Serological identification of Salmonella species is of great importance in clinical microbiology, epidemiological studies, and public health surveillance, as it helps in accurate diagnosis, outbreak investigation, and disease control.
The classification and identification of Salmonella are mainly based on the presence of specific surface antigens that are capable of stimulating distinct immune responses in the host.
The major antigens used for serological testing of Salmonella include the Somatic O antigen, Flagellar H antigen, and Vi antigen (capsular antigen), each having a specific structural location and diagnostic significance.
Among these antigens, the O antigen and H antigen are the most commonly and routinely used markers in serotyping of Salmonella in diagnostic laboratories.
A clear understanding of the differences between O and H antigens is essential for microbiology students, laboratory professionals, and researchers, as it aids in correct interpretation of serological tests and accurate identification of Salmonella serotypes.
What is O Antigen?
The O antigen, also known as the somatic antigen, is a heat-stable antigenic component found on the surface of Gram-negative bacteria such as Salmonella.
It is a polysaccharide component of the lipopolysaccharide (LPS) present in the outer membrane of the bacterial cell wall.
The O antigen forms the outermost part of the LPS structure and is directly exposed to the host immune system.
It is primarily responsible for the antigenic specificity of the bacterial cell wall, allowing differentiation of bacteria into distinct serogroups during serological testing.
Due to its structural stability and surface location, the O antigen plays a key role in serogrouping and identification of Salmonella in diagnostic microbiology laboratories.
Key Features of O Antigen
The O antigen is located on the surface of the bacterial cell, forming part of the outermost layer that directly interacts with the host immune system.
It is composed of polysaccharides, which constitute the outer side chain of the lipopolysaccharide (LPS) molecule.
The O antigen is heat stable and resistant to alcohol, allowing it to retain its antigenic properties even after exposure to heat or chemical treatment.
It induces an early antibody response during infection, making it useful for early serological detection.
In agglutination reactions, the O antigen produces granular clumping, which is a characteristic pattern used for laboratory identification.
Compared to the H antigen, the O antigen is less immunogenic, resulting in the production of antibodies with relatively lower titres.
O antigens play a critical role in the serogrouping of Salmonella into groups such as A, B, C, D, and others, according to the Kauffmann–White classification scheme, which is widely used for epidemiological typing and diagnosis.
What is H Antigen?
The H antigen, also known as the flagellar antigen, is an antigenic component associated with the flagella of motile bacteria, including Salmonella.
It is proteinaceous in nature, as it is composed of flagellin proteins that form the bacterial flagella.
The H antigen is heat-labile, meaning it is easily destroyed by heat and loses its antigenic activity when exposed to high temperatures.
It is highly immunogenic, stimulating a strong immune response in the host.
Antibodies produced against the H antigen are strong and sustained, persisting for a longer duration compared to those produced against the O antigen, which makes H antigen useful for detailed serotyping and epidemiological studies.
Key Features of H Antigen
The H antigen is located on the bacterial flagella, which are responsible for motility in Salmonella and other motile bacteria.
It is proteinaceous in nature, being composed mainly of flagellin proteins.
The H antigen is heat labile and sensitive to alcohol, and therefore loses its antigenic properties when exposed to heat or chemical agents.
Antibodies formed against the H antigen persist for a longer period, making them useful for detecting past or ongoing infections.
In agglutination tests, the H antigen produces a characteristic cottony or fluffy precipitate, which helps in its laboratory identification.
The H antigen is highly immunogenic, inducing a strong immune response with the production of high-titre antibodies.
H antigens play an important role in differentiating Salmonella species and serotypes into specific strains, providing finer classification beyond serogrouping.
H antigens help in differentiating Salmonella species and serotypes further into specific strains.
Difference Between O Antigen and H Antigen
Type: The O antigen is a somatic antigen present on the bacterial cell surface as part of the cell wall, whereas the H antigen is a flagellar antigen associated with the flagella of motile bacteria.
Composition: The O antigen is polysaccharide in nature, forming part of the lipopolysaccharide (LPS) complex, while the H antigen is proteinaceous, composed mainly of flagellin proteins.
Antibody response: Antibodies against the O antigen are produced rapidly and early during infection, whereas antibodies against the H antigen are produced rapidly but are sustained for a longer duration.
Antibody level: The antibody levels against the O antigen decline quickly, while antibodies against the H antigen persist for longer periods in the serum.
Agglutination pattern: In agglutination tests, the O antigen produces granular clumps, whereas the H antigen produces cottony, fluffy precipitates.
Observation of agglutination: Agglutination due to the O antigen is observed using round-bottom Felix tubes, while H antigen agglutination is observed using conical-bottom Dreyer’s tubes.
Heat sensitivity: The O antigen is heat stable and retains its antigenic properties after heating, whereas the H antigen is heat labile and is destroyed by heat.
Alcohol sensitivity: The O antigen is resistant to alcohol, while the H antigen is sensitive to alcohol and may lose its antigenic activity.
Extraction method: The O antigen is extracted using trichloroacetic acid, whereas the H antigen is extracted using formaldehyde.
Immunogenicity: The O antigen is less immunogenic, producing a weaker immune response, while the H antigen is highly immunogenic.
Antibody titre: Antibodies formed against the O antigen are usually of low titre, whereas antibodies against the H antigen are produced in high titres, making them more useful for detailed serotyping and epidemiological investigations.
Role in Serological Testing of Salmonella
Both O and H antigens play a crucial role in the serological testing and serotyping of Salmonella by means of agglutination reactions.
The O antigen is primarily used to identify the serogroup of Salmonella, which helps in the initial classification of the organism based on somatic antigenic structure.
The H antigen is used to determine the specific serotype, allowing further differentiation of Salmonella strains within the same serogroup.
The combined use of O and H antigen information is essential for accurate laboratory diagnosis of Salmonella infections, particularly enteric fever.
Serological identification using these antigens is also important for tracing disease outbreaks, monitoring the spread of infection, and conducting epidemiological studies in public health settings.
Conclusion
O and H antigens differ markedly in their structure, stability, immunogenicity, and roles in serological reactions used for the identification of Salmonella.
The O antigen is mainly responsible for group-level identification, allowing Salmonella isolates to be classified into specific serogroups.
The H antigen provides finer, strain-level differentiation, helping to distinguish between different serotypes within the same serogroup.
When used together, O and H antigens form the backbone of Salmonella serological classification, enabling precise identification in diagnostic laboratories.
Their combined application is fundamental for accurate laboratory diagnosis, epidemiological surveillance, and effective control of Salmonella-associated diseases.
