurs. Both daughter cells from the division do not necessarily survive. However, if the number surviving exceeds unity on average, the microbial population undergoes exponential growth. The measurement of an exponential microbial growth curve in batch culture was traditionally a part of the training of all microbiologists; The basic means requires bacterial enumeration (cell counting) by direct and individual (microscopic, flow cytometry), direct and bulk (biomass), indirect and individual (colony counting), or indirect and bulk (most probable number, turbidity, nutrient uptake) methods. Since there are limits on space, food, and other factors, actual growth never matches actual measured growth.

Refraction and Magnification

The underlying principal of a microscope is that lenses refract light which allows for magnification.

Learning Objectives

Describe refraction and distinguish between convex and concave lenses

Key Takeaways

Key Points

  • Convex lenses allow light to converge.
  • Concave lenses spread light that travels through it.
  • There are limits to the amount of refraction that can be done by a material, and therefore, limits to the amount a microscope can magnify a sample.

Key Terms

  • concave: Curved like the inner surface of a sphere or bowl.
  • convex: Curved or bowed outward like the outside of a bowl or sphere or circle.
  • specular: Pertaining to mirrors; mirror-like, reflective.

Refraction and Magnification

Refraction occurs when light travels through an area of space that has a changing index of refraction. The simplest case of refraction occurs when there is an interface between a uniform medium with an index of refraction and another medium with an index of refraction.

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Refraction: As the light is reflected off the pencil we see that, due to the different refraction indexes of water and air, the pencil appears to bend in the water. However, the pencil is straight. It is actually the water acting much like a lens in a microscope that gives it the appearance of bending.

Some media have an index of refraction that varies gradually with position. Therefore, light rays curve through the medium rather than travelling in straight lines. This effect is what is responsible for mirages seen on hot days where the changing index of refraction of the air causes the light rays to bend creating the appearance of specular reflections in the distance (as if on the surface of a pool of water).

Taking advantage of the principle of refraction, devices can be built that can focus light. A device that produces converging or diverging light rays due to refraction is known as a lens. In general, two types of lenses exist: convex lenses, which cause parallel light rays to converge, and concave lenses, which cause parallel light rays to diverge. The former property of convex lenses is of special interest to microbiologists. In essence, a convex lens allows magnification. Light reflecting off an object is focused to a point. The simplest example of this that most people know is a magnifying glass. A magnifying glass is one convex lens, and this by itself allows the magnification of objects.

A microscope is basically a series of lenses that take advantage of the nature of refraction. Due to the nature of light, and the maximum amount of refraction that can be possible by a material, there are limits to the amount of magnification that can be done by a light microscope.

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The Microscope: Notice the blue areas, which represent lenses. Note also that many of the lenses are convex, thus the light that goes through a specimen is focused and therefore magnified. Labels: A) Ocular, B) Objective, C) slide holder, D) Illumination Lenses, E) Slide Stage, and F) Illumination Mirror

Magnification and Resolution

Magnification is the enlargement of an image; resolution is the ability to tell two objects apart.

Learning Objectives

Define magnification and resolution

Key Takeaways

Key Points

  • Magnification is the ability to make small objects seem larger, such as making a microscopic organism visible.
  • Resolution is the ability to distinguish two objects from each other.
  • Light microscopy has limits to both its resolution and its magnification.

Key Terms

  • airy disks: In optics, the Airy disk (or Airy disc) and Airy pattern are descriptions of the best-focused spot of light that a perfect lens with a circular aperture can make, limited by the diffraction of light.
  • diffraction: the breaking up of an electromagnetic wave as it passes a geometric structure (e.g., a slit), followed by reconstruction of the wave by interference
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Aging Tissue and Vision Loss: These are micrographs of a section of a human eye. Using computer algorithms and other technology the panel on the right has a higher resolution and is therefore clearer. It should be noted that both panels are at the same magnification, yet the panel on the right has a higher resolution and gives more information on the sample. The labels represent various parts of the human eye: Bruch membrane (B); choroid (C); retinal pigment epithelium (RPE); and retinal rod cells (R). The scale bar is 2um.

Magnification is the process of enlarging something only in appearance, not in physical size. This enlargement is quantified by a calculated number also called “magnification. ” The term magnification is often confused with the term “resolution,” which describes the ability of an imaging system to show detail in the object that is being imaged. While high magnification without high resolution may make very small microbes visible, it will not allow the observer to distinguish between microbes or sub-cellular parts of a microbe. In reality, therefore, microbiologists depend more on resolution, as they want to be able to determine differences between microbes or parts of microbes. However, to be able to distinguish between two objects under a microscope, a viewer must first magnify to a point at which resolution becomes relevant.

Resolution depends on the distance between two distinguishable radiating points. A microscopic imaging system may have many individual components, including a lens and recording and display components. Each of these contributes to the optical resolution of the system, as will the environment in which the imaging is performed. Real optical systems are complex, and practical difficulties often increase the distance between distinguishable point sources.

At very high magnifications with transmitted light, point objects are seen as fuzzy discs surrounded by diffraction rings. These are called Airy disks. The resolving power of a microscope is taken as the ability to distinguish between two closely spaced Airy disks (or, in other words, the ability of the microscope to distinctly reveal adjacent structural detail). It is this effect of diffraction that limits a microscope’s ability to resolve fine details. The extent and magnitude of the diffraction patterns are affected by the wavelength of light (λ), the refractive materials used to manufacture the objective lens, and the numerical aperture (NA) of the objective lens. There is therefore a finite limit beyond which it is impossible to resolve separate points in the objective field. This is known as the diffraction limit.