Serratia marcescens is a widely distributed bacterium, a single-celled microorganism. It belongs to the Enterobacteriaceae family, a large group of Gram-negative bacteria. This bacterium is often studied in various scientific fields due to its unique characteristics.
Macroscopic and Microscopic Appearance
Serratia marcescens exhibits distinctive features. When grown on laboratory media at room temperature, it often produces a striking red pigment called prodigiosin. This pigment can range from dark red to pale pink, depending on the age of the bacterial colonies. Some strains, however, may not produce this pigment and can appear white or colorless.
Under a microscope, Serratia marcescens appears as rod-shaped bacteria, also known as bacilli. These individual rods typically measure about 0.5 to 0.8 micrometers in diameter and 1 to 2 micrometers in length. They are often observed individually or in small clusters, sometimes forming short chains. A defining microscopic characteristic is its Gram-negative nature. Many strains are motile, possessing flagella that allow movement. Some strains also have a capsule, which can enhance their ability to form biofilms.
Common Habitats and Significance
Serratia marcescens is widespread in various environments, commonly found in moist, damp places. Its natural habitats include soil, water, and on the surfaces of plants and insects. It has also been identified in the digestive tracts of both humans and animals. This bacterium is a mesophilic saprophyte, thriving in moderate temperatures (5 to 40 degrees Celsius) and obtaining nutrients from decaying organic matter.
Despite its prevalence in the environment, Serratia marcescens holds significance as an opportunistic pathogen, particularly in healthcare settings. While generally harmless in its natural environmental state, it can cause hospital-acquired infections, such as pneumonia, urinary tract infections, and bloodstream infections. Historically, it was considered non-pathogenic and even used as a biological marker in experiments due to its easily recognizable red colonies. However, since the 1960s, its role as an opportunistic human pathogen has been recognized.
Basic Microscopic Observation Techniques
Observing Serratia marcescens under a microscope involves fundamental steps to prepare the sample for visualization. A common approach is preparing a bacterial smear, which involves spreading a thin layer of bacteria onto a glass slide. The slide is then air-dried and heat-fixed, a process that adheres the bacteria and kills them.
Staining is then applied to enhance visibility, as bacteria are mostly transparent. Gram staining is particularly useful for identifying Serratia marcescens. This procedure involves applying crystal violet dye, followed by iodine, a decolorizer (alcohol or acetone), and finally a counterstain, safranin. As Serratia marcescens is Gram-negative, its cells will appear pink or red after this process, distinguishing them from Gram-positive bacteria which retain crystal violet and appear purple.