Vibrio mimicus are the reservoirs of the heat-stable enterotoxin gene (nag-st) among species of the genus Vibrio

Molecular and Genomic Characterization of Vibrio mimicus Isolated from a Frozen Shrimp Processing Facility in Mexico

Vibrio mimicus is a gram-negative bacterium responsible for diseases in humans. Three strains of V. mimicus identified as V. mimicus 87, V. mimicus 92 and V. mimicus 93 were isolated from a shrimp processing facility in Guaymas, Sonora, Mexico. The strains were analyzed using several molecular techniques and according to the cluster analysis they were different, their similarities ranged between 51.3% and 71.6%. ERIC-PCR and RAPD (vmh390R) were the most discriminatory molecular techniques for the differentiation of these strains. The complete genomes of two strains (V. mimicus 87, renamed as CAIM 1882, and V. mimicus 92, renamed as CAIM 1883) were sequenced. The sizes of the genomes were 3.9 Mb in both strains, with 2.8 Mb in ChI and 1.1 Mb in ChII. A 12.7% difference was found in the proteome content (BLAST matrix). Several virulence genes were detected (e.g. capsular polysaccharide, an accessory colonization factor and genes involved in quorum-sensing) which were classified in 16 categories. Variations in the gene content between these genomes were observed, mainly in proteins and virulence genes (e.g., hemagglutinin, mobile elements and membrane proteins). According to these results, both strains were different, even when they came from the same source, giving an insight of the diversity of V. mimicus. The identification of various virulence genes, including a not previously reported V. mimicus gene (acfD) in ChI in all sequenced strains, supports the pathogenic potential of this species. Further analysis will help to fully understand their potential virulence, environmental impact and evolution.

Distribution of Virulence-Associated Genes inVibrio mimicusIsolates from Clinical and Environmental Origins

Distribution of virulence-associated genes in Vibrio mimicus was studied including the toxin genes ctxA, tdh, st and vmh and the genes necessary for regulation of toxin production, toxR, toxS, toxT, tcpA and tcpP. Approximately half of clinical V. mimicus isolates possessed one or more genes encoding V. cholerae enterotoxic factors such as ctxA, tdh and st. All of the clinical and environmental isolates possessed vmh encoding V. mimicus hemolysin (VMH). The ctxA encoding cholera toxin was detected in only 2 strains, 5% of the clinical isolates. Furthermore, there were very few strains possessing tcpP and toxT needed for the expression of ctxA. These results may suggest that VMH is a more important pathogenic factor than well recognized toxins such as cholera toxin (CT) in V. mimicus infection.

Enterobacter sakazakii: infectivity and enterotoxin production in vitro and in vivo

Enterobacter sakazakii has been implicated as the causal organism in a severe form of neonatal meningitis, with reported mortality rates of 40 to 80%. Dried infant formula has been identified as a potential source of the organism in both outbreaks and sporadic cases. In this study, clinical and foodborne isolates of E. sakazakii were evaluated for enterotoxin production by the suckling mouse assay. In addition, suckling mice were challenged both orally and by intraperitoneal injection. Of 18 E. sakazakii strains evaluated, four were found to test positive for enterotoxin production. All strains of E. sakazakii were lethal to suckling mice at 10(8) CFU per mouse by intraperitoneal injection, while two strains caused death by the peroral route. In in vitro assays, CHO, Vero, and Y-1 cells demonstrated both cell lysis and rounding when exposed to E. sakazakii strain LA filtrates. This is the first report describing any putative virulence factors of E. sakazakii.

Detection of virulence associated genes in clinical strains of vibrio mimicus

A total of 42 clinical strains of Vibrio mimicus were examined for the presence of virulence associated genes toxR, toxS, toxT, tcpP, ctx and tcpA by PCR assay. Almost all strains were shown to have the toxR gene, while the toxS gene was found in 27 strains. On the other hand, five strains possessed both toxT and tcpP genes, but others had neither. Only two strains were positive for amplification of the ctx gene, whereas no PCR product with tcpA primers was detected. The results indicate the incomplete copies of virulence cascade in V mimicus strains. The pathogenesis and epidemic potential of this species is also discussed.

Detection of genes encoding cholera toxin (CT), zonula occludens toxin (ZOT), accessory cholera enterotoxin (ACE) and heat-stable enterotoxin (ST) in Vibrio mimicus clinical strains

A total of 51 clinical strains of Vibrio mimicus were searched for the presence of virulence-associated genes, like ctx, zot or ace genes which locate in "cholera virulence cassette," and the st gene by polymerase chain reaction. Moreover, the pathological potential of each clinical strain was also examined by rabbit ileal loop (RIL). Three strains showed to have the ctx gene, of which only one strain was zot gene-positive. Meanwhile, one other strain was zot+ but ctx-. All of these four strains were found to have the ace gene and to belong to serogroup O115. Nine strains showed to carry the st gene. However, none of these ST-gene-positive strains was indicated to contain the genes located in the "cholera virulence cassette." It is of interest to note that all of the RIL-positive and/or virulence gene-positive strains were restricted to three serogroups, O20, O41 and O115. These results suggest a significant association between O antigens and enterotoxic activities in V. mimicus clinical strains, and clearly demonstrate multifactorial virulence potentials of this human pathogen.

Pathogenic vibrios in the natural aquatic environment

In recent years, members belonging to the genus Vibrio of the family Vibrionaceae have acquired increasing importance because of the association of several of its members with human disease. The most feared of the Vibrio species is Vibrio cholerae, the causative agent of cholera, a devastating disease of global significance. Other important vibrios of medical importance are V. parahemolyticus, V. vulnificus, V. mimicus, and to a lesser extent V. fluvialis, V. furnissii, V. hollisae, and V. damsela. Recent studies have also implicated V. alginolyticus and V. metschnikovii in human disease, although their complete significance has not yet been established. The virulence of all medically important vibrios is aided by a variety of traits that help breach human defenses. In this review, we provide an overview of the environmental distribution of the pathogenic vibrios and the important virulence traits that enable them to cause disease.

Expression of virulence-related properties by, and intestinal adhesiveness of, Vibrio mimicus strains isolated from aquatic environments

A study of the major pathogenic characteristics of Vibrio mimicus was carried out with 77 strains isolated from aquatic environments in Okayama, Japan. Of the strains tested, 96% demonstrated in vitro adherence to the rabbit intestinal mucosa, of which 36, 20, and 43% belonged to the strongly, moderately, and weakly adhesive groups, respectively. Of the 27 strains which appeared to be enterotoxigenic in the experiments using rabbit ileal loops, 74% belonged to the strongly adhesive group. All strains of V. mimicus at early log phase showed cell-mediated hemagglutination, and 70% of strongly hemagglutinative strains belonged to the strongly adhesive group, implying a possible correlation between cell-mediated hemagglutination and bacterial adherence. However, no significant correlation could be detected in the production of putative exocellular pathogenic factors and bacterial adherence or enterotoxigenicity.

Detection of heat-stable enterotoxin genes among Australian Vibrio cholerae O1 strains

DNA probes derived from the heat-stable enterotoxin gene of Vibrio cholerae non-O1 (stn), and the cholera toxin gene (ctx), were used to screen 199 strains of V. cholerae O1, which were isolated within Australia from 1977-1986. 13 environmental strains isolated from the riverine environment in Southeast Queensland in 1980 and 1981, hybridized with the stn and ctx DNA probes. The concentrated supernatant of 6 of these strains elicited fluid accumulation in the infant mouse assay both before and after heating at 100 degrees C for 5 min. Genetic relationships among the 13 stn+ strains were studied by a comparison of the rRNA-RFLPs (ribotyping) and by Southern blot analysis with a stn gene probe. The results indicate that there is a clonal relationship among the Australian stn+ strains and that there is an environmental reservoir of stn genes among Australian V. cholerae O1 isolates.

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.