O antigen and H antigen- Definition and 21 Key Differences

Bacteria are microscopic organisms that have diverse shapes, sizes, and metabolic abilities. They are surrounded by a cell membrane that regulates the transport of molecules in and out of the cell. Most bacteria also have a cell wall that provides structural support and protection from environmental stresses. However, bacteria can also have additional layers or structures outside the cell wall that contribute to their survival and interaction with other organisms. These surface structures include capsules, slime layers, S-layers, fimbriae, pili, and flagella. They are often composed of proteins, polysaccharides, or glycoproteins that are secreted by the cell and assembled on the outer part of the cell wall or membrane.

One of the important functions of these surface structures is to act as antigens, which are molecules that can elicit an immune response from a host organism. Antigens are recognized by specific antibodies or immune cells that bind to them and trigger a series of reactions to eliminate or neutralize the foreign invaders. However, bacteria have evolved various mechanisms to evade or modulate the host immune response by altering their surface antigens. This phenomenon is known as antigenic variation or antigenic alteration.

Antigenic variation can occur by changing the expression, structure, or composition of the surface antigens. For example, bacteria can switch on or off the production of certain antigens, modify them by adding or removing chemical groups, or recombine different segments of DNA to generate new variants of antigens. Antigenic variation can result from gene conversion, site-specific DNA inversions, hypermutation, or recombination of sequence cassettes. The advantage of antigenic variation is that it allows bacteria to escape from the host immune recognition and memory, and to adapt to different environmental conditions or niches.

Antigenic variation is especially common among bacterial pathogens that cause chronic or recurrent infections in humans and animals. For instance, Neisseria meningitidis and Neisseria gonorrhoeae can vary their pili and outer membrane proteins; Streptococcus pyogenes can vary its M protein; Borrelia burgdorferi can vary its surface lipoprotein VlsE; and Salmonella enterica can vary its O and H antigens . These antigens play a crucial role in bacterial virulence, adhesion, invasion, and colonization.

In this article, we will focus on the O and H antigens of Salmonella enterica, which are used as part of a serologic classification system for this genus. We will explain what these antigens are, how they are expressed and varied by the bacteria, and how they affect the host immune response. We will also compare and contrast the O and H antigens in terms of their structure, function, distribution, and properties.

Serologic classification is a method of identifying bacteria based on their antigenic properties. Antigens are molecules that can elicit an immune response in a host organism. Bacteria have various antigens on their surface structures, such as cell wall, capsule, flagella, pili, and fimbriae. These antigens can be detected by specific antibodies that bind to them and cause agglutination (clumping) or precipitation (settling) of the bacterial cells.

Serologic classification is useful for distinguishing different strains or serotypes of bacteria within the same species or genus. For example, Escherichia coli can be classified into more than 150 serotypes based on the combination of O and H antigens. O antigen refers to the polysaccharide component of the lipopolysaccharide (LPS) layer that covers the outer membrane of gram-negative bacteria. H antigen refers to the protein component of the flagella that enable bacterial motility.

Serologic classification can also provide information about the epidemiology, pathogenicity, and immunity of bacterial infections. For example, some serotypes of Salmonella are associated with specific diseases or hosts, such as S. Typhi (typhoid fever in humans), S. Paratyphi (paratyphoid fever in humans), S. Enteritidis (gastroenteritis in humans and animals), and S. Gallinarum (fowl typhoid in poultry). Some serotypes of bacteria may also have different virulence factors or resistance mechanisms that affect their ability to cause disease or respond to treatment. Furthermore, some serotypes of bacteria may induce cross-reactive immunity or protection against other serotypes with similar antigens.

Serologic classification is usually performed by using agglutination tests with specific antisera that contain antibodies against known antigens. The antisera can be prepared from animals or humans that have been immunized with bacterial antigens or infected with bacterial strains. The agglutination tests can be done in tubes or on slides, depending on the type and amount of antigen and antibody involved. The results are interpreted by observing the presence or absence of visible clumps or precipitates of bacteria after incubation with antisera.

Serologic classification is a simple, rapid, and inexpensive method of identifying bacteria, but it has some limitations. It requires the availability and quality control of specific antisera, which may vary in their specificity and sensitivity. It also depends on the expression and stability of bacterial antigens, which may change due to genetic or environmental factors. Moreover, it may not detect all the antigenic variations or subtypes of bacteria that exist in nature. Therefore, serologic classification should be complemented by other methods of bacterial identification, such as biochemical tests, molecular typing, or phenotypic characterization.

Salmonellae are Gram-negative bacilli that belong to the family Enterobacteriaceae. They are responsible for various diseases in humans and animals, such as gastroenteritis, typhoid fever, and septicemia.

One of the methods used to classify and identify different Salmonellae is based on their surface antigens, namely O and H antigens.

O antigen is a part of the lipopolysaccharide (LPS) layer that covers the outer membrane of the bacterial cell wall. It consists of repeating units of oligosaccharides that vary in their chemical structure and number among different Salmonellae. O antigen is heat-stable and can be detected by agglutination with specific antisera.

H antigen is a protein component of the flagella that enable the bacteria to move. It can be either phase 1 or phase 2, depending on the expression of different flagellar genes. H antigen is heat-labile and can be detected by slide agglutination or tube agglutination with specific antisera.

Based on the combination of O and H antigens, Salmonellae are divided into more than 2000 serotypes or serovars. For example, Salmonella enterica serovar Typhi has the antigenic formula 9,12:d:-, which means it has O antigens 9 and 12, phase 1 H antigen d, and no phase 2 H antigen.

The serologic classification of Salmonellae is useful for epidemiological purposes, as it can help to trace the source and transmission of infections, as well as to monitor the prevalence and distribution of different serotypes.

Virulence is the ability of a pathogen to cause disease in a host. Different pathogens have different levels of virulence, depending on the unique virulence factors they produce. Virulence factors are molecules or structures that help the pathogen to adhere, invade, evade, damage, or manipulate the host cells or immune system. Some examples of virulence factors are toxins, enzymes, capsules, fimbriae, flagella, and surface antigens.

Surface antigens are molecules that are exposed on the outer membrane of the pathogen and can be recognized by the host immune system. They can also be used for serologic classification of bacteria, which is based on the identification of specific antigens by using antibodies. For example, Salmonellae are classified into more than 2000 serotypes based on their O (LPS side chain) and H (flagellar) antigens.

The antigenic type of the bacteria may be a marker for virulence, meaning that it may indicate the potential of the pathogen to cause disease. This is because some antigenic types are associated with certain virulence factors or pathogenic clones that have evolved to infect specific hosts or tissues. For example, Salmonella Typhi, which causes typhoid fever in humans, has a single O antigen (O9) and a single H antigen (d), whereas Salmonella Typhimurium, which causes gastroenteritis in humans and animals, has multiple O antigens (O4, O5, O12) and multiple H antigens (i and 1,2).

However, the antigenic type itself may not be the actual virulence factor that causes disease. Rather, it may reflect the genetic background of the pathogen that carries other virulence factors. For example, some strains of E. coli that cause urinary tract infections have type 1 fimbriae that allow them to adhere to uroepithelial cells. These strains also express a specific O antigen (O6) and a specific H antigen (K2), but these antigens are not directly involved in adhesion or invasion.

Another way that antigenic type can affect virulence is by allowing the pathogen to escape or evade the host immune response. Some pathogens can change their surface antigens by mechanisms such as gene conversion, site-specific DNA inversions, hypermutation, or recombination of sequence cassettes. This process is called antigenic variation and it results in a heterogeneous population of pathogens with different antigenic profiles. Antigenic variation can help the pathogen to avoid recognition by antibodies or immune cells that target specific antigens. For example, some strains of Neisseria gonorrhoeae can switch their expression of type IV pili and Opa proteins, which are involved in adhesion and invasion of mucosal cells. By changing these antigens, the bacteria can evade the immune response and persist in the host.

Therefore, antigenic type can play an important role in virulence by indicating the presence of other virulence factors, by reflecting the evolutionary history of the pathogen, or by enabling the pathogen to escape the host immune response.

O antigen and H antigen are two types of surface structures that are used for the serologic classification of bacteria, especially Salmonellae. They differ in their composition, location, extraction, immunogenicity, agglutination, and role in virulence. The following table summarizes some of the key differences between O antigen and H antigen:

Some additional differences between O antigen and H antigen are:

• O antigens have no phases, while flagellar antigens exist in two alternative phases- Phase I and II. Most of them are biphasic except S. Typhi which is monophasic.
• O antigens are more specific than H antigens, meaning that they can distinguish between more serotypes of bacteria.
• O antigens are more conserved than H antigens, meaning that they are less likely to change due to mutation or recombination.

Boivin antigen is another name for O antigen, which is derived from the French bacteriologist Paul Boivin who first demonstrated its presence in Salmonella typhi in 1916. Boivin antigen is a part of the lipopolysaccharide (LPS) molecule that forms the outer layer of the cell wall of gram-negative bacteria. It consists of a repeating unit of sugar molecules called O-specific polysaccharide (OPS) that varies in composition and length among different bacterial strains. Boivin antigen is responsible for the serological specificity of gram-negative bacteria and can elicit an immune response in the host. Boivin antigen can also protect the bacteria from the action of complement and phagocytosis by masking the more immunogenic core and lipid A regions of the LPS molecule.

Boivin antigen can be detected by agglutination tests using specific antisera that contain antibodies against the OPS. The agglutination reaction indicates that the bacteria and the antisera share the same O antigen type. Boivin antigen can also be extracted from the bacterial cells by boiling them in saline solution or treating them with organic solvents. The extracted Boivin antigen can then be used to produce specific antisera or to identify unknown bacterial strains by comparing their agglutination patterns with known reference strains.

Boivin antigen is a useful tool for the classification and identification of gram-negative bacteria, especially Salmonella species. There are more than 50 different O antigen types among Salmonella, which are designated by numbers or letters. For example, Salmonella typhi has O antigens 9 and 12, while Salmonella enteritidis has O antigens 9 and 12 as well as O antigens 1 and 4. Some Salmonella strains may also have more than one OPS chain attached to their LPS molecule, resulting in a complex O antigen structure. For example, Salmonella paratyphi A has O antigens 1, 2, and 12.

Boivin antigen is also important for the pathogenesis and epidemiology of gram-negative infections. Some O antigens may be associated with certain diseases or geographical regions. For example, Salmonella typhi, which causes typhoid fever, has O antigens 9 and 12 that are common in Asia and Africa, while Salmonella paratyphi B, which causes paratyphoid fever, has O antigens 1, 4, and 5 that are common in Europe and North America. Some O antigens may also confer resistance or susceptibility to certain antibiotics or bacteriophages. For example, Salmonella typhimurium has O antigens 4 and 5 that make it resistant to phage P22, while Salmonella enterica has O antigens 8 and 20 that make it susceptible to phage P22.

In summary, Boivin antigen is a term used to describe the O antigen of gram-negative bacteria, which is a component of their LPS molecule that determines their serological specificity and plays a role in their virulence and immunity. Boivin antigen can be detected by agglutination tests or extracted by chemical methods for further analysis. Boivin antigen is useful for the classification and identification of gram-negative bacteria, especially Salmonella species, as well as for understanding their pathogenesis and epidemiology.

As mentioned earlier, most S. enterica serovars have two alternately expressed H antigens, also referred to as ‘phases’. The phase-1 and phase-2 flagellin proteins are encoded by fliC and fljB, respectively. The phase switch is regulated by the invertase hin and the fliC repressor gene fljA. However, some serovars have only one flagellin gene and are therefore called monophasic variants. For example, S. Enteritidis does not possess a second flagellin gene and has the antigenic formula 1,9,12:g,m:-. Similarly, some variants of S. Typhimurium have only one flagellin gene (usually fliC) and have the antigenic formula 1,4,,12:i:-. These monophasic variants are epidemiologically important as they can cause outbreaks of salmonellosis in humans and animals.

The O antigen, on the other hand, does not have phases. It is determined by the oligosaccharides associated with the lipopolysaccharide (LPS) in the cell wall. The O antigen is heat-stable and alcohol-resistant, unlike the H antigen which is heat-labile and alcohol-sensitive. The O antigen is also less immunogenic than the H antigen and produces lower antibody titers. The O antigen is the most important virulence factor of S. enterica as it protects the bacteria from phagocytosis and complement-mediated killing. The O antigen also contributes to the endotoxic activity of LPS.

Therefore, the biphasic and monophasic properties of S. enterica are related to the H antigen, not the O antigen. The presence or absence of a second flagellin gene determines whether a serovar is biphasic or monophasic. The O antigen is constant and does not change with phases.

O and H antigens are useful for the identification and classification of bacteria, especially those belonging to the Enterobacteriaceae family, such as Salmonella, Escherichia, Shigella, and Yersinia. These bacteria can cause various diseases in humans and animals, such as gastroenteritis, typhoid fever, dysentery, plague, and urinary tract infections. By detecting the specific O and H antigens of these bacteria in clinical samples or cultures, the diagnosis can be confirmed and the appropriate treatment can be given.

O and H antigens are also important for the epidemiological surveillance and outbreak investigation of bacterial infections. By comparing the O and H antigen profiles of different isolates of the same species, the source and transmission of the infection can be traced and the relatedness of the strains can be determined. This can help to prevent further spread of the disease and to implement control measures.

O and H antigens are also used as targets for vaccine development against some bacterial pathogens. For example, a conjugate vaccine that combines the O antigen of Salmonella Typhi with a carrier protein has been shown to be effective in preventing typhoid fever in children. Similarly, a vaccine that contains the O antigen of Yersinia pestis has been developed to protect against plague.

O and H antigens are therefore valuable tools for the characterization, diagnosis, epidemiology, and prevention of bacterial infections. However, there are some limitations and challenges associated with their use, such as antigenic variation, cross-reactivity, and technical difficulties in serotyping. Therefore, other methods such as molecular typing and genomics are also needed to complement the serological approach.

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