When identifying bacteria, it is necessary to know the different microbial species in samples in clinical and laboratory settings. This paper will discuss three major categories used to classify bacteria: gram staining, need for oxygen to grow, and shape. These categories have allowed researchers to identify unknown bacteria into broad categories and have allowed for easy identification of what tests should follow.
In clinical medicine and diagnostic settings, the gram stain is often the first procedure used to classify bacteria 1. This is because the staining procedure is a rather simple and fast test to identify bacteria. The results from a gram stain divide species into two specific categories: gram positive and gram negative 2. For example, an unknown bacterial sample would first be identified as gram-negative or gram-positive in the following procedure. The procedure (for a gram stain) involves staining samples with crystal violet, a stain that binds to peptidoglycan in both gram-negative and gram-positive bacteria 3. The next step is to apply iodine solution that binds with crystal violet and creates an insoluble combination. A decoloration step follows, which removes the purple color from gram-negative bacteria due to an outer-membrane covering the peptidoglycan, while gram-positive retain their purple color. Finally, safranin is added to bacterial samples as a counter stain, in which gram–negative bacteria’s peptidoglycan can now bind to the stain, becoming a pink color 4. However, there are some examples in which gram-staining is not effective due to the microbe’s structure, such as anaerobic bacteria and archaea species 1, 4. In these cases, in addition to the gram stain, other steps must be taken; such as using potassium hydroxide with anaerobic bacteria for stringing to occur and indicate a positive result (gram-negative bacteria), while gram-positive bacteria displayed no reaction 1. In a laboratory or clinical setting, known gram-negative or gram-positive organisms are used to compare to unknown bacteria 2. The controlled organisms should be stained while comparing to unknown bacteria to make sure the procedure and reagents used work correctly. With the results of gram-staining, researchers determine what should then be the subsequent identification procedures 1.
Another broad category to classify bacteria into, would be on if the species needs or does not need oxygen to grow. This is referred to as the bacteria surviving in an anaerobic or aerobic environment. Aerobic bacteria require oxygen to grow, while anaerobic bacteria survive in environments where there is a minimal or no amount of oxygen 5. For example, while classifying unknown bacteria selective media is used to identify if the microbe is either anaerobic or aerobic 6. A meat-peptone broth is a common medium used for aerobic bacteria to identify growth, while thioglycollate medium with reducing agents was a common medium to identify growth of anaerobic bacteria. Finally, the classification of anaerobic and aerobic bacteria is important in clinical settings to identify bacteria that cause human infections and disease 7. In some cases, while classifying an unknown bacterium into anaerobic or aerobic in a clinical setting, anaerobic or aerobic jars are used. This creates oxygen-free (or oxygen-sensitive) environments to identify the category the bacteria fall into. This identification can allow researchers and clinicians, as with gram-staining, which subsequent identification test should be used. Many times, it becomes necessary to identify a third category for the unknown bacteria, the morphology.
Morphology, along with their need for oxygen to grow and gram staining is a broad category that can be used to classify bacteria. Currently, there are three main shapes used to classify many bacteria species: bacillus (rod-shape), coccus (sphere-shape), and spirillum (spiral-shape) 8. This can be a quick way to classify microbes, with the use of a stain and and microscope. Besides easily being identified, morphology has also been found to be specific to a bacteria genus 9. This can be due to bacteria modifying to their environment or pathogenesis. Through natural selection, bacterial species will develop a unique shape that benefits that species survive, which over time makes it specific to the species and helps when classifying. Morphology may also indicate phylogenies and evolutionary lines 9. For example, there are similarities between bacteria and archaea in morphology, which shows this is a broad category for classifying microbes in two different domains. By using the gram stain result and shape, many scientists, clinicians, and researchers are able to follow a dichotomous key that helps to pinpoint the exact bacteria species to classify 5.
As mentioned above, gram staining, need for oxygen to grow, and shape are important when identifying bacteria. It is necessary to know the different microbial species in samples in clinical and laboratory settings. By classifying an unknown bacterium into these three broad categories it allows clinicians and researchers to easily identify what differential tests should be used next. With the number of examples provided above, it shows that these categories aid in quickly identifying bacterial species.