Biochemical Test and Identification of Serratia marcescens
Serratia marcescens is a ubiquitous Gram-negative, rod-shaped bacterium belonging to the family Yersiniaceae within the Enterobacterales order. While commonly found in soil, water, and plants, it is clinically significant as an opportunistic pathogen primarily associated with nosocomial (hospital-acquired) infections, including urinary tract infections, respiratory tract infections, and bacteremia. Accurate and timely identification of S. marcescens in a clinical microbiology laboratory relies heavily on a standard panel of biochemical tests that differentiate it from other Gram-negative rods, particularly other members of the Enterobacterales. Its distinguishing feature is the occasional production of a characteristic red pigment, prodigiosin, though many clinical isolates are non-pigmented, necessitating a robust biochemical profile for definitive identification. The full suite of tests helps to map the organism’s metabolic capabilities, ensuring correct classification and guiding appropriate therapeutic decisions.
General and Morphological Characteristics
Initial identification begins with basic morphological and staining procedures. S. marcescens is a non-spore-forming, motile organism, typically exhibiting peritrichous flagella, which is confirmed by a positive Motility test. Its cell wall structure dictates a Gram-negative result, appearing as pink or red rods under microscopy. Culturally, it can be grown on common media like Blood Agar and MacConkey Agar (MAC). On MAC, it is generally considered a non-lactose fermenter, though some strains can be slow or late fermenters, producing smooth, raised, circular colonies. The hallmark characteristic—pigment production—involves the synthesis of prodigiosin, a bright red, non-diffusible pigment, which is most reliably observed when cultures are incubated at room temperature (20-30°C). However, the pigmentation test is not definitive, as a significant portion of clinically isolated strains, particularly those grown at the standard human body temperature of 37°C, are non-pigmented, making biochemical testing crucial for confirmation.
The Core Diagnostic Test Scheme
A minimal and highly reliable scheme for the tentative identification of S. marcescens often centers on a few key enzymatic hydrolysis tests that are overwhelmingly positive for this species. These tests are Deoxyribonuclease (DNase) activity, Tween 80 hydrolysis (TEH), and the Voges-Proskauer (VP) test, coupled with Citrate utilization. S. marcescens is one of the few Enterobacterales species that is consistently and strongly positive for both DNase and Tween 80 hydrolysis. The DNase test detects the enzyme that hydrolyzes DNA, resulting in a clear zone around the bacterial growth on DNase test agar. Similarly, Tween 80 hydrolysis indicates the presence of lipase activity, breaking down the polyoxyethylene sorbitan monooleate substrate. These two tests alone provide a very high degree of specificity for the genus Serratia.
The Voges-Proskauer (VP) test, which is part of the IMViC series, is also characteristically positive for S. marcescens, indicating the organism’s ability to ferment glucose to acetylmethylcarbinol (acetoin). Furthermore, the species is an excellent utilizer of citrate as its sole carbon source, resulting in a positive reaction on Simmons Citrate Agar, turning the medium blue. When these four reactions (DNase+, TEH+, VP+, Citrate+) are observed together, a strong presumptive identification can be made.
Metabolic and Enzymatic Profile
A broader biochemical profile is used for complete species identification and differentiation from related organisms. The Triple Sugar Iron (TSI) test typically yields an alkaline slant/acid butt (K/A) reaction, demonstrating glucose fermentation but not lactose or sucrose fermentation (though variability in sucrose fermentation is sometimes noted). Importantly, S. marcescens is non-reactive for hydrogen sulfide production (H2S negative). In the remainder of the IMViC series, S. marcescens is reliably Methyl Red (MR) negative, as its primary glucose fermentation path is toward neutral products (VP positive) rather than stable acids (MR positive). It is also Indole negative, as it lacks the enzyme tryptophanase. The Oxidase test, which checks for the presence of cytochrome c oxidase, is uniformly negative, confirming its classification within the oxidase-negative Enterobacterales.
Tests related to nitrogen metabolism provide further confirmation. S. marcescens is positive for Nitrate Reduction, possessing the enzyme to reduce nitrate to nitrite. Conversely, the Phenylalanine Deaminase test is negative. The Urease test is known for its variability; while classical profiles list it as negative, clinical isolates sometimes demonstrate a positive reaction, which can be an important differentiating point from other members of the Serratia genus.
Carbohydrate Fermentation and Decarboxylase Activity
The ability of S. marcescens to ferment various carbohydrates is a stable characteristic. It is known to ferment common sugars such as glucose, fructose, galactose, and sorbitol, with acid production. A crucial enzymatic test is the ONPG test (O-nitrophenyl-β-D-galactopyranoside), which is positive. A positive ONPG test indicates the presence of the enzyme $beta$-galactosidase, even in slow or non-lactose fermenting strains, effectively linking S. marcescens metabolically to lactose-utilizing organisms.
Decarboxylase and Dehydrolase tests are essential for distinguishing S. marcescens from other Gram-negative organisms. It is positive for both Lysine Decarboxylase and Ornithine Decarboxylase, but negative for Arginine Dehydrolase. These reactions confirm the species identity and are particularly useful when using commercial multi-test systems. The combination of its motility, DNase positivity, VP positivity, and positive Lysine/Ornithine decarboxylase reactions forms a highly specific biochemical signature.
Clinical and Epidemiological Implications
The identification of S. marcescens using biochemical tests is not merely an academic exercise; it has significant clinical relevance. Its prevalence as a nosocomial agent, particularly in the urinary and respiratory tracts, makes rapid and accurate diagnosis critical for infection control. Furthermore, S. marcescens is notorious for its intrinsic and acquired resistance to multiple antibiotics. It is often resistant to cephalosporins and polymyxins, and various proportions of strains are resistant to aminoglycosides like kanamycin, though it frequently remains susceptible to gentamicin. The biochemical identification often precedes the antibiotic susceptibility testing, providing the necessary context for interpreting resistance patterns. The non-pigmented strains, which are common in clinical settings, often pose an identification challenge to labs relying solely on color; therefore, the robust, enzymatic biochemical profile—especially the positive DNase and VP—remains the gold standard for reliable differentiation and management of infections caused by Serratia marcescens.
In summary, the biochemical fingerprint of S. marcescens is characterized by its Gram-negative, motile rod morphology, its characteristic K/A TSI reaction, and its unique, strongly positive enzymatic activities for DNase, Tween 80 hydrolysis, Citrate utilization, and Voges-Proskauer. These characteristics, combined with positive Lysine and Ornithine decarboxylation and negative H2S and Indole production, allow for its definitive identification in a laboratory setting, supporting effective patient care and public health efforts.