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Colony Morphology

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Colony morphology describes the characteristics of cultures. It refers to characteristics visible to the naked eye of both bacterial and fungal colonies growing on an agar plate. It helps researchers to choose the appropriate confirmatory tests. While the methods might look simple, careful observation and examination of colony characteristics are essential in microbiology. When identifying an unknown microbe, observing the colony morphology is often the first step. From colour to size, the characteristics provide necessary clues for further testing and identification. Colony morphology of microbes became a prevalent practice in 1887 when the Petri dish was invented by Julius Richard Petri. It remains a core part of microbiological research since.

Colony Morphology


When identifying bacteria or fungi species, a lot of weight is put on how the organism grows on the solid media. The method is an important and cost-effective tool in microbiology. Bacteria grow on agar plates in colonies. Colonies are defined as a collection of cells, all originating from a single mother cell. All cells within a single colony are therefore genetically identical. Colony morphology is often used as a preliminary identification tool for microbes. Here are some key aspects on why the method matters:

  • Early identification of microbes - The characteristics exhibited by colonies give microbiologists clues about the organism's identity.
  • Guiding further testing - By examining the colony size, shape, colour, opacity, and consistency, microbiologists can determine which further tests are required.
  • Distinguishing between microbial species - Various microbe species produce specific colonies. This aids in differentiation of organism, strains, or even whether the species is pathogenic or not.
  • Detecting contamination in cultures - Colony morphology aids in identifying unwanted microbial contaminants that may be a risk to health.
  • Assessing growth conditions - The growth pattern of specific colonies can indicate how well a microorganism survives under specific environments. This provides an insight into the optimal culture conditions.
  • Recognising pathogenic microbes - Some pathogenic bacteria exhibit specific haemolysis pattern on blood agar, which helps speed up the identification.
  • Supporting epidemiological studies - Microbial outbreaks can be minimised by identifying bacterial growth based on the colony morphology.

Features of Colony Morphology


Colony morphology might change as the microbial colonies grow. Therefore, it is important to carry out the observations at specific time points after inoculation. This usually occurs at 18-24 hours post-inoculation. However, certain microbes, such as fungi, grow at a slower rate and require longer incubation. Microbiologists regularly assess various characteristics of microbial colonies to gather insights into their identity. These features include:

Feature Description
Size The diameter of the colony
Shape Colonies may appear circular, irregular or punctiform (pin-point sized)
Colours Some microbes produce pigments, affecting the colony's appearance
Opacity Colonies can appear transparent, translucent or opaque
Elevation Elevation types include flat, raised, convex, pulvinate (very convex), umbilicate (central depression), or umbonate (central bump)
Edge/Margin The perimeter may be smooth, rough, irregular or filamentous
Consistency The colonies can be brittle, creamy, sticky or dry
Haemolysis (blood agar)
  • Alpha haemolysis - partial digestion of blood resulting in a green halo
  • Beta haemolysis - complete digestion of blood, creating clear zone around colonies
  • Gamma haemolysis - no haemolysis, indicating no blood digestion
Odour Some microbes have distinctive smell
Motility and pigment Specific bacteria move across the agar or produce characteristic pigments

Steps in Identifying Microbe Species using Colony Morphology


  1. Choose a plate with well-isolated colonies and examine their shape and colour. This provides the initial assessment of colony morphology, which helps identify the obvious characteristics. These are then used to distinguish between microbes.
  2. Use dissecting microscope for detailed observation. This allows the observation of characteristics that might not be visible to the naked eye. Among others, these include surface texture, edge structure and minor colour variation.
  3. If condensation is present, carefully remove excess water before examination. Water droplets on the lid of the petri dish can effect the view of the colonies. This could lead to unwanted spreading of microbes, which may affect accurate observation.
  4. Use a sterile inoculating loop to assess colony consistency by gently touching and lifting the colony. This step helps determine whether the colonies are sticky, brittle or creamy. If the colony adheres to the loop, it may determine a mucous-like texture. While if the colonies are brittle, it suggest dry and friable colony.
  5. Observe colony elevation by tilting the plate at an angle. This helps distinguish between flat, raised, convex, or umbonate structures.
  6. Take a note of any haemolysis you observe on the blood agar. The presence and specific type of haemolysis gives the microbiologist important information about pathogenicity and species classification.
  7. Document all observations carefully. Keeping detailed notes of colony morphology, growth conditions, and any additional characteristics will help with accurate identification and reproducibility of the experiments.

Colony Morphology in Bacteria


Form

Colony morphology in bacteria is one of the fundamental aspects of microbiology. One of the most noticed characteristics is the colony shape. Each of these shapes has unique characteristics. Circular colonies are the most common. The colonies are usually uniform and symmetrical, with smooth edges. Bacteria that grow rapidly and evenly usually form circular colonies. These colonies are usually the easiest to identify, thanks to their regularity. Filamentous colonies are very distinct thanks to their thread-like structure. Bacteria with mycelial growth patterns are usually responsible for forming these colonies. The filamentous shape indicated a complex colony structure which is associated with soil-dwelling bacteria. Irregular colonies have a more complex morphology with uneven and jagged edges. It is unsure what causes the irregularity, however, it is thought that the type of agar, bacterial species or environmental condition may have an effect. Rhizoid colonies are similar to filamentous colonies. However, they are more branched and root-like. The colonies grow in a pattern resembling a root of a plant, which is easily distinguishable on agar.

Elevation

The elevation of bacterial colonies serves as an extra layer of detail that might be necessary for the identification of the species. This characteristic represents the rise of colonies above the surface. Slow-growing bacteria often exhibit flat colonies that barely rise above the agar. Raised colonies are often associated with bacteria secreting extracellular substances. These colonies are elevated above the agar and often form a very distinct dome-like structure. Convex colonies form a more rounded, bulging shape and often point out good growth conditions. On the other hand, umbonate colonies are more complex with a small bump in the middle.

Margin

The margin/edge of a bacterial colony is another feature that microbiologists observe. The appearance of the colony's edge can reveal important details about the growth pattern and environmental interactions of the bacterial species. Smooth/entire edges are associated with their even and uninterrupted edges, and bacteria with consistent metabolic processes. Undulating margins are wavy and irregular that are more complex and eye-catching. Lobate margins feature finger-like projections that extend away from the colony. Arguably the most complex looking margins are filamentous margins. Although they have structures pulling away from the colony, similar to lobate margins, filamentous margins are more thread-like.

Types of bacterial colonies
Types of bacterial colonies

Colony Morphology in Fungi


Fungi, as eukaryotic organisms, display unique growth patterns, appearing either as unicellular yeasts or multicellular filamentous moulds. To study fungal colonies, microbiologists commonly use specialised media such as potato dextrose agar. When describing fungal colony morphology, key characteristics to observe are the colour of the surface, the colour on the reverse side of the colony, texture and growth rate. These features helps differentiate fungal species and provide valuable insights into their identification and behaviour.

Factors that Influence Colony Morphology


Colony morphology is not only dependent on the species. Other factors such as the media and growth conditions can influence the appearance of a microbe. While selective or differential media can influence specific colony characteristics, nutrient-rich media promote fast microbial growth. Some bacteria prefer to grow at body temperature (37°C), while others develop pigments and grow at a slower rate when at room temperature. Therefore, the temperature plays a crucial role in colony morphology. The period of incubation has an impact on the size and features of the colonies. Longer incubation times often result in further development of the colonies. Lastly, interactions with neighbouring microbes can have an influence on colony morphology. This is because competition or symbiotic relationships may have an effect on patterns and characteristics of colony morphology.

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References


  1. J. Jackman The Microbe: The basics of structure, morphology, and physiology as they relate to microbial characterization and attribution Chemical and physical signatures for microbial forensics, 2011
  2. A.M. Sousa, I. Machado, A. Nicolau & M.O. Pereira Improvements on colony morphology identification towards bacterial profiling Journal of Microbiological Methods, Volume 95, Issue 3, 2013

Contributors


Written by

Linda Vidova

Scientific Writer

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