Production of cytolethal distending toxin and other virulence characteristics of Escherichia coli strains of serogroup O86

Production of Cytolethal Distending Toxin and Other Virulence Characteristics of Escherichia coli Strains of Serogroup O86

Vol. 96(5): 703-708, July 2001

Ângela Cristina Rodrigues Ghilardi/⁺, Tânia Aparecida Tardelli Gomes*, Luiz Rachid Trabulsi**

Seção de Bacteriologia, Instituto Adolfo Lutz, Av. Dr Arnaldo 351, 9° andar, 01246-902 São Paulo, SP, Brasil*Departamento de Microbiologia, Universidade Federal de São Paulo, São Paulo, SP, Brasil

**Laboratório Especial de Microbiologia, Instituto Butantã, São Paulo, SP, Brasil

Genetic and phenotypic virulence markers of different categories of diarrhoeagenic Escherichia coli were investigated in 106 strains of enteropathogenic E. coli (EPEC) serogroup O86. The most frequent serotype found was O86:H34 (86%). Strains of this serotype and the non motile ones behaved as EPEC i.e., carried eae, bfpA and EAF DNA sequences and presented localised adherence to HeLa cells. Serotypes O86:H2, O86:H6, O86:H10, O86:H18, O86:H27 and O86:H non determined, belonged to other categories. The majority of the strains of serotype O86:H34 and non motile strains produced cytolethal-distending toxin (CDT). The ribotyping analysis showed a correlation among ribotypes, virulence markers and serotypes, thus suggesting that CDT production might be a property associated with a universal clone represented by the O86:H34 serotype.

Key words: enteropathogenic Escherichia coli - serogroup O86 - cytolethal distending toxin - virulence markers - diarrhoea

Escherichia coli strains associated with diarrhoeal disease were first classified on the basis of their somatic (O) and flagelar (H) antigens. In 1987, the World Health Organization (WHO) recognized as enteropathogenic E. coli (EPEC) the following serogroups: O26, O55, O86, O111, O114, O119, O125, O126, O127, O128, O142 and O158 (WHO 1987). With the introduction of tissue culture assays and DNA probes to study the virulence of E. coli strains associated with diarrhoea, it has been possible to demonstrate the existence of at least six established or putative categories of diarrhoeagenic E. coli: enteropathogenic E. coli (EPEC), enterohemorragic E. coli (EHEC), enterotoxigenic E. coli (ETEC), enteroinvasive E. coli (EIEC), enteroaggregative E. coli (EAEC) and difuselly adhering E. coli (DAEC). Recently, a new category termed atypical EPEC has been proposed (Nataro & Kaper 1998). In addition to the defining characteristics, strains of some serogroups might present other virulence properties that could contribute to their pathogenesis.

Studies carried out by several authors in the last few years, have shown that the EPEC serogroups are very heterogeneous. In general, they include more than one of the seven diarrhoeagenic E. coli categories (with the exception of EIEC) and each category have specific H antigens, corresponding to defined E. coli serotypes (Campos et al. 1994, Rodrigues et al. 1996, Gonçalves et al. 1997, Valle et al. 1997).

The first E. coli strain of serogroup O86 (E. coli E990) was identified by Taylor and Charter (1952) as the cause of an outbreak of acute diarrhoea in children committed to day care in London. Since then, strains of this serogroup, mainly of serotype O86:H34, have been isolated in outbreaks and sporadic cases of diarrhoea in many parts of the world (Toledo et al. 1983, Gomes et al. 1989, Scotland et al. 1996, Cravioto et al. 1996).

Cytolethal Distending Toxin (CDT) is a heat-labile factor which induces a progressive distension and death of various cell lines cultivated in vitro. CDT production has been detected in the supernatants of some E. coli, Shigella spp., Campylobacter spp., Haemophilus ducrey and Actinobacillus actinomycetemcomitans strains (Johnson & Lior 1987, 1988a,b, Cope et al. 1997, Mayer et al. 1999). However, so far, CDT production has not been associated to any particular category of diarrhoeagenic E. coli, except for a few strains of EPEC serogroups including the O86:H34 serotype (Johnson & Lior 1988a, Guth et al. 1994).

The purpose of this study was to determine the virulence characteristics of serogroup O86 strains isolated in Brazil and in other countries and to investigate the relationship between serotypes and ribotypes in this serogroup.

Bacterial strains and serotyping - The study employed 90 E. coli strains which were isolated from faeces of children with diarrhoea at Instituto Adolfo Lutz, São Paulo, Brazil, between 1977 and 1991. For comparative purposes we included 14 E. coli strains from other countries, kindly provided by Dr Fleming Schultz, from Department of Gastrointestinal Infections, Denmark and E. coli strains from Chile and Centers for Disease Control (CDC), Atlanta, Ga, included in Dr Luiz R Trabulsi's collection.

The original strains kept at room temperature were grown in Triptic Soy Broth (TSB) and streaked out onto nutrient agar for serogroup confirmation and determination of H antigens (Edwards 1986).

Adherence to HeLa cells - Adherence was assayed in the presence of 1% D-mannose (Cravioto et al. 1979). Briefly monolayers were examined after 3 h of incubation, and poorly adherent and non-adherent strains were submitted to a period of assay of 6 h E. coli strains E2348/69, O42, and C1845 were used as control of localized, aggregative and diffuse adherence, respectively (Valle et al. 1997).

Hybridisation with DNA probes - All strains were submitted to colony hybridisation assays (Maas 1983), using specific DNA probes for different categories of diarrhoeagenic E. coli: eae (E. coli attaching and effacing gene encoding intimin), EAF (EPEC adherence factor plasmid), bfpA (bundle forming pilus), daaC (associated with diffuse adherence), EAEC (associated with aggregative adherence), INV (E. coli invasiveness), Stx-1 and 2 (Shiga-toxin types 1 and 2), LT-I and II (heat-labile enterotoxin types I and II), ST-I p and h (heat-stable enterotoxin type I, from E. coli of porcine and human origin, respectively) (Rodrigues et al. 1996). The CDT probe used was the 1,375 bp- Acc I fragment of pCVD448 derived from E. coli E6468/62 (O86:H34) (Scott & Kaper 1994). Probes were isotopically labelled and used in colony hybridisation assays as described elsewhere (Rigby et al. 1977).

Citotoxicity assays - CDT production was investigated in CDT probe positive and negative strains by testing in HeLa cells as described previouly (Johnson & Lior 1988a). Briefly, strains were grown statically in 2 ml of Brain Heart Infusion broth (BHI) at 37°C, for 48 h. The cells were pelleted and the supernatant fraction was retained and filtered. To prepare the sonicate, the pellet was resuspended in 1 ml of Eagle's Minimal Essential Medium, modified with Earle's salts (MEM) and the suspension was sonicated as described by Pickett et al. (1994). Supernatants and sonicates were diluted 1:4 and 1:10 respectively for the assays and morphological changes were monitored every 24 h for 5 days. Strain 0741-4 (O86:H34) (Guth et al. 1994) was used as positive control. Sonicates presenting activity similar to that of the CDT control were submitted to neutralisation assays with rabbit anti-CDT serum, kindly provided by Dr H Lior. Equal volumes of sonicate diluted 1:5 and antisera (imunne and non-imunne) diluted 1:128 in MEM, were mixed and incubated at 37°C. After 2 h the mixture was submitted to citotoxicity assays.

Ribotyping - The genomic DNA of 33 strains carrying different H antigens and virulence markers, was extracted by the method described by Brenner et al. (1982). Approximately 2 µg of DNA were digested with BglI (Sigma) and eletrophoresed with a marker (Haemophilus aegyptius strain, 320/86), digested with EcoRI (Dalla-Costa et al. 1998). The DNA was transferred onto a nylon membrane (Magnagraph, USA) in a vacuum blotting system (Vacum Gene XL, Pharmacia), according to the manufacture's instructions. A cDNA probe prepared by reverse transcriptation of 16S plus 23S rRNA (Boerhinger, Germany) and labelled using the digoxigenin DNA labelling and detection Kit (Boerheinger) as described (Popovic et al. 1993).

The 106 strains tested, belonged to six distinct serotypes. Seven strains were non motile (O86:H) and the H antigen of one strain could not be determined (O86:H?) (Table I).

Table I

The data in Fig. 1 showed that O86:H34 was the predominant serotype in São Paulo, between 1977 and 1982.

Figure 1

None of the strains studied reacted with the probes for daaC, INV, LT-I and II, ST-Ip and h, and Stx 1 and 2. Table II shows the results obtained with the other probes and the adherence patterns presented by different strains. Presence of EPEC sequences (eae, EAF and bfpA) was detected in most strains of serotype O86:H34 (72 of 78 Brazilian strains and all 6 strains of other countries). Moreover, these probe positive strains produced localised adherence (LA) in HeLa cells. Four of 7 O86:H- strains had these same properties. All 10 strains of serotypes O86:H18 and O86:H2 produced the aggregative adherence (AA). Strains of the remaining serotypes were eae positive; (O86:H6, O86:H?) or lacked any virulence property searched for; (O86:H27, O86:H10).

Table II

Most of the strains (79 of 85) of serotypes O86:H34 and O86:H- isolated in São Paulo and all strains of serotype O86:H34 from other countries had the CDT sequence and produced a progressive distention on HeLa cells after 48 h of incubation. The distending activity of the sonicates was completely neutralized by rabbit antiserum to CDT.

Among the E. coli O86 strains isolated in São Paulo we observed five ribotypes (RT) named RT-A to RT-E. The O86:H34 and the virulent O86:H- strains, including the prototype strain E990 (O86:H-), belonged to RT-A. The O86:H- with no virulence sequences belonged to RT-B. The serotypes O86:H10, O86:H18 and O86:H6 belonged to RT-C, RT-D and RT-E, respectively. The O86 E. coli strains isolated in other countries belonged to four ribotypes, RT-I to RT-IV. The O86:H2, O86:H18, O86:H34 and O86:H27 strains belonged to RT-I, RT-II, RT-III and RT-IV, respectively (Fig. 2).

Figure 2

In this study, we examined a diverse collection of E. coli O86 strains isolated from human diarrhoea, in different countries. We observed that the most frequent serotype of serogroup O86, found in São Paulo (O86:H34) (Toledo et al. 1983, Gomes et al. 1989) is not the most frequent in other countries (Scotland et al. 1996, Giamanco et al. 1996). Therefore, geographic variation in the predominance of the EPEC serotypes may occur. The serotype O86:H34 was predominant in São Paulo between 1977 and 1991 and since then, it has been isolated no more frequently than the other less common serotypes of this serogroup, found in São Paulo. This kind of distribution suggested the ocurrence of an outbreak, which started before or at the beginning of 1977 and disappeared in 1982, or that this serotype was endemic until 1990, when its frequency started to fall (Fig. 1).

Typical EPEC strains, i.e., strains carrying EAF, bfpA, eae and presenting LA in the 3 h adherence assay, were found in 82 (77.4%) of the strains studied, which were comprised in the serotypes O86:H34 and O86:H-. The strains of serotypes O86:H18 and 6 of 8 O86:H2 strains were categorised as EAEC, since they presented the aggregative adherence pattern and hibridised with the EAEC probe. Lack of hybridisation with the EAF probe in four O86:H34 strains and with the EAEC probe in two O86:H2 strains, is probably attributed to genetic alterations that often occur in E. coli stored for long periods (Rodrigues et al. 1996, Gonçalves et al. 1997). Strains of serotypes O86:H6 and O86:H? showed the characteristics of atypical EPEC (i.e., eae only). No sequences associated with intestinal infections were found in the strains of serotypes O86:H27 and O86:H10 (Table II). The data described above, agreed with the findings reported by other authors (Scotland et al. 1996, Cravioto et al. 1996, Giamanco et al. 1996).

Of particular interest in this study was the demonstration that besides having all the virulence characteristics of typical EPEC, most of the strains of serotypes O86:H34 and the virulent O86:H- strains produced CDT, and it could act as an additional virulence factor of diarrhoea. Although O86:H34 and O86:H- may be considered as distinct serotypes, it is likely that the virulent H- strains were derived from O86:H34 strains, because they have identical virulence characteristics and were isolated in the same period of time (Fig. 1, Table II). Until now, among the EPEC serotypes, frequent production of CDT seemed to be an exclusive characteristic of the serotype O86:H34. In fact, in a survey of CDT production by EPEC serogroups O55, O86, O119, O126, O127 and O142, Guth et al. (1994) found that only O86:H34 strains produced CDT. CDT producing strains have been reported among E. coli strains isolated in Canada and India, although in one of these studies serotyping was not performed (Bouzari et al. 1990) and in the other one, a single CDT positive strain of serotype O86:H34 was found (Johnson & Lior 1988a). A significant percentage of CDT producing strains belonging to the same serotype has not been reported so far. The role of CDT in the pathogenicity of E. coli strains has not been studied. It is possible that CDT production alone, has no implication for pathogenesis but its production by E. coli strains in combination with other virulence factors, such as the ability to adhere to the intestinal mucosa, may be significant. Albert et al. (1996) did not find an important association between CDT positive E. coli strains and diarrhoea but it has been shown in a recent study that this toxin has diarrhoeagenic properties in an animal model (Okuda et al. 1997).

We found an association between O:H serotypes and ribotypes among the studied strains. This relationship allowed the identification of non motile strains (O86:H-) by comparing their fingerprints with those of the motile strains, i.e., RT-B (O86:H-) and RT-I (O86:H2), the prototype strain E990 (O86:H-), and RT-A and RT-III (O86:H34) (Fig. 2). This finding confirmed that this kind of flagellar variation occurred with relatively high frequency, a phenomenon that has also been observed in other EPEC serogroups (Rodrigues et al. 1996, Gonçalves et al. 1997). Despite of that, the ribotype presented by the O86:H34 strains isolated in Brazil (RT- A) was the same of the O86:H34 strains isolated in other countries (RT- III) (Fig. 2), suggesting that CDT production could be associated with a clone distributed all over the world represented by strains belonging to the serotype O86:H34.

To James B Kaper (University of Maryland) for providing us with the CDT probe and Dr Fleming Schultz (Departament of Gastrointestinal Infections, Denmark) who kindly sent us the strains from other countries.

Fig. 1 | Fig. 2 | Table I | Table II

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Publication Dates

• Publication in this collection
  26 July 2001
• Date of issue
  July 2001

History

• Accepted
  22 Feb 2001
• Received
  28 Oct 2000