Plasmid profiles of Shigella dysenteriae type 1 isolates from Ethiopia with special reference to R-plasmids

Plasmid profiles of 80 Shigella dysenteriae type 1 (Shiga's bacillus) strains, collected between 1974 and 1985 in Ethiopia, were examined. Strains with the dominant antibiotic-resistance (R-) type--resistance to ampicillin (A), chloramphenicol (C), streptomycin (S), sulphadiazine (Su) and tetracycline (T)--showed two distinct plasmid profiles (PP). Six plasmid bands were demonstrated in "Ethiopian strains" with PP-1A isolated between 1974 and 1982. In mating experiments with these strains, Escherichia coli K12 recipients showed plasmids pYH10a (72 Mda, atypical Inc FIme, coding for ACSSuT resistance). Eight plasmid bands were demonstrated in strains with PP-2A. These strains were first isolated in 1980 and carried plasmid pYH11a (40 Mda, Inc X, coding for ACT resistance). Strains with PP-2A were identical with a "Zairian strain" described elsewhere. Strains with R-type ACT were "Zairian strains" lacking the 5.1- and 4.2-Mda plasmids. Those with R-type CSSuT were temporally clustered in 1978-1980 and carried plasmid pYH12 (58 Mda, Inc B, coding for the same R-type). A trimethoprim-resistant strain (Gimira strain) had a pattern of small plasmids similar to those of the "Zairian strain" and is probably a subclone of the latter. The fact that a limited number of plasmid profiles have remained constant over many years shows the limitation of plasmid profile analysis as an epidemiological tool. However, when the usual profile is known for a given area, identification of a distinctly different pattern becomes easy and epidemiologically useful.     

Transfer of plasmid-borne aminoglycoside-resistance determinants in staphylococci

Aminoglycoside-resistance determinants in staphylococci are borne on conjugative and non-conjugative plasmids. The conjugative plasmids were found in methicillin-resistant strains of Staphylococcus aureus isolated recently in Darwin and Sydney, Australia and in Houston, Texas, USA. These plasmids and the class-2 conjugative plasmid reported by Archer and Johnston (1983) had similar patterns of EcoR1 restriction-endonuclease fragments, encoded resistance to gentamicin, kanamycin and neomycin, transferred to a non-lysogenic recipient in conditions that promoted close cell-to-cell contact and mobilised a small, non-conjugative plasmid. A further plasmid, pWG14, encoding resistance to kanamycin, neomycin, streptomycin, erythromycin and lincomycin, also displayed conjugative properties but did not mobilise the small, non-conjugative plasmid. The transfer frequency of all conjugative plasmids was stimulated by the addition of polyethylene glycol, particularly at concentrations above 20%, to mixtures of donor and recipient broth cultures. Polyethylene glycol appeared to promote close cell-to-cell contact between donor and recipient cells. A representative of the most common aminoglycoside-resistance plasmids in Australian isolates of methicillin-resistant S. aureus was non-conjugative and transferred by a bacteriophage-mediated system to a lysogenic recipient. With the exception of plasmid pWG14, the conjugative plasmids were also transferred by a bacteriophage-mediated system. Furthermore, cultural conditions that favoured conjugative transfer of plasmids inhibited bacteriophage-mediated transfer and vice versa. The efficacy of the two transfer systems for analysing the plasmids of gentamicin-resistant, methicillin-resistant isolates of S. aureus has been compared.     

Molecular characterization of vancomycin-resistant enterococcus faecium isolates from mainland China

Little is known about vancomycin-resistant enterococci in China. Thirteen pulsed-field gel electrophoresis-confirmed heterogeneous VanA-type vancomycin-resistant Enterococcus faecium (VRE) isolates were obtained from five Chinese hospitals from 2001 to 2005. The isolates were typed by multilocus sequence typing into nine different sequence types (STs), including five new STs (ST18, ST25, ST78, ST203, ST320, ST321, ST322, ST323, and ST335). Vancomycin resistance in each isolate was encoded on conjugative plasmids; two of the plasmids, pZB18 (67 kbp) and pZB22 (200 kbp), were highly conjugative and were able to transfer at high frequencies of around 10(-4) and 10(-7) per donor cell in broth mating, respectively. None of the plasmids identified in these isolates carried traA, which is usually conserved in the pMG1-like highly conjugative plasmid for E. faecium, implying that pZB18 and pZB22 were novel types of a highly conjugative plasmid in enterococci. Thirteen Tn1546-like elements encoding VanA-type VRE on the conjugative plasmids were classified into six types (types I to VI), and most of them contained both IS1216V and IS1542 insertions. The isolates carrying the type II element were predominant. The six type elements were different from that of a VanA-type Enterococcus faecalis strain isolated from Chinese chicken meat. The results suggested that the disseminations of VRE in these areas were by Tn1546-like elements being acquired by the conjugative plasmids and transferred among E. faecium strains.     

Possible connection between a widely disseminated conjugative gentamicin resistance (pMG1-like) plasmid and the emergence of vancomycin resistance in Enterococcus faecium

A total of 640 vancomycin-resistant Enterococcus faecium (VRE) isolates, which were obtained between 1994 and 1999 from the Medical School Hospital of the University of Michigan, Ann Arbor, were used in this study. Of the 640 strains, 611 and 29 were VanA and VanB VRE, respectively, based on PCR analysis. Four hundred ninety-two (77%) of the strains exhibited resistance to concentrations of gentamicin from 64 micro g/ml (MIC) to more than 1,024 micro g/ml (MIC). The gentamicin resistance of each of 261 (53%) of the 492 gentamicin-resistant strains was transferred to E. faecium at a frequency of about 10(-5) to 10(-6) per donor cell in broth mating. More than 90% of vancomycin resistances of the 261 strains cotransferred with the gentamicin resistances to E. faecium strains by filter mating. The conjugative gentamicin resistance plasmids were identified and were classified into five types (A through E) with respect to their EcoRI restriction profiles. The transfer frequencies of each type of plasmid between E. faecium strains or Enterococcus faecalis strains were around 10(-3) to 10(-5) per donor cell or around 10(-6) to 10(-7) per donor cell, respectively, in broth mating. Type A and type B were the most frequently isolated, at an isolation frequency of about 40% per VRE isolate harboring the gentamicin resistance conjugative plasmid. The plasmids did not show any homology in Southern hybridization with the pheromone-responsive plasmids and broad-host-range plasmids pAMbeta1 and pIP501. The EcoRI or NdeI restriction fragments of each type of plasmids hybridized to the conjugative gentamicin resistance plasmid pMG1 (65.1 kb), which was originally isolated from an E. faecium clinical isolate, and transfer efficiently in broth mating.     

Temporal shifts in the dominance of serotypes of Shigella dysenteriae from 1999 to 2002 in Dhaka, Bangladesh

A total of 358 Shigella dysenteriae strains isolated from patients attending the Dhaka treatment center of the International Centre for Diarrheal Disease Research, Bangladesh, between the years 1999 and 2002 were included in this study. S. dysenteriae type 1, the dominant serotype in 1999 (76.4%), declined to 6.5% in 2002. On the other hand, S. dysenteriae types 2 to 12 were isolated with increasing frequencies of 19, 67, 73.5, and 87% in 1999, 2000, 2001, and 2002, respectively. Of these, types 2 and 4 were the most dominant serotypes, accounting for more than 18.7 and 28.5% of the total isolates, respectively. There was no isolation of serotypes 5, 7, 8, and 13 during this period. Twenty-eight (7.8%) of the isolates were atypical and agglutinated only with the polyvalent antiserum of S. dysenteriae. More than 98% of type 1 strains isolated between 1999 and 2001 were resistant to ampicillin, sulfamethoxazole-trimethoprim, and nalidixic acid. Among other serotypes of S. dysenteriae, Nal(r) type 2 strains were isolated in 2001 and 2002. Although heterogeneous plasmid profiles were obtained depending on the presence or absence of a single plasmid, core plasmids were defined for particular serotypes. On the other hand, the same plasmid profile was found to be shared by different serotypes. Interestingly, plasmid patterns of types 2 and 4 were almost identical except that a middle-range plasmid of 70 to 60 MDa was present in type 4 in addition to the core plasmids. All the strains harboring the 140-MDa plasmid were positive for the ipaH gene, had Congo red binding abilities, and were positive by the Sereny test, demonstrating their invasive properties.     

Intra- and inter-species mobilisation of non-conjugative plasmids in staphylococci.

The ability of Staphylococcus aureus conjugative plasmids to mobilise non-conjugative resistance plasmids from clinical isolates of S. aureus and S. epidermidis was studied. Plasmids which could not be transferred by transduction or mixed-culture transfer were transferred from phage-typable and non-typable S. aureus and from S. epidermidis. Plasmids encoding single resistance determinants were transferred by mobilisation whereas multiple-resistance plasmids were transferred as co-integrates between the conjugative and non-conjugative plasmids. This study demonstrates that mobilisation is a useful tool for the transfer and study of staphylococcal plasmids and illustrates how antibiotic resistance could be transferred between staphylococci in vivo.     

Distribution of conjugative-plasmid-mediated 16S rRNA methylase genes among amikacin-resistant Enterobacteriaceae isolates collected in 1995 to 1998 and 2001 to 2006 at a university hospital in South Korea and identification of conjugative plasmids mediating dissemination of 16S rRNA methylase

The distribution of conjugative-plasmid-mediated 16S rRNA methylase genes among amikacin-resistant Enterobacteriaceae collected between 1995 and 1998 and between 2001 and 2006 at a university hospital in South Korea was examined, and conjugative plasmids carrying the 16S rRNA methylase genes were characterized by PCR-based replicon typing and by determination of their antimicrobial resistance pattern. Among the 7,127 isolates, 463 isolates showed a high level of resistance to amikacin, and 218 of the 463 isolates transferred amikacin resistance by conjugation. Among the 218 isolates, armA was detected in 153 isolates (88 Klebsiella pneumoniae, 28 Escherichia coli, 19 Enterobacter cloacae, and 6 Serratia marcescens isolates and 12 isolates of other organisms), and rmtB was detected in 51 isolates (32 K. pneumoniae isolates, 18 E. coli isolates, and 1 Citrobacter freundii isolate). The first appearance of armA was in 1997. The armA gene was carried by conjugative plasmids of replicon groups IncL/M, IncFIIAs, IncF, IncA/C, IncHI2, and Inc(unidentified) in 38, 20, 7, 9, 4, and 75 strains, respectively. The rmtB gene was carried by conjugative plasmids of groups IncA/C, IncF, and IncI1-Iγ in 43 strains, 7 strains, and 1 strain, respectively. Transconjugants that received the IncL/M plasmid carrying armA or the IncA/C plasmid carrying rmtB showed an additional resistance to cefotaxime. Transconjugants that received the IncFIIA plasmid or Inc(unidentified) plasmid carrying the armA gene showed an additional resistance to cefoxitin and a high MIC₅₀ (0.25 mg/liter) of ciprofloxacin. In conclusion, this study demonstrated that the dissemination of 16S rRNA methylase genes among the Enterobacteriaceae is mediated by conjugative plasmids of various incompatibility groups that confer resistance to multiple drugs, including aminoglycosides, extended-spectrum β-lactams, and/or quinolones.     

Antimicrobial resistance of Shigella sonnei in Korea during the last two decades.

Eighty-eight strains of Shigella sonnei isolated in Korea during the period 1980 to 1999 were tested for susceptibility to 13 antimicrobial agents. S. sonnei isolates demonstrated high frequencies of resistance to sulfamethoxazole (97.7%), tetracycline (96.6%), and trimethoprim (95.5%). S. sonnei isolates from the 1990s were more resistant to nalidixic acid than isolates from the 1980s (100 vs 7.7%), while isolates from the 1990s were more susceptible to chloramphenicol than isolates from the 1980s (0 vs 100%). Ampicillin-resistant S. sonnei isolates produced the TEM-1 beta-lactamase with a pI of 5.4. The TEM-1 gene was located on conjugally transferable plasmids in the majority of isolates. S. sonnei isolates were all susceptible to cefotaxime, cefoxitin, ceftazidime, ciprofloxacin, and norfloxacin. These results indicate that cephalosporins and quinolones may be alternative antibiotics for the treatment of S. sonnei infections in Korea.     

Enterotoxin plasmid from Clostridium perfringens is conjugative

Clostridium perfringens enterotoxin is the major virulence factor involved in the pathogenesis of C. perfringens type A food poisoning and several non-food-borne human gastrointestinal illnesses. The enterotoxin gene, cpe, is located on the chromosome of food-poisoning isolates but is found on a large plasmid in non-food-borne gastrointestinal disease isolates and in veterinary isolates. To evaluate whether the cpe plasmid encodes its own conjugative transfer, a C. perfringens strain carrying pMRS4969, a plasmid in which a 0.4-kb segment internal to the cpe gene had been replaced by the chloramphenicol resistance gene catP, was used as a donor in matings with several cpe-negative C. perfringens isolates. Chloramphenicol resistance was transferred at frequencies ranging from 2.0 x 10(-2) to 4.6 x 10(-4) transconjugants per donor cell. The transconjugants were characterized by PCR, pulsed-field gel electrophoresis, and Southern hybridization analyses. The results demonstrated that the entire pMRS4969 plasmid had been transferred to the recipient strain. Plasmid transfer required cell-to-cell contact and was DNase resistant, indicating that transfer occurred by a conjugation mechanism. In addition, several fragments of the prototype C. perfringens tetracycline resistance plasmid, pCW3, hybridized with pMRS4969, suggesting that pCW3 shares some similarity to pMRS4969. The clinical significance of these findings is that if conjugative transfer of the cpe plasmid occurred in vivo, it would have the potential to convert cpe-negative C. perfringens strains in normal intestinal flora into strains capable of causing gastrointestinal disease.     

Interaction of drug resistance plasmids and bacteriophages with diarrheagenic strains of Escherichia coli.

Seven transfer-derepressed plasmids from different incompatibility groups in Escherichia coli K-12 were tested for their ability to enter 43 strains of diarrheagenic E. coli (mostly enteropathogenic E. coli clinical isolates) representing 12 serogroups and including rough and semirough mutants (characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis). Strains in some serogroups were more competent as recipients of plasmids than were those in others. Five test plasmids in an E. coli K-12 (rough) donor transferred significantly less efficiently to two smooth strains than to their rough or semirough isogenic derivatives. When the same smooth and rough strains were used as donors, the plasmids transferred to E. coli K-12 equally well. These results suggested that the O-antigenic lipopolysaccharide side chains of diarrheagenic E. coli isolates shielded the outer membrane receptors for conjugative pili, thus preventing plasmid entry. The different receptors for eight bacteriophages were also covered by O side chains. In addition, a limited survey of clinical isolates for drug resistance markers and resident plasmids was carried out.     

Molecular analysis of ampicillin-resistant sporadic Salmonella typhi and Salmonella paratyphi B clinical isolates

Objective: To study the mechanisms of antibiotic resistance in Salmonella typhi and Salmonella paratyphi B clinical isolates, and the clonality of resistant strains.
Method: Antibiotic susceptibility was tested by disk-agar diffusion. Conjugation experiments and plasmid analysis by agarose gel electrophoresis after Eco RI digestion were followed by hybridization to a digoxigenin-labeled TEM-type β-lactamase probe. DNA fingerprints were obtained by pulsed-field gel electrophoresis of Xba I-digested chromosomal DNA.
Results: Three S. typhi isolates (7% of the isolates studied), of which one was ampicillin resistant and the other two multiresistant (ampicillin, chloramphenicol, tetracycline, sulfamethoxazole/trimethoprim and streptomycin), and two ampicillin-resistant S. paratyphi B isolates (25% of the isolates studied) were further evaluated. A 34-MDa conjugative plasmid, previously isolated from Salmonella enteritidis , conferred ampicillin resistance. A 100-MDa conjugative plasmid encoded resistance to chloramphenicol, tetracycline and sulfamethoxazole/trimethoprim, as well as ampicillin. Chromosomal fingerprinting revealed two distinct resistant strains for each serovar which were different from a matched set of sensitive S. typhi strains.
Conclusions: Two conjugative, TEM-type β-lactamase-encoding plasmids conferred ampicillin resistance to S. typhi and S. paratyphi B. The 34-MDa plasmid was identical to that previously characterized from S. enteritidis , while the 100-MDa plasmid also encoded resistance to chloramphenicol, tetracycline and sulfamethoxazole/trimethoprim. Resistant isolates did not belong to a single clone but rather represented distinct strains.     

Plasmid analysis of Shigella dysenteriae type 1 isolates obtained from widely scattered geographical locations.

Plasmid profiles and antimicrobial susceptibility patterns of 343 strains of Shigella dysenteriae type 1, obtained from 18 different geographical locations, were analyzed. Three plasmids, with molecular sizes of 140, 6, and 2 megadaltons (MDa), were present in 94, 98, and 96%, respectively, of the 343 strains isolated during either epidemic or nonepidemic periods from 1965 to 1987. In addition to these plasmids, 83% of the strains harbored a 4-MDa plasmid and 25% harbored a 20-MDa plasmid. Various plasmid profiles were observed in which the 140-, 6-, and 2-MDa plasmids occurred commonly, irrespective of the place of isolation and drug resistance pattern of the strains. Certain profiles showed significant association with drug resistance patterns. These findings suggest that three plasmids, of molecular sizes 140, 6, and 2 MDa, are unique to S. dysenteriae type 1 strains and may indicate the global spread of a pathogenic bacterial clone. Additionally, these core plasmids, plus plasmids of various other sizes, could be used to identify emerging subclones which are causing both epidemic and sporadic disease. Thus, plasmid profiles of S. dysenteriae type 1 strains can be used to monitor possible pandemic strains as well as individual epidemic strains.     

Molecular evolution of large virulence plasmid in Shigella clones and enteroinvasive Escherichia coli.

Three genes, ipgD, mxiC, and mxiA, all in the invasion region of the Shigella virulence plasmid, were sequenced from strains representing a range of Shigella serotypes and from two enteroinvasive Escherichia coli (EIEC) isolates. The plasmids can be classified into two relatively homogeneous sequence forms which are quite distinct. pINV A plasmids are found in Shigella flexneri strains F6 and F6A, S. boydii strains B1, B4, B9, B10, B14, and B15, S. dysenteriae strains D3, D4, D6, D8, D9, D10, and D13, and the two EIEC strains (M519 and M520). pINV B plasmids are present in S. flexneri strains F1A, F2A, F3A, F3C, F4A, and FY, two S. boydii strains (B11 and B12), and S. sonnei. The D1 pINV plasmid is a recombinant with ipgD gene more closely related to those of pINV A but with mxiA and mxiC genes more closely related to those of pINV B. The phylogenetic relationships of the plasmid and those of the chromosomal genes of Shigella strains are largely consistent. The cluster 1 and cluster 3 strains tested (G.M. Pupo, R. Lan, and P. R. Reeves, Proc. Natl. Acad. Sci. USA 97:10567-10572, 2000) have pINV A and pINV B plasmids, respectively. However, of the three cluster 2 strains (B9, B11, and B15), B9 and B15 have pINV A while B11 has a pINV B plasmid. Those Shigella (D8 and D10 and S. sonnei) and EIEC strains which do not group with the main body of Shigella strains based on chromosomal genes were found to have plasmids belonging to one or the other of the two types and must have acquired these by lateral transfer.     

Co-transfer of plasmids in association with conjugative transfer of mupirocin or mupirocin and penicillin resistance in methicillin-resistant Staphylococcus aureus

Two distinct strains of methicillin-resistant Staphylococcus aureus (MRSA) isolated from patients in a dermatology ward were also resistant to mupirocin. The mupirocin resistance plasmids from both strains were indistinguishable by EcoRI and HindIII restriction digest analysis, except for the presence of genes apparently mediating penicillinase production in some transconjugants. Conjugative transfer of the plasmid mediating mupirocin resistance from one of these strains to a recipient S. aureus was accompanied in some cases by co-transfer of plasmids mediating resistance to tetracycline or erythromycin; in some instances a plasmid which possessed no apparent resistance markers was also transferred. The second strain demonstrated conjugative transfer of penicillin and mupirocin resistance as well as transfer of a plasmid mediating gentamicin resistance, but transfer of erythromycin resistance was not apparently plasmid-mediated.     

Outbreak of dysentery Is Kenya due to multi-resistant polyclonal strain of Shigella dysenteriae type 1

Sixty-eight strains of multi-drug resistant Shigella dysenteriae type 1 were isolated from outbreaks of severe dysentery in three areas of Kenya, namely, Kisumu, Mombassa and Nairobi. The strains were tested for their susceptibility to seven antibiotics. Plasmids were extracted to study their variety and conjugated with Escherichia coli K12 to identify the plasmid coding for the resistance gese. All the strains were resistant to ampicillin, trimethoprim-sulfamethoxazole (S-T), tetracyciine and chloramphenicol. They were sensitive to gentamicin, kanamycin and nalidixic acid. Part of the resistance was found to be plasmid mediated and the sizes of plasmids coding resistance gene wereSMDaand 19MDa. These piasmids coded resistance for ampicillin, trimethoprim-sulfamethoxazole, tetracyciine and chloramphenicol. Verotoxin (VT) production was demonstrated in 80% of the strains by oligonucleotide DNA-DNA gybridization of whole cells with the VT-probe. In view of the extent of drug resistance exhibited by Shigella dysenteriae Type 1 in this study, we recommend the use of nalidixic acid as a first choice of treatment in multiresistant S. dysenteriae Type 1 dysentery outbreaks. Compared to the other two agents that the pathogen was sensitive to, nalidixic acid is relatively cheap with the cost of 7 days treatment for an adult being approximately five US dollars. Resistance to nalidixic acid has to be, however, closely monitored as S. dysenteriae Type 1 has been reported to develop resistance to this drug over time.     

Plasmids coding for drug resistance and localized adherence to HeLa cells in enteropathogenic Escherichia coli O55:H- and O55:H6.

Plasmids coding for drug resistance and localized adherence (LA) to HeLa cells were found in two enteropathogenic Escherichia coli strains belonging to serotypes O55:H- and O55:H6. Strain 49-81 HSJ (O55:H-) carries two plasmids, one coding for both ampicillin resistance (Apr) and LA (pMS49). Strain 71-82 HSJ (O55:H6) harbors only one plasmid, coding for resistance to sulfadiazine, chloramphenicol, kanamycin, ampicillin, and LA (pMS71). Plasmids pMS49 and pMS71 were transferred to E. coli K-12 711 and from this strain to E. coli K-12 J53. Curing with acridine orange of an Apr LA+ transconjugant showed that both characteristics were lost simultaneously. The plasmids have a molecular weight of approximately 55 X 10(6) and are the first naturally recombinant plasmids coding for adherence and drug resistance described in enteropathogenic E. coli.     

Large plasmids associated with virulence in Shigella species have a common function necessary for epithelial cell penetration.

Large plasmids (120 to 140 megadaltons) associated with virulence of Shigella sonnei, S. flexneri 2a and S. dysenteriae 1 were transferred from each strain into Escherichia coli K-12 and avirulent S. flexneri 1b strains by ampicillin transposon (Tn1)-mediated conduction. Strains with the virulence plasmid could penetrate tissue culture cells irrespective of the original host of the plasmid.     

Plasmids coding for colonization factor antigen I and heat-stable enterotoxin production isolated from enterotoxigenic Escherichia coli: comparison of their properties.

We examined seven enterotoxigenic Escherichia coli strains which produced colonization factor antigen I (CFA/I). Four of these strains were from South Africa (three serotype O78:H12 and one serotype O63:H-), one was from Ethiopia (O78:H12), and two were from Bangladesh (O78:H11 and O78:H12). Plasmids coding for CFA/I were mobilized from six of these strains by using resistance or enterotoxin factors. No plasmid was mobilized from the serotype O78:H12 Bangladesh strain. The transconjugants obtained from crosses with the O78 strains also produced heat-stable enterotoxin (ST), and additional investigations showed that CFA/I and ST were coded for by a single non-autotransferring plasmid. These plasmids were fertility inhibition negative, did not restrict any of the coliphages with which they were tested, and were incompatible with each other. Four had molecular weights of approximately 60 X 10(6), and one had a molecular weight of 52 X 10(6). Like the other CFA/I plasmids, the CFA/I plasmid transferred from the O63:H- strain coded for ST, but this plasmid also coded for heat-labile enterotoxin. In most other respects the properties of this plasmid were similar to those of the CFA/I-ST plasmids previously described. The molecular weight of this plasmid was 65 X 10(6). The IncT R-factor Rtsl was marked with a transposon for tetracycline resistance and then transferred into the two Bangladesh wild-type strains. Plasmids which coded for tetracycline resistance, CFA/I, and ST were transferred from these strains. These plasmids were incompatible with Rtsl and with the CFA/I-ST plasmids described above and were recombinants between Rtsl and a CFA/I-ST plasmid. Their properties are also described.     

Plasmids coding for enterotoxins,K88 antigen and colicins in porcineEscherichia coli strains of O-group 149

This study was carried out to determine whether the strong epidemiological correlation observed in Sweden between production of the adhesin K88, the heat-stable (ST) and the heat-labile (LT) enterotoxins inE. coli strains of O-group 149 isolated from piglet diarrhea might be explained by linkage of their genetic determinants. From 22 different isolates plasmids coding for these virulence factors were investigated by conjugation and transduction experiments and analysis on agarose gels. The genes coding for ST production could be transferred by selection for antibiotic resistance, but behaved as transposable elements most often residing on a 55 Mdal plasmid coding for colicin B. The genes coding for raffinose fermentation and K88 antigen production were located on a 45 Mdal plasmid and the genes coding for LT production on plasmids within the 45–70 Mdal size. Thus the epidemiological importance and spread of this O-group in Sweden was explained by its stable content of two or three virulence plasmids, which could be transferred independently of one another.     

Clonal relationships among Shigella serotypes suggested by cryptic flagellin gene polymorphism

The presence of cryptic fliC alleles in the genomes of 120 strains representative of the four Shigella species was investigated. One fragment was obtained by PCR amplification of fliC, with a size varying from 1.2 to 3.2 kbp, depending on the species or serotype. After digestion with endonuclease HhaI, the number of fragments in patterns varied from three to nine, with sizes of between 115 and 1,020 bp. Patterns sharing most of their bands were grouped to constitute an F type. A total of 17 different F types were obtained from all strains included in this study. A unique pattern was observed for each the following serotypes: Shigella dysenteriae 1, 2, 8, and 10 and S. boydii 7, 13, 15, 16, and 17. On the contrary, S. dysenteriae serotype 13 and S. sonnei biotype e were each subdivided into two different F types. S. flexneri serotypes 3a and X could be distinguished from the cluster containing S. flexneri serotypes 1 to 5 and Y. S. flexneri serotype 6 clustered with S. boydii serotypes 1, 2, 3, 4, 6, 8, 10, 11, 14, and 18 and S. dysenteriae serotypes 4, 5, 6, 7, 9, 11, and 12. Two other clusters were outlined: one comprising S. dysenteriae serotypes 3, 12, 13 (strain CDC598-77), 14, and 15 and the other one joining S. boydii serotypes 5 and 9. None of the 17 fliC patterns was found in the fliC HhaI pattern database previously described for Escherichia coli. Overall, this work supports the hypothesis that Shigella evolved from different ancestral strains of E. coli. Moreover, the method outlined here is a promising tool for the identification of some clinically important Shigella strains as well as for confirmation of atypical isolates as Shigella spp.