Citrobacter freundii: Morphology, Staining, and Biochemical Traits

Citrobacter freundii is a Gram-negative bacterium commonly found in the environment, especially in soil, water, and the intestines of animals. It holds significant medical relevance due to its association with various infections, including urinary tract infections, respiratory conditions, and sepsis.

Accurate identification of Citrobacter freundii is crucial for effective treatment and management of these infections. Understanding its morphology, staining properties, and biochemical traits can provide essential insights into its behavior and interaction within different environments.

Morphological Characteristics

Citrobacter freundii exhibits distinct morphological features that aid in its identification and study. Typically, this bacterium appears as rod-shaped cells, a common trait among members of the Enterobacteriaceae family. These rods are generally straight and can vary in length, often measuring between 1 to 5 micrometers. The size and shape of these cells can be influenced by environmental conditions, which may affect their growth and division.

The bacterium is known for its motility, facilitated by the presence of peritrichous flagella. These flagella are distributed over the entire surface of the cell, allowing for movement in liquid environments. This motility is not only a defining characteristic but also plays a role in the bacterium’s ability to colonize and adapt to various habitats. The presence of flagella can be observed under a microscope using specific staining techniques, which highlight these structures against the cell body.

In addition to its motility, Citrobacter freundii can form biofilms, a feature that enhances its survival in diverse environments. Biofilms are complex communities of microorganisms that adhere to surfaces, providing protection against external threats such as antibiotics and the host immune system. This ability to form biofilms is significant in clinical settings, where it can contribute to persistent infections and complicate treatment efforts.

Gram Staining

The Gram staining technique is a fundamental tool used to classify bacteria into two main groups: Gram-positive and Gram-negative. This categorization is based on the structural differences in the bacterial cell wall. Citrobacter freundii, a Gram-negative bacterium, presents a thinner peptidoglycan layer and an outer membrane containing lipopolysaccharides. These structural elements play a significant role in its classification and influence how it reacts to the Gram stain.

During the Gram staining process, a series of dyes are applied to the bacterial sample. Initially, a primary stain, such as crystal violet, is used, followed by the application of iodine, which acts as a mordant. This step helps in fixing the dye to the bacterial cell walls. Subsequently, the sample is treated with alcohol or acetone, serving as a decolorizer. The unique cell wall structure of Gram-negative bacteria causes them to lose the crystal violet stain during this step. As a result, Citrobacter freundii appears colorless at this stage.

To visualize the bacteria, a counterstain, typically safranin, is applied, imparting a pink or red hue to the cells. This final coloring provides a clear distinction between Gram-negative and Gram-positive bacteria under a microscope, allowing for accurate identification. This differentiation is not merely a matter of classification but has practical implications in clinical diagnostics and treatment decisions.

Biochemical Properties

Exploring the biochemical properties of Citrobacter freundii provides valuable insights into its metabolic capabilities and ecological roles. This bacterium is adept at utilizing a wide range of substrates, which is reflected in its ability to ferment lactose, a trait it shares with other members of the Enterobacteriaceae family. The fermentation process results in the production of acid and gas, a characteristic that can be observed through specific laboratory tests such as the lactose fermentation test on MacConkey agar, where colonies appear pink due to acid production.

In addition to lactose fermentation, Citrobacter freundii can also metabolize citrate as its sole carbon source. This ability is determined using the Simmons citrate test, where the bacterium’s growth results in a color change of the medium, indicating citrate utilization. Such metabolic versatility allows it to thrive in diverse environments, contributing to its widespread distribution. Furthermore, the bacterium’s enzymatic activity includes the production of hydrogen sulfide, as detected in triple sugar iron agar, where a black precipitate forms in the presence of this gas.

The urease test is another critical biochemical assay for Citrobacter freundii. Although not all strains produce urease, those that do can hydrolyze urea into ammonia and carbon dioxide, leading to an alkaline shift in the medium. This enzymatic activity provides a nitrogen source, aiding survival in nutrient-limited settings. The ability to produce urease is particularly relevant in the context of urinary tract infections, where it can contribute to pathogenesis.