Serotypes and Their Significance

Serotypes are groups within a single species of microorganisms, such as bacteria or viruses, which share distinctive surface structures.

For example, all Salmonella bacteria look alike under a microscope but they can be separated into many serotypes based on the presence of a type of somatic (O) antigen and a flagellar (H) antigen. Another member of the Enterobacteriaceae family, E.coli has also serotyped on the basis of the presence of these antigens. E. coli O157:H7 is a serotype of a virulent strain of E. coli associated with hemorrhagic colitis and hemolytic–uremic syndrome (HUS).

As microorganisms from the same species can differ into in the antigenic determinants expressed on the cell surface, they can be grouped on the basis of serological typing. Serotyping is one of the classic tools for epidemiological study and can be applied to any species that has multiple serotypes such as Escherichia coli, Salmonella, Vibrio, Shigella, Streptococcus pneumoniae </em>etc.

For example, there are > 84 strains of Streptococcus pneumoniae, each differing in the nature of its capsular material. These differences can be detected by capsular swelling (termed the Quellung reaction) if antisera specific for the capsular types are used.

Having multiple serotypes of a pathogen possess challenges to eradicate as well as to adopt measures to prevent an infection.  Re-infection with the same pathogen is common, as the immunity formed against infecting serotype fails to protect it from another serotype.  This is the reason why so many people encounter multiple episodes of the common cold in their lives.  There are over 115 distinct serotypes of rhinoviruses (which causes about 50% of the common cold cases) and infections with rhinoviruses result in the development of serotype-specific protective immunity. Repeated infections with these pathogens occur because of the presence of a large number of distinct serotypes of each virus.

It is difficult to make an effective vaccine if the pathogen of interest has many serotypes. At least 100 distinct rhinovirus serotypes cause the common cold, making the development of a preventive vaccine impractical. Vaccines are not available for streptococcal diseases other than streptococcal pneumonia because of the large number of serotypes. People get flu shot in every season to protect themselves from an influenza virus serotype that is most likely to circulate in this particular season.

Pathogens and their serotypes

Name of the organism 
Number of serotypes 
Further notes 


Streptococcus pneumoniae
>90
Pneumococcal conjugate vaccine 7 (PCV7) contains serotypes 4, 6B, 9V,
14, 18C, 19F and 23F. PCV10 contains three additional serotypes 1, 5 and 7F.


Poliovirus
3
Oral polio vaccine (OPV) was initially trivalent and later WHO removed type 2, making it bivalent. IPV contains all three serotypes of poliovirus. (ironman.greaterzion.com/)  


Rhinoviruses
>115
  Serotype-specific immunity


C. trachomatis
15 (A, B, Ba, C, D, E, F, G, H, I, J, K, L1, L2 and L3)
Diseases vary according to the serotypes (serovars) involved* (check the table below).  


Adenovirus
52
  Serotype-specific immunity


Dengue
4 (DEN-1, DEN-2, DEN-3, DEN-4)
 


Salmonella</em>
>2500
Less than 100 serotypes account for most human infections. Enteric fever is mainly caused by Salmonella enterica
serotype Typhi and Paratyphi.


Shigella
43
Shigella has 4 major subgroups and 43 recognized serotypes.


Haemophilus influenzae </em>
6 (a, b, c, d, e, and f)
H.influenzae serotype b is considered the most pathogenic.


Herpes Simplex Virus
2
Type 1 infections are most common in the upper body, whereas type 2 infections produce genital lesions.

Chlamydia trachomatis serotypes (serovars) and diseases

References and further reading

• Color Atlas and Textbook of Diagnostic Microbiology, Koneman, 5th edition