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Amazing discovery: How to identify bacteria by the smell of colonies?
In microbiology, colony morphology refers to the visual appearance of bacterial or fungal colonies on an agar plate. Examination of colony morphology is the first step in identifying unknown microorganisms. Microbiologists systematically evaluate the appearance of colonies, focusing on characteristics such as size, shape, color, opacity, and consistency, which provide clues to identifying the organism and allow them to select appropriate tests for definitive identification.
When the sample arrives at the microbiology laboratory, it is inoculated onto an agar plate and placed in an incubator to encourage microbial growth. The appearance of colonies changes as they grow, so check colony morphology at specific times after inoculation.
Typically, plates are observed 18 to 24 hours after inoculation, but slow-growing organisms (such as fungi) may require longer. A microbiologist will examine the colony's appearance and note specific characteristics like size, color, shape, consistency, and opacity. A handheld magnifying glass or magnifying glass can be used to view colonies in more detail.
The transparency of a colony can be described as transparent, translucent, or opaque. Staphylococci, for example, are usually opaque, while many Streptococcus species are translucent.
The overall shape of a colony may be characterized as round, irregular, or small points (such as pinpoints). Vertical growth or protrusion of a colony is also another identifying characteristic, which can be assessed on a side-slanted agar plate and the shape will be described as flat, raised, rounded, raised (very raised), centrally depressed, or with a raised point, etc. Colony edges can be described by terms such as smooth, rough, irregular, and filamentous. For example, the filamentous appearance of Bacillus anthracis is sometimes described as resembling the head of Medusa.
Consistency is checked by physically manipulating the colonies with sterile tools and can be described using terms such as crispy, creamy, sticky, and dry. Staphylococci are considered to have a creamy consistency, while some Neisseria species are slimy, and colonies of diphtheriae and beta-hemolytic Streptococci are often dry.
Certain capsule-producing bacteria often have a viscous (mucus-like) consistency. When certain microorganisms grow on blood agar, they may digest the blood in the medium, causing visible hemolysis (destruction of red blood cells) on the agar plate. In colony morphology, hemolysis is classified into three types: alpha hemolysis, beta hemolysis, and gamma hemolysis. In alpha hemolysis, the blood is partially digested, causing the area around the colonies to turn green. In beta hemolysis, the organism completely digests the blood, leaving a clear area around each colony. Organisms that do not produce hemolysis are called gamma hemolytics.
Odor-producing colonies are sometimes considered part of the colony morphology. While intentionally sniffing microbial cultures is not recommended, certain organisms produce distinctive odors that can be detected during routine inspection of cultures.
For example, Pseudomonas aeruginosa has a grape-like odor; Staphylococcus aureus is thought to smell like old socks, and Proteus' odor has been alternately described as rancid or like chocolate cake. Other unique features of colony morphology include motility and pigment production. Pseudomonas aeruginosa produces cyanine and green pigments, which give the colonies a green sheen.
Certain Rhodoproteobacteria produce an orange-red pigment called red. Some organisms capable of extended motility (e.g., Proteus species) exhibit concentric waves of growth extending outward from the site of inoculation.
Colony morphology serves as the first step in identifying microbial species from clinical samples. Based on the visual appearance of colonies, microbiologists are able to narrow down the list of possible organisms, allowing them to select the appropriate test for a definitive diagnosis.
For example, if a microbiologist observes a colony that resembles a Staphylococcus species in shape, they might perform a catalytic enzyme test to confirm that it is a Staphylococcus species and a coagulase test to determine whether it is a coagulase-negative Staphylococcus or More pathogenic species such as Staphylococcus aureus. Observation of hemolysis is very useful for the prior identification of streptococci because they are classified based on the hemolytic reaction.
For example, Streptococcus pyogenes, which cause sore throats and scarlet fever, show beta hemolysis, while Streptococcus pneumonia, which can cause pneumonia and meningitis, shows alpha hemolysis. The highly pathogenic Staphylococcus aureus typically shows beta hemolysis, whereas Staphylococcus epidermidis, which is part of the normal flora of the skin and occasionally becomes an opportunistic pathogen, shows weak or no hemolysis.
Although modern techniques such as MALDI-TOF are increasingly used to identify microorganisms in clinical laboratories, colony morphology is still very useful for distinguishing potential pathogens from normal flora that must be identified (definitive identification is not required) , and can be used to confirm identification when automated techniques give inconclusive results.
In these huge microbial worlds, can the shape and smell of bacterial colonies serve as inspiration for your in-depth exploration of microbial identification?
