|Abstract||This National Standard Method (NSM) describes the identification of Vibrio species.|
The genus Vibrio is a member of the family Vibrionaceae and consists of at least 34 recognised
species. Vibrio cholerae can be serogrouped into 155 groups on the basis of somatic antigens.
Epidemic strains usually belong to serogroup O1, which can be further subdivided into Inaba, Ogawa and Hikojima subtypes. Epidemic strains of V. cholerae O1 can be further differentiated into El Tor and Classical biotypes. Strains not belonging to serogroup O1 are generally referred to as V. cholerae non-O1. In 1993 an outbreak of epidemic cholera began in Bengal caused by a new serogroup of non-O1 V. cholerae. Although initial isolates of this serogroup (O139) were resistant to vibriostatic agent O129, recently isolated strains are sensitive.
Vibrio species are curved, Gram-negative rods. They produce colonies 2 - 3 mm in diameter on blood agar and colonies on thiosulphate citrate bile salt sucrose (TCBS) are either yellow or green. Vibrio species are facultative anaerobes, motile by a single polar flagellum, and are oxidase-positive (except Vibrio metschnikovii). They are usually sensitive to the vibriostatic agent O129 (2,4-diamino-6, 7-diisopropylpteridine phosphate – 150 ìg disc). Growth is stimulated by sodium ions (halophilic) - the concentration required is reflected in the salinity of their natural environment. V. cholerae (the causative agent of cholera) is not halophilic.
V. cholerae O1 depends on the detection of the O1 antigen on the surface of the bacterium, and
therefore does not identify V. cholerae O139 strains.
V. cholerae O1 classical biotype is VP-negative and is sensitive to polymyxin (50 IU disc). V. cholerae O1 El Tor biotype is VP-positive and is resistant to polymyxin.
Twelve species of the genus Vibrio have been incriminated in gastrointestinal and extra-intestinal
diseases in man - the most important of these is cholera.
Principles of identification:
Isolates from primary culture are identified by colonial appearance, Gram’s stain, serology
(agglutination with specific antisera) and biochemical testing. If confirmation of identification is
required, isolates should be sent to the Reference Laboratory. All identification tests should ideally be performed from non-selective agar. The oxidase test may give false negative results if performed from TCBS agar.
It should be noted that V. hollisae will not grow on TCBS.