What Is a Blood Agar and How Does It Work?

Blood agar is a nutrient-rich culture medium made by mixing animal blood, usually from sheep, into a molten agar base at a concentration of about five percent. Laboratories rely on it to grow bacteria from clinical samples and, just as importantly, to classify those bacteria by how they destroy, damage, or leave alone the red blood cells embedded in the plate. That interaction between a bacterial colony and the surrounding blood creates visible color changes that a trained eye can read almost at a glance, making blood agar one of the most informative and widely used tools in diagnostic microbiology.

What Goes Into a Blood Agar Plate

At its simplest, a blood agar plate starts with a nutrient base, often trypticase soy agar or Columbia agar, that supplies amino acids, vitamins, and minerals bacteria need to grow. The base is autoclaved to sterilize it, then cooled to roughly 45–50 °C before sterile blood is gently mixed in. Pouring the blood into agar that is too hot would burst the red blood cells and turn the medium brown, which defeats the purpose. Once poured into petri dishes and solidified, the plates have a characteristic opaque, cherry-red surface.

Sheep blood is the standard in most clinical laboratories worldwide. Its red blood cells produce clean, predictable hemolysis patterns with the widest range of medically important bacteria. Human blood, by contrast, tends to give smaller colony sizes and weaker hemolysis, making results harder to interpret. A comparative study found that characteristic colony morphology was not evident on human blood agar, and hemolysis was poor enough that the researchers recommended against using it for isolation or susceptibility testing of common pathogens.¹

The blood can be prepared in two main ways: defibrinated (mechanically agitated to remove the clotting protein fibrin) or citrated (treated with sodium citrate to prevent clotting chemically). Defibrination requires manual labor and specialized equipment. Research has shown that citrated hair sheep blood performs just as well as defibrinated blood for diagnostic tests, which simplifies production in settings where resources are limited.²

How Hemolysis Patterns Work

The real diagnostic power of blood agar comes not from growing bacteria but from what those bacteria do to the red blood cells around them. Microbiologists classify this damage into three hemolysis patterns, and learning to read them is one of the first skills taught in a clinical microbiology lab.

Beta-Hemolysis

Beta-hemolysis is the most dramatic. Bacteria that produce it completely destroy the red blood cells in their vicinity, leaving a clear, transparent zone around the colony. You can hold the plate up to a light and see right through the cleared area. Group A Streptococcus, the bacterium behind strep throat, is the classic beta-hemolytic organism. Identification protocols for it typically start by looking for that distinctive clearing on a sheep blood agar plate, then confirm with additional tests like bacitracin sensitivity.³

The toxins responsible for beta-hemolysis vary by species. Group A streptococci, for instance, produce two hemolysins called streptolysin O and streptolysin S. Streptolysin O is inactivated by oxygen and can be reactivated by reducing agents, while streptolysin S remains fully active in the presence of oxygen.⁴ This is why surface colonies on an agar plate, where oxygen is freely available, still show clearing: streptolysin S handles the job topside. Subsurface colonies, shielded from oxygen in the agar itself, rely on streptolysin O. Some laboratories stab the agar with an inoculating loop before streaking the surface specifically to enhance the subsurface hemolysis and make reading easier.

The story of beta-hemolysis gets more nuanced with Staphylococcus aureus. S. aureus produces a toxin called β-hemolysin (confusingly named with the Greek letter beta, but distinct from the hemolysis pattern category). Research has found that this toxin degrades a lipid in the red blood cell membrane but does not actually burst the cells under normal body-temperature conditions. Significant cell lysis only appeared when the temperature dropped to 4 °C.⁵ This means the hemolysis you observe on a plate incubated at 37 °C is driven by other S. aureus toxins, not by this particular one.

Alpha-Hemolysis

Alpha-hemolysis produces a greenish or brownish discoloration around a colony rather than a clear zone. The red blood cells are not completely destroyed; instead, the hemoglobin inside them is chemically altered. For Streptococcus pneumoniae, one of the most medically important alpha-hemolytic bacteria, the mechanism has been pinned down: the organism produces hydrogen peroxide, which oxidizes the iron in hemoglobin from its normal oxygen-carrying form to a form called met-hemoglobin that cannot carry oxygen. It is this chemical change that creates the green hue.⁶

Because the green zone is subtler than the clearing seen in beta-hemolysis, reading alpha-hemolysis requires some experience. The discoloration extends only a short distance beyond the colony edge, and its shade can vary depending on plate thickness, blood concentration, and incubation time.

Gamma-Hemolysis

Gamma-hemolysis is essentially the absence of hemolysis. The agar around the colony looks unchanged, with no clearing and no discoloration. Organisms that show gamma-hemolysis are sometimes called non-hemolytic, and the finding is itself diagnostically useful because it rules out the more aggressive species. Lactic acid bacteria isolated from poultry, for example, have been shown to exhibit gamma-hemolysis on blood agar, marked by the absence of any clear zone around their colonies.⁷ This characteristic matters in food safety screening, where you want to confirm that the organisms present are benign.

Using Hemolysis to Identify Pathogens in the Clinic

Hemolysis patterns are rarely the final word on identification, but they narrow the field enormously. When a throat swab comes into a lab, the technician streaks it onto a sheep blood agar plate, incubates it overnight at 37 °C in an atmosphere enriched with about five percent carbon dioxide, and checks for beta-hemolytic colonies the next morning.³ If beta-hemolytic colonies are present, a few quick follow-up tests, like confirming the bacteria are Gram-positive cocci that do not produce the enzyme catalase and are sensitive to the antibiotic bacitracin, can confirm Group A Streptococcus with high confidence.⁸

For alpha-hemolytic organisms, the workflow branches differently. S. pneumoniae is the main clinical target, and the traditional confirmatory test is optochin susceptibility: a small disc impregnated with the chemical ethylhydrocupreine is placed on the plate, and if the organism’s growth is inhibited around the disc, pneumococcus is presumed.⁹ The catch is that the agar base matters. One study compared three different blood agar bases and found that trypticase soy agar with sheep blood misidentified zero percent of pneumococcal isolates, while Columbia agar misidentified over fifteen percent and Mueller-Hinton agar misidentified over twenty percent.¹⁰ The wrong base can lead to the wrong diagnosis, which is why standardizing the agar formulation is not a trivial detail.

An additional complication is that optochin-resistant strains of S. pneumoniae have been reported with increasing frequency over the past two decades, which can lead to misidentification if optochin susceptibility is the only test used.¹¹ Modern labs often back up the optochin test with bile solubility testing or molecular methods for definitive confirmation.

Why Sheep Blood and Not Something Else

Sheep blood became the gold standard largely by historical convention backed by practical results. Sheep red blood cells are small and uniform, they lyse cleanly in response to bacterial hemolysins, and they do not contain antibodies against human pathogens that might interfere with growth. But not every lab on the planet has easy access to sheep blood, and researchers have examined alternatives.

A study that tested blood agar made from cow, goat, rabbit, chicken, human, and sheep blood found that the hemolytic reactions of some bacteria shifted depending on the blood source. Beta-hemolytic streptococci looked the same regardless of blood type, which is reassuring. But Staphylococcus aureus, which typically shows alpha-hemolysis on sheep blood, did not show that pattern on chicken blood agar. Pseudomonas aeruginosa was beta-hemolytic on cow blood but only alpha-hemolytic on goat and rabbit blood. On sheep and human blood, it started alpha-hemolytic at 24 hours and shifted to beta-hemolytic by 48 hours. Overall, hemolysis was least consistent on chicken and cow blood agar.¹²

These inconsistencies explain why sheep blood agar remains the recommended standard for routine clinical work. When you read results from a different blood source, hemolysis patterns can be misleading, and the diagnostic algorithms built into clinical guidelines assume sheep blood. Labs that must substitute another animal’s blood need to validate their results carefully against known organisms before relying on the plates clinically.

Chocolate Agar and Other Heated Variants

Chocolate agar looks like its name suggests: dark brown and smooth. It is made by adding blood to molten agar and then heating the mixture to about 80 °C, which bursts the red blood cells and releases their internal contents into the medium. The “chocolate” color comes from denatured hemoglobin. The heating step also inactivates enzymes that would otherwise break down NAD, a growth factor that certain fastidious bacteria need to survive in culture.¹³

Because the red cells are already destroyed, you lose the ability to read hemolysis patterns on chocolate agar. That is the trade-off: what you gain is the ability to grow organisms that cannot grow on standard blood agar because they need those released intracellular nutrients. Haemophilus influenzae is the textbook example, requiring both hemin (X factor) from hemoglobin and NAD (V factor) to grow. On a standard blood agar plate, the NAD is trapped inside intact red blood cells where Haemophilus cannot access it.

An interesting workaround is the satellitism test. If you streak Staphylococcus aureus across a blood agar plate and then cross-streak the suspected Haemophilus near it, the staph colonies lyse red blood cells and release NAD into their immediate neighborhood. Haemophilus colonies will grow larger near the staph streak and smaller or not at all farther away, creating a satellite pattern. This test has practical value but can also produce misleading results depending on factors like the staph strain used and the blood concentration in the plate.¹⁴

Selective Blood Agar Formulations

Plain blood agar grows almost anything, which is its strength and its limitation. A clinical sample from a wound or from stool may contain dozens of bacterial species, and the organism you care about can be overgrown by faster-growing contaminants. To deal with this, microbiologists add antibiotics, antifungals, or other inhibitory agents to blood agar to create selective versions that suppress unwanted organisms while allowing the target pathogen to grow.

Neomycin blood agar, for instance, has been used to screen for vancomycin-resistant enterococci from stool samples. The neomycin kills most Gram-negative bacteria and many Gram-positive species, letting the resistant enterococci survive and form visible colonies. However, the concentration of neomycin matters: set it too high, and some resistant isolates are suppressed along with the contaminants.¹⁵

For isolating Brucella species from field samples, a selective medium called CITA was developed with a cocktail of vancomycin, colistin, nystatin, nitrofurantoin, and amphotericin B added to the blood agar base. This combination inhibited most contaminants while allowing all major Brucella species to grow at levels comparable to a non-selective control medium.¹⁶ Another formulation targeting Capnocytophaga species used a mix of polymyxin B, vancomycin, trimethoprim, and amphotericin B in a chocolate agar base and recovered the target organisms from clinical specimens at a rate of 96 percent, compared to just six percent on plain blood agar.¹⁷

These examples illustrate a general principle: the “right” blood agar plate depends entirely on the clinical question. A lab investigating a strep throat uses plain sheep blood agar. A lab screening stool for resistant organisms uses a selective variant. A lab hunting for slow-growing, easily overgrown species may need an entirely different antibiotic cocktail. The base medium is similar in each case, but the additives transform its selectivity.

Storage, Shelf Life, and Quality Control

Blood agar plates are perishable. The blood component degrades over time, and contamination risk increases with storage. Prepared plates are typically stored at 4 °C and should not sit for more than about four weeks before use. In one validation study involving drug-susceptibility testing for tuberculosis, the investigators explicitly limited their plate storage to four weeks at 4 °C as part of quality control.¹⁸

Quality control in a clinical lab involves more than just shelf life tracking. Each new batch of plates should be tested with reference strains whose hemolysis patterns are already known. If a batch of sheep blood agar does not produce a clean beta-hemolytic zone around a Group A strep control strain, or a clear alpha-hemolytic halo around a pneumococcal control, the batch fails and must be discarded. Temperature of the blood when added, final blood concentration, agar depth, and even the age of the blood itself can all influence results. Labs that prepare their own plates in-house must monitor these variables consistently; labs that purchase commercially prepared plates still need to verify performance on arrival.

When Blood Agar Falls Short

For all its versatility, blood agar cannot grow everything. Obligate intracellular pathogens like Chlamydia and Rickettsia do not grow on any conventional media because they require living host cells. Mycoplasma species lack a cell wall and need special formulations with cholesterol. Many fungi grow poorly or not at all on standard blood agar without extended incubation times and adjusted pH. Anaerobic bacteria require incubation without oxygen, which is a function of the atmosphere rather than the medium itself, though blood agar is often the medium used inside an anaerobic chamber.

Even among organisms that do grow on blood agar, interpretation is not always straightforward. As the animal blood comparison study showed, hemolysis patterns can shift between blood sources and between incubation times.¹² A colony that looks alpha-hemolytic at 24 hours may look beta-hemolytic at 48 hours, which changes the diagnostic interpretation completely. Reading plates at a standardized time, usually 18 to 24 hours, is therefore part of the protocol, not just a convenience.

Modern clinical labs increasingly supplement or even replace traditional culture with molecular methods like PCR, mass spectrometry (MALDI-TOF), and automated identification systems. These methods can identify organisms in hours rather than the overnight incubation that culture requires, and they do not depend on subjective visual interpretation. Still, blood agar culture remains indispensable for antimicrobial susceptibility testing, because knowing what organism you have is only half the clinical problem. You also need to know which antibiotics will kill it, and for that, you typically need living bacteria growing on a plate.

Blood Agar Beyond the Hospital Lab

While clinical diagnostics consume the bulk of blood agar production, the medium also appears in food safety, veterinary medicine, water quality testing, and pharmaceutical manufacturing. Food safety labs use blood agar to screen for hemolytic organisms in raw milk, meat, and ready-to-eat products. Veterinary labs use it to diagnose mastitis in cattle, wound infections in companion animals, and respiratory infections in poultry. Water treatment facilities culture environmental samples on blood agar to check for fecal contamination or pathogenic bacteria in drinking water supplies.

In pharmaceutical quality assurance, blood agar plates serve as part of environmental monitoring programs in cleanrooms where sterile products are manufactured. Any bacterial colony that grows on a monitoring plate needs to be identified, and hemolysis on blood agar is the first screening step. The versatility of the medium, its ability to support a vast range of species while simultaneously providing diagnostic information through hemolysis, is what keeps it in widespread use more than a century after it was first developed.