--- title: "Serratia marcescens in Microbiology" description: "Serratia marcescens is a Gram-negative opportunistic bacterium that makes red prodigiosin pigment and is linked to hospital-acquired infections." canonical: "https://fiveable.me/microbio/key-terms/serratia-marcescens" type: "key-term" subject: "Microbiology" unit: "Unit 19" --- # Serratia marcescens in Microbiology ## Definition Serratia marcescens is a Gram-negative, rod-shaped bacterium in Microbiology known for red prodigiosin pigment. It is an opportunistic pathogen often tied to hospital-acquired urinary, wound, and respiratory infections. ## What It Is Serratia marcescens is a Gram-negative rod bacterium that microbiology classes usually meet as an opportunistic pathogen with a very recognizable red pigment called prodigiosin. That pigment can make colonies stand out on a plate, which is one reason this organism gets remembered in lab work. Structurally, it is part of the Enterobacterales group, so it fits the general pattern of a Gram-negative cell envelope with an outer membrane, thin peptidoglycan layer, and endotoxin-containing outer membrane components. In lab identification, that means you are not just looking at color. You are also thinking about Gram stain result, colony appearance, and where the organism tends to show up. The red pigment is a big clue, but it is not the whole story. S. marcescens can lose pigment under some growth conditions, so a non-red colony is not automatically a different species. Culture temperature, media, and nutrient conditions can all affect whether prodigiosin is obvious, which is why microbiologists do not identify it from color alone. In infection settings, S. marcescens matters because it can survive in water, on surfaces, and on medical equipment. That makes it a classic nosocomial organism, especially in catheter-associated urinary tract infections, bloodstream infections, respiratory infections, and wound infections. The course takeaway is that the bacterium is both a lab ID organism and a real-world hospital contaminant. Its disease-causing side comes from a mix of virulence factors, including proteases and hemolysins, plus intrinsic resistance traits such as beta-lactamase production. That combination lets it damage tissue, avoid being easy to treat, and persist in healthcare environments where vulnerable patients are exposed. In microbiology, this is a good example of how structure, metabolism, ecology, and clinical impact all connect in one species. ## Why It Matters Serratia marcescens shows how a microbe can be identified, spread, and treated based on a few linked traits instead of one single clue. In microbiology, that makes it a useful example of how phenotype and pathogenicity overlap. The red pigment is memorable, but the more useful lesson is that colony appearance can point you toward an organism while still leaving room for confirmation by Gram stain, culture behavior, and clinical context. It also gives you a concrete way to think about nosocomial infection. A bacterium that survives in water sources, sinks, tubing, or equipment can move into patients during catheter use or wound care, so infection control is part of the story, not just antibiotics. That is why S. marcescens comes up in hospital epidemiology and environmental contamination discussions. This term also connects to antibiotic resistance in a very practical way. If a bacterium produces beta-lactamase, some beta-lactam drugs will not work well, so the species becomes a good reminder that treatment choice depends on mechanism, not just on whether something is a bacterium. In class, this often shows up when you are asked to explain why an organism is hard to eradicate or why a hospital outbreak spreads so quickly. ## Connections ### Prodigiosin Prodigiosin is the red pigment that makes Serratia marcescens easy to recognize in culture. The connection matters because pigment production is a phenotype, not a guarantee, so you may see bright colonies under some conditions and pale ones under others. That is a good example of how environmental conditions can affect bacterial appearance without changing the species identity. ### Nosocomial Infection Serratia marcescens is a classic nosocomial pathogen because it can persist on surfaces, in water, and on medical equipment. When you connect the term to nosocomial infection, focus on transmission in healthcare settings, especially catheter-related and device-related spread. The case shows why hospital organisms are tracked differently from community-acquired ones. ### Beta-lactamase Beta-lactamase helps explain why S. marcescens can be resistant to many beta-lactam antibiotics. In microbiology, this is a mechanism question: the enzyme breaks down the drug before the drug can reach its target. That makes the organism a strong example of how bacterial enzymes can change treatment outcomes. ### [CD4+ T cells](/microbio/key-terms/cd4-cells) CD4+ T cells matter because patients with weakened adaptive immunity are more vulnerable to opportunistic infections like S. marcescens. The link is not that the bacterium targets CD4 cells directly, but that low immune reserve makes it easier for ordinarily manageable microbes to cause disease. This is a useful way to connect pathogen behavior with host defense. ## On the AP Exam A lab quiz or image ID question may show a Gram-negative rod with red colonies and ask you to name the organism or explain why it fits Serratia marcescens. You may also need to connect the species to a hospital infection case, especially if the prompt mentions catheters, urinary tract infection, or contaminated equipment. If the question asks about treatment failure, beta-lactamase is the mechanism to bring in. In a short-answer or case analysis, the best move is to link phenotype, environment, and infection site instead of just naming the bacterium. ## Serratia marcescens vs Bacillus cereus Both can show up as organisms with memorable colony features, but they are not the same type of bacterium. Serratia marcescens is Gram-negative, while Bacillus cereus is Gram-positive and forms spores. If a question gives you a red-pigmented rod with hospital-associated infection, Serratia is the better fit. ## Key Takeaways - Serratia marcescens is a Gram-negative rod bacterium in Microbiology that is known for red prodigiosin pigment. - It is an opportunistic pathogen, so it matters most when it infects people with weakened defenses or enters through medical devices. - Its hospital spread is tied to water, surfaces, and equipment, which is why it shows up in nosocomial infection discussions. - Pigment can help with identification, but microbiologists still use Gram stain, colony traits, and clinical context together. - Beta-lactamase production and other virulence factors help explain why this organism can be hard to treat and able to damage tissue. ## FAQs ### What is Serratia marcescens in Microbiology? Serratia marcescens is a Gram-negative, rod-shaped bacterium that often produces a red pigment called prodigiosin. In Microbiology, it is usually discussed as an opportunistic, hospital-associated pathogen that can cause urinary, wound, respiratory, and bloodstream infections. ### Why does Serratia marcescens make red colonies? The red color comes from prodigiosin, a pigment made by the bacterium under certain growth conditions. The color can help with identification in culture, but it is not always visible, so a missing red color does not rule out the organism. ### Is Serratia marcescens a Gram-negative or Gram-positive bacterium? It is Gram-negative. That means it has a thin peptidoglycan layer and an outer membrane, which affects how it stains, how it interacts with the host, and how some antibiotics work against it. ### Why is Serratia marcescens a problem in hospitals? It can survive in water and on medical equipment, which makes it good at spreading in healthcare settings. It is especially linked to catheter-associated infections and can be harder to treat because of antibiotic resistance mechanisms like beta-lactamase. ## Related Study Guides - [19.4 Immunodeficiency](/microbio/unit-19/4-immunodeficiency/study-guide/FKFBFiKRlqm6FykC) - [10.4 Structure and Function of Cellular Genomes](/microbio/unit-10/4-structure-function-cellular-genomes/study-guide/eyl9Aov6fpTczQdH) ## About This Document Canonical Fiveable pages are available as Markdown at the same path plus `.md`. - [llms.txt](https://fiveable.me/llms.txt): index of Fiveable's sections and URL patterns - [llms-full.txt](https://fiveable.me/llms-full.txt): complete subject and unit listing - [MCP server](https://fiveable.me/mcp): call Fiveable as tools instead of fetching pages (`https://fiveable.me/api/mcp`) - [MCP server for AP teachers](https://fiveable.me/mcp/teachers): a teacher's classes, assignments and AP-rubric grading (`https://fiveable.me/api/mcp/teacher`) ## Structured Data ```json {"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/microbio/key-terms/serratia-marcescens#resource","name":"Serratia marcescens in Microbiology","url":"https://fiveable.me/microbio/key-terms/serratia-marcescens","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/microbio/key-terms/serratia-marcescens#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-07-03T02:23:51.977Z","isPartOf":{"@type":"Collection","name":"Microbiology Key Terms","url":"https://fiveable.me/microbio/key-terms"},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/microbio/key-terms/serratia-marcescens#term","name":"Serratia marcescens","description":"Serratia marcescens is a Gram-negative, rod-shaped bacterium in Microbiology known for red prodigiosin pigment. 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