The definitive answer to the question of whether Proteus vulgaris is citrate positive or negative is that the organism is typically Citrate Negative. This biochemical characteristic is a fundamental tool used in the microbiology laboratory to identify and distinguish this bacterium from other closely related species. The citrate utilization test is a standard part of a battery of assays designed to determine the metabolic capabilities of an unknown microbial sample. This test helps scientists determine if the bacterium can use a specific organic compound as its sole source of carbon. Understanding this specific negative result requires looking closely at the organism itself, the biological mechanism of the test, and how this data point fits into a complete microbial profile.
Understanding Proteus vulgaris
Proteus vulgaris is a Gram-negative, rod-shaped bacterium belonging to the order Enterobacterales, specifically within the family Morganellaceae. This organism is commonly found in the natural environment, inhabiting soil and water, but it also colonizes the intestinal tract of humans and animals. As an opportunistic pathogen, P. vulgaris can cause significant clinical issues when it moves outside of its normal habitat, most notably being a cause of urinary tract infections (UTIs) and wound infections.
A hallmark feature of Proteus species is their remarkable, rapid, and coordinated movement known as swarming motility. This behavior allows the bacterial colony to spread quickly across the surface of a solid agar plate, creating a thin, concentric film of growth. Another prominent metabolic trait is its potent production of the enzyme urease. This enzyme rapidly breaks down urea into ammonia and carbon dioxide, a reaction that causes the urine to become highly alkaline during a UTI, which can promote the formation of infectious kidney stones.
In the laboratory setting, P. vulgaris is also characterized by being indole-positive, meaning it can break down the amino acid tryptophan to produce indole. The combination of its Gram-negative morphology, swarming motility, positive urease activity, and positive indole test provides a strong initial indication of its identity. The negative result on the citrate test then further refines this identification, distinguishing it from other organisms that share some of these characteristics.
The Mechanics of the Citrate Utilization Test
The citrate utilization test, most often performed using Simmons’ Citrate Agar, is a measure of a bacterium’s ability to metabolize citrate as its only carbon source. The medium is chemically defined, containing sodium citrate, ammonium dihydrogen phosphate, and the pH indicator bromothymol blue. The uninoculated medium appears a deep forest green due to the neutral pH of the agar.
For a bacterium to grow on this medium and yield a positive result, it must possess a specific transport enzyme called citrate permease. This enzyme is responsible for transporting the citrate molecule from the agar outside the cell into the bacterial cytoplasm. Once inside, the citrate is broken down, entering the cell’s metabolic pathways, which allows the organism to grow and multiply.
The metabolism of citrate produces alkaline byproducts, primarily sodium carbonate. Additionally, the bacterium utilizes the ammonium dihydrogen phosphate as its sole nitrogen source, which also contributes to the medium’s increasing alkalinity by producing ammonia. This increase in pH from neutral (around 6.9) to alkaline (above 7.6) is what triggers the color change. The bromothymol blue indicator changes from green to a distinct, bright blue color, signaling a positive test.
Since Proteus vulgaris typically registers as citrate negative, it means the organism cannot effectively utilize the sodium citrate in the medium. This inability stems from the fact that P. vulgaris generally lacks the necessary citrate permease enzyme to transport the citrate into the cell. Without the ability to transport and metabolize this compound, the organism cannot grow sufficiently, and the medium remains its original green color.
Using Citrate Results for Bacterial Identification
The negative citrate result for P. vulgaris is a small yet significant piece of a larger puzzle used in the clinical microbiology lab to achieve a final identification. In the context of the Enterobacterales, a group that includes many common human pathogens, several species share similar characteristics like being Gram-negative rods or fermenting glucose. The citrate test is therefore used as a differential test to separate these look-alike organisms.
The ability to utilize citrate is a defining feature that separates P. vulgaris from certain other clinically relevant bacteria. Organisms such as Klebsiella pneumoniae and Enterobacter aerogenes are both known to be citrate positive, meaning they turn the agar blue. The P. vulgaris negative result immediately rules out these possibilities, narrowing the list of potential identifications.
In a diagnostic setting, the citrate result is rarely used in isolation; it is combined with the results of other biochemical assays. The classic profile for P. vulgaris is a combination of a positive urease test, positive indole test, and negative citrate test. This specific biochemical fingerprint makes it possible to differentiate P. vulgaris from its close relative, Proteus mirabilis, which is typically urease positive but indole negative. The systematic application of these tests allows laboratory technicians to create a precise metabolic profile.