Vibrio cholerae in disguise?a disturbing entity

Detection of ctx gene positive non-O1/non-O139 V. cholerae in shrimp aquaculture environments

Water and post-larvae samples from black tiger (Penaeus monodon) shrimp hatcheries; pond water, pond sediment and shrimp from aquaculture farms were screened for the presence of V. cholerae. A V. cholerae-duplex PCR method was developed by utilizing V. cholerae species specific sodB primers and ctxAB genes specific primers. Incidence of V. cholerae was not observed in shrimp hatchery samples but was noticed in aquaculture samples. The incidence of V. cholerae was higher in pond water (7.6%) than in pond sediment (5.2%). Shrimp head (3.6%) portion had relatively higher incidence than shrimp muscle (1.6%). All the V. cholerae isolates (n = 42) belonged to non-O1/non-O139 serogroup, of which 7% of the V. cholerae isolates were potentially cholera-toxigenic (ctx positive). All the ctx positive V. cholerae (n = 3) were isolated from the pond water. Since, cholera toxin (CT) is the major contributing factor for cholera gravis, it is proposed that the mere presence of non-O1/non-O139 V. cholerae need not be the biohazard criterion in cultured black tiger shrimp but only the presence of ctx carrying non-O1/non-O139 V. cholerae may be considered as potential public health risk.

Aetiology of cholera in Tamil Nadu: recent observations

Vibrio cholerae was isolated from 1008 of 3496 stool samples (28.8%) examined in Tamil Nadu State, India, between November 1992 and December 1995. During November and December 1992, 363 of the 370 isolates serotyped (98%) were V. cholerae O139 (Bengal). The epidemic predominantly affected adults (91%; 597/656). Both V. cholerae O1 and O139 serotypes were sometimes isolated in the same locality from different individuals. From January 1993 onwards, the rate of isolation of V. cholerae O139 declined, and in 1995 V. cholerae E1 Tor (serotype O1) was isolated from most of the cases (85.6%; 131/153). V. cholerae E1 Tor has clearly not been replaced by serotype O139, but can survive during inter-epidemic periods and reappear at an opportune moment. The decline of serotype O139 may be due to the development of immunity as a result of repeated exposure.

Characterization of phage phi O139, a Vibrio cholerae O139 temperate bacteriophage with cohesive DNA termini

A temperate bacteriophage isolated from Vibrio cholerae O139, the new epidemic strain of cholera, was found to have a polyhedral head 65 nm in diameter and a rigid contractile tail 120 nm in length. The phage chromosome was a double-stranded DNA of 35 kb, with unique cohesive ends and had a G + C content of 58.8%. A restriction map of the phage DNA was constructed using the restriction endonucleases AvaI and BstEII. The phage, whose presence could be detected in nine out of 13 V. cholerae O139 isolates tested, was found to have identical chromosomal integration sites in all the strains. The phage attachment site (attP) was found to be located very close to one end of the genome.

Characterization of phenotypic, serological, and toxigenic traits of Vibrio cholerae O139 bengal

Biochemical and physiological traits of a collection of strains of Vibrio cholerae O139 Bengal isolated from India, Bangladesh, and Thailand showed that these strains formed a phenotypically homogeneous group with identical characteristics that were essentially similar to those of the O1 serogroup. Resistance to 150 micrograms of the vibriostatic agent O/129 (2,4-diamino-6,7-diisopropylpteridine) and Mukherjee's El Tor phage 5 and classical phage IV and the nonagglutinability of the strains with O1 antiserum were the only discernible differences between the O139 and O1 serogroups. Extensive serological characterization further revealed the O139 serogroup to be distinct from the existing 138 serogroups of V. cholerae. Antiserum raised against the O139 serogroup required absorption with the R reference strain CA385 and with the reference strain representing serogroup O22 to remove cross-reacting agglutinins. All of the 223 representative strains of V. cholerae O139 examined hybridized with DNA probes specific for the cholera toxin (CT) gene, zonula occludens toxin gene, and El Tor hemolysin gene but not with the probe specific for the heat-stable enterotoxin gene. The amount of CT present in stool samples of patients infected with the O139 serogroup was higher than that found in stools of patients infected with O1 El Tor, and this echoed findings that the amount of CT produced by O139 strains in vitro was higher than that produced by the O1 El Tor strains. The nucleotide sequences of the genes encoding the A and B subunits of CT of the O139 serogroup were identical to the sequences reported for the CT gene of O1 El Tor. The CT gene of O139 strains could be amplified by using primers developed for detection of the CT gene of the O1 serogroup by a PCR assay, which could also be used to detect the CT gene in stool samples of patients infected with strains of the O139 serogroup.