Acquisition Of Plasmids Research Articles

The Complete Genome Sequence of Yersinia pseudotuberculosis IP31758, the Causative Agent of Far East Scarlet-Like Fever

The first reported Far East scarlet-like fever (FESLF) epidemic swept the Pacific coastal region of Russia in the late 1950s. Symptoms of the severe infection included erythematous skin rash and desquamation, exanthema, hyperhemic tongue, and a toxic shock syndrome. The term FESLF was coined for the infection because it shares clinical presentations with scarlet fever caused by group A streptococci. The causative agent was later identified as Yersinia pseudotuberculosis, although the range of morbidities was vastly different from classical pseudotuberculosis symptoms. To understand the origin and emergence of the peculiar clinical features of FESLF, we have sequenced the genome of the FESLF-causing strain Y. pseudotuberculosis IP31758 and compared it with that of another Y. pseudotuberculosis strain, IP32953, which causes classical gastrointestinal symptoms. The unique gene pool of Y pseudotuberculosis IP31758 accounts for more than 260 strain-specific genes and introduces individual physiological capabilities and virulence determinants, with a significant proportion horizontally acquired that likely originated from Enterobacteriaceae and other soil-dwelling bacteria that persist in the same ecological niche. The mobile genome pool includes two novel plasmids phylogenetically unrelated to all currently reported Yersinia plasmids. An icm/dot type IVB secretion system, shared only with the intracellular persisting pathogens of the order Legionellales, was found on the larger plasmid and could contribute to scarlatinoid fever symptoms in patients due to the introduction of immunomodulatory and immunosuppressive capabilities. We determined the common and unique traits resulting from genome evolution and speciation within the genus Yersinia and drew a more accurate species border between Y. pseudotuberculosis and Y. pestis. In contrast to the lack of genetic diversity observed in the evolutionary young descending Y. pestis lineage, the population genetics of Y. pseudotuberculosis is more heterogenous. Both Y. pseudotuberculosis strains IP31758 and the previously sequenced Y. pseudotuberculosis strain IP32953 have evolved by the acquisition of specific plasmids and by the horizontal acquisition and incorporation of different genetic information into the chromosome, which all together or independently seems to potentially impact the phenotypic adaptation of these two strains.

Emergence of multidrug-resistant Gram-negative bacteria during selective decontamination of the digestive tract on an intensive care unit—authors' response

Sir, We thank Abecasis and colleagues for their comments on our paper on the detection of extended-spectrum b-lactamase (ESBL)-producing positive aerobic Gram-negative bacilli (ESBL-GNB) in patients on the intensive care unit (ICU) of our hospital, where selective decontamination of the digestive tract (SDD) is routinely used in mechanically ventilated patients. Their main comment is that the link that we suggest between SDD use and the detection of these strains is only circumstantial; they also argue that the occurrences of such isolates is related to their resistance to tobramycin, an antibiotic in the SDD regimen which could be replaced by neomycin or paromomycin. We agree that during treatment with SDD, ESBL-GNB isolates will only reach detectable numbers in cultures from patients if they have decreased susceptibility to tobramycin. We isolated four genetically different strains that carried identical plasmids, had identical resistance patterns, and carried identical ESBL genes. As we noted in our paper, the emergence of the four strains could be due to: (i) an increase in these strains from below to above detection level; (ii) acquisition of the strains carrying the resistance plasmid; or (iii) acquisition of the resistance plasmid by strains that belong to the normal flora of the patients. We do not state that the emergence of these strains would not have occurred if these patients had not received SDD; we only want to show that SDD does not prevent the emergence of such strains and possibly, like any other antimicrobial treatment including cephalosporins, even promotes their emergence. Possibility (i) is unlikely, as it implies that isolates with decreased cefotaxime and tobramycin susceptibility are frequently missed during screening in an SDD setting. Acquisition of strains with identical plasmids seems improbable because this presumes a widespread presence of plasmids with identical RFLP patterns in different isolates. RFLP analyses of plasmids isolated from two ESBL-positive and one ESBL-negative bacterial strains isolated during the same period in our ICU, showed different patterns. Since four independent events of acquisition of different strains with similar plasmids is unlikely, acquisition of the resistance plasmid by different strains seems the most likely explanation for our observation. The multidrug-resistant (MDR) strains described in our study were intermediately susceptible to polymyxin E. Of note, in our hospital, outside the ICU, polymyxin E is not usually used to treat infections caused by Escherichia coli or Klebsiella pneumoniae. Cefotaxime, the most common systemic antibiotic used in SDD regimens has been reported to be associated with the emergence of ESBL-producing strains and with an increase in ESBL prevalence. In our view, an important part of the problem is the colonization with strains carrying MDR plasmids, as every colonized patient forms a potential source which may facilitate further dissemination of ESBL genes. Although adjustment of the SDD regimen or isolation measures can eventually contain outbreaks, detection of ESBL plasmid-mediated outbreaks is more complex than that of clonal outbreaks. In all studies that evaluated emergence of resistance with SDD wherein cefotaxime is used, there was no optimal screening for ESBLs. Therefore, we suggest that for optimal evaluation of SDD regimens with respect to ESBL-producing strains and emergence of resistance, optimal screening techniques for detection of ESBLs is warranted. Obviously, optimal screening for ESBL production is also needed in ICUs where SDD is not used.

Prevalence, distribution and characterisation of ceftiofur resistance in Salmonella enterica isolated from animals in the USA from 1999 to 2003

Third-generation cephalosporin (3GC) antimicrobials are the drugs of choice for treatment of salmonellosis in children. Salmonella isolated in the USA are assayed by the National Antimicrobial Resistance Monitoring System (NARMS) for resistance to antimicrobials including first-, second- and third-generation cephalosporins. From 1999 to 2003, 34,411 Salmonella were isolated from animals in the USA, of which 10.9% were found to be resistant to ceftiofur, a 3GC used in animals, whilst only 0.3% were resistant to ceftriaxone, a 3GC used in human medicine. Ceftiofur resistance rose from 4.0% in 1999 to 18.8% in 2003. Isolates from diagnostic laboratories had higher levels of resistance (18.5%), whereas levels in isolates from on-farm (3.4%) and slaughter (7.1%) sources were lower. Animals with a higher than average proportion of resistant Salmonella included cattle (17.6%), horses (19.2%) and dogs (20.8%). Levels in turkeys (6.8%), chickens (7.1%), eggs (3.6%) and swine (4.6%) were lower. Resistance varied between Salmonella serotypes. A few serotypes had significantly high levels, e.g. S. Newport was 70.4% ceftiofur resistant. Resistance was predominantly associated with bla CMY-2-encoding plasmids. These data suggest that the acquisition of resistance plasmids and the spread of specific serotypes harbouring these plasmids are driving the observed resistance to ceftiofur in Salmonella animal isolates.

Investigation of plasmid DNA and antibiotic resistance in some pathogenic organisms

Twenty-eight strains of Salmonella, Pseudomonas, and Escherichia coli isolated from cultures of stool, urine and wound were tested for their susceptibility to various antimicrobial agents. All the strains were resistant to erythromycin and tetracycline. Nineteen Salmonella isolates were susceptible to chloramphenicol and gentamycin. All the Pseudomonas and Escherichia coli isolates were resistant to chloramphenicol and cloxacillin while one Pseudomonas species and three Escherichia coli isolates were sensitive to gentamycin. The antibiotic resistance determinants in each strain were encoded by plasmid. These isolates were typed by plasmid profile analysis according to their different molecular weights. Several similar and distinct profiles were identified for most resistant and sensitive isolates. It appeared that a single strain containing a plasmid conferring multiple drug resistance emerged within the bacterial population and was able to adapt and to survive the challenges of antibiotics as they were introduced into clinical medicine. Therefore, the acquisition of plasmid has greatly contributed to the rapid spread of antibiotic resistance genes in the bacterial population.

Investigation of plasmid DNA and antibiotic resistance in some pathogenic organisms

Twenty-eight strains of Salmonella, Pseudomonas, and Escherichia coli isolated from cultures of stool, urine and wound were tested for their susceptibility to various antimicrobial agents. All the strains were resistant to erythromycin and tetracycline. Nineteen Salmonella isolates were susceptible to chloramphenicol and gentamycin. All the Pseudomonas and Escherichia coli isolates were resistant to chloramphenicol and cloxacillin while one Pseudomonas species and three Escherichia coli isolates were sensitive to gentamycin. The antibiotic resistance determinants in each strain were encoded by plasmid. These isolates were typed by plasmid profile analysis according to their different molecular weights. Several similar and distinct profiles were identified for most resistant and sensitive isolates. It appeared that a single strain containing a plasmid conferring multiple drug resistance emerged within the bacterial population and was able to adapt and to survive the challenges of antibiotics as they were introduced into clinical medicine. Therefore, the acquisition of plasmid has greatly contributed to the rapid spread of antibiotic resistance genes in the bacterial population.

Kinetics of plasmid transfer among Bacillus cereus group strains within lepidopteran larvae

The cry toxin encoding plasmid pHT73 was transferred from Bacillus thuringiensis subspecies kurstaki KT0 to six B. cereus group strains in three lepidopteran (Spodoptera exigua, Plutella xyllostella and Helicoverpa armigera) larvae by conjugation. The conjugation kinetics of the plasmid was precisely studied during the larval infection using a new protocol. The infections were performed with both vegetative and sporulated strains. However, larval death only occurred when infections were made with spore and toxin preparations. Likewise, spore germinations of both donor and recipient strains were only observed in killed larvae, 44-56 h post-infection. Accordingly, kinetics showed that gene transfer between B. thuringiensis strain KT0 and other B. cereus strains only took place in dead larvae among vegetatively growing bacteria. The conjugational transfer ratios varied among different strain combinations and different larvae. The highest transfer ratio reached 5.83 x 10(-6) CFU/donor between the KT0 and the AW05R recipient in Helicoverpa armigera, and all transconjugants gained the ability to produce the insecticidal crystal. These results indicated that horizontal gene transfer among B. cereus group strains might play a key role for the acquisition of extra plasmids and evolution of these strains in toxin susceptible insect larvae.

Evolution of Mycobacterium ulcerans and Other Mycolactone-Producing Mycobacteria from a Common Mycobacterium marinum Progenitor

It had been assumed that production of the cytotoxic polyketide mycolactone was strictly associated with Mycobacterium ulcerans, the causative agent of Buruli ulcer. However, a recent study has uncovered a broader distribution of mycolactone-producing mycobacteria (MPM) that includes mycobacteria cultured from diseased fish and frogs in the United States and from diseased fish in the Red and Mediterranean Seas. All of these mycobacteria contain versions of the M. ulcerans pMUM plasmid, produce mycolactones, and show a high degree of genetic relatedness to both M. ulcerans and Mycobacterium marinum. Here, we show by multiple genetic methods, including multilocus sequence analysis and DNA-DNA hybridization, that all MPM have evolved from a common M. marinum progenitor to form a genetically cohesive group among a more diverse assemblage of M. marinum strains. Like M. ulcerans, the fish and frog MPM show multiple copies of the insertion sequence IS2404. Comparisons of pMUM and chromosomal gene sequences demonstrate that plasmid acquisition and the subsequent ability to produce mycolactone were probably the key drivers of speciation. Ongoing evolution among MPM has since produced at least two genetically distinct ecotypes that can be broadly divided into those typically causing disease in ectotherms (but also having a high zoonotic potential) and those causing disease in endotherms, such as humans.

Transposon-aided capture (TRACA) of plasmids resident in the human gut mobile metagenome

The bacterial community of the human gut is a complex ecosystem composed of >1,000 species, the majority of which are Gram positive and uncultured. To study plasmids resident within this community, we developed a culture-independent transposon aided capture method (TRACA), which does not rely on any plasmid-encoded traits. TRACA facilitated acquisition of plasmids from metagenomic DNA extracts, and subsequent maintenance and selection in an Escherichia coli host. We confirmed the presence of the transposon in captured plasmids and demonstrate that these plasmids are mainly of a Gram-positive origin. Sequencing of plasmids designated pTRACA10 (7 kb) and pTRACA17 (2.7 kb) revealed genes involved in plasmid mobilization and replication. From the homologies of these genes we conclude that pTRACA17 originates from a Gram-positive host belonging to the Firmicute division. pTRACA10 had two additional open reading frames with similarity to a conserved hypothetical protein and phosphoesterase or phosphohydrolase enzymes (Clusters of Orthologous Groups number 4186). Both plasmids lacked any conventional selectable markers.

Identification and characterization of “pathoadaptive mutations” of the cadBA operon in several intestinal Escherichia coli

The dysenteric Shigella spp. and enteroinvasive Escherichia coli (EIEC) have evolved from commensal E. coli by the acquisition of a virulence plasmid and inactivation of genes of the cad locus encoding lysine decarboxylase (LDC) by so-called pathoadaptive mutation. As horizontal gene transfer and recombination occurs frequently in E. coli we were interested to see if similar pathoadaptive mutations are commonly present in other intestinal pathotypes. Therefore, we examined 140 intestinal E. coli strains of various pathotypes and the ECOR collection for their ability to decarboxylate lysine, and identified 25 strains that were unable to do so. Complementation of a Shiga toxin-producing E. coli and two enteropathogenic E. coli strains, both LDC-negative, with the intact cad locus restored LDC activity and resulted in a reduction in adherence to tissue culture cells. We investigated the cad locus for possible alterations by using hybridization and PCR techniques and compared the results with the alterations reported for Shigella spp. and EIEC strains. Interestingly, the alterations of the cad genes were similar to those previously reported, pointing towards a parallel evolution of LDC silencing in different intestinal E. coli pathotypes.

Resistance in gram-negative bacteria: Enterobacteriaceae

The emergence and spread of resistance in Enterobacteriaceae are complicating the treatment of serious nosocomial infections and threatening to create species resistant to all currently available agents. Approximately 20% of Klebsiella pneumoniae infections and 31% of Enterobacter spp infections in intensive care units in the United States now involve strains not susceptible to third-generation cephalosporins. Such resistance in K pneumoniae to third-generation cephalosporins is typically caused by the acquisition of plasmids containing genes that encode for extended-spectrum beta-lactamases (ESBLs), and these plasmids often carry other resistance genes as well. ESBL-producing K pneumoniae and Escherichia coli are now relatively common in healthcare settings and often exhibit multidrug resistance. ESBL-producing Enterobacteriaceae have now emerged in the community as well. Salmonella and other Enterobacteriaceae that cause gastroenteritis may also be ESBL producers, which is of relevance when children require treatment for invasive infections. Resistance of Enterobacter spp to third-generation cephalosporins is most typically caused by overproduction of AmpC beta-lactamases, and treatment with third-generation cephalosporins may select for AmpC-overproducing mutants. Some Enterobacter cloacae strains are now ESBL and AmpC producers, conferring resistance to both third- and fourth-generation cephalosporins. Quinolone resistance in Enterobacteriaceae is usually the result of chromosomal mutations leading to alterations in target enzymes or drug accumulation. More recently, however, plasmid-mediated quinolone resistance has been reported in K pneumoniae and E coli, associated with acquisition of the qnr gene. The vast majority of Enterobacteriaceae, including ESBL producers, remain susceptible to carbapenems, and these agents are considered preferred empiric therapy for serious Enterobacteriaceae infections. Carbapenem resistance, although rare, appears to be increasing. Particularly troublesome is the emergence of KPC-type carbapenemases in New York City. Better antibiotic stewardship and infection control are needed to prevent further spread of ESBLs and other forms of resistance in Enterobacteriaceae throughout the world.

Resistance in Gram-Negative Bacteria: Enterobacteriaceae

The emergence and spread of resistance in Enterobacteriaceae are complicating the treatment of serious nosocomial infections and threatening to create species resistant to all currently available agents. Approximately 20% of Klebsiella pneumoniae infections and 31% of Enterobacter spp infections in intensive care units in the United States now involve strains not susceptible to third-generation cephalosporins. Such resistance in K pneumoniae to third-generation cephalosporins is typically caused by the acquisition of plasmids containing genes that encode for extended-spectrum β-lactamases (ESBLs), and these plasmids often carry other resistance genes as well. ESBL-producing K pneumoniae and Escherichia coli are now relatively common in healthcare settings and often exhibit multidrug resistance. ESBL-producing Enterobacteriaceae have now emerged in the community as well. Salmonella and other Enterobacteriaceae that cause gastroenteritis may also be ESBL producers, which is of relevance when children require treatment for invasive infections. Resistance of Enterobacter spp to third-generation cephalosporins is most typically caused by overproduction of AmpC β-lactamases, and treatment with third-generation cephalosporins may select for AmpC-overproducing mutants. Some Enterobacter cloacae strains are now ESBL and AmpC producers, conferring resistance to both third- and fourth-generation cephalosporins. Quinolone resistance in Enterobacteriaceae is usually the result of chromosomal mutations leading to alterations in target enzymes or drug accumulation. More recently, however, plasmid-mediated quinolone resistance has been reported in K pneumoniae and E coli, associated with acquisition of the qnr gene. The vast majority of Enterobacteriaceae, including ESBL producers, remain susceptible to carbapenems, and these agents are considered preferred empiric therapy for serious Enterobacteriaceae infections. Carbapenem resistance, although rare, appears to be increasing. Particularly troublesome is the emergence of KPC-type carbapenemases in New York City. Better antibiotic stewardship and infection control are needed to prevent further spread of ESBLs and other forms of resistance in Enterobacteriaceae throughout the world.

Evaluation of O-antigen inactivation on Pla activity and virulence of Yersinia pseudotuberculosis harbouring the pPla plasmid

Yersinia pestis is a species that emerged recently from Yersinia pseudotuberculosis and gained an exceptional pathogenicity potential. Among the major genetic differences between the plague bacillus and its ancestor is the acquisition of the pPla plasmid, which has been associated with the increased virulence of Y. pestis. In a previous study, introduction of pPla into Y. pseudotuberculosis did not lead to any modification of the virulence of the host bacterium. However, it was subsequently demonstrated that the presence of smooth lipopolysaccharide (LPS) inhibits the activity of Pla. In this study, pPla was introduced into a Y. pseudotuberculosis strain expressing smooth LPS, and into a variant in which a mutation that abrogates the formation of O-antigen (O-Ag) repeats (as in natural isolates of Y. pestis) was generated. It was found that in both strains, Pla was synthesized, exported to the bacterial membrane and processed as in Y. pestis. However, the ability of Pla to activate plasminogen was weak and observed only at 37 degrees C in the smooth strain, while this activity was similar to that of Y. pestis and expressed at both 28 and 37 degrees C in the O-Ag mutant strain. Similarly, Pla-mediated inactivation of the antiprotease alpha2-antiplasmin was not detected in the smooth Y. pseudotuberculosis strain grown at 28 degrees C, but was expressed at both temperatures in the O-Ag mutant strain. Despite the more efficient activity of Pla, the Y. pseudotuberculosis O-Ag mutant strain exhibited a lower pathogenicity upon subcutaneous infection of mice. The results thus indicate that, although abrogation of O side chain synthesis in a Y. pseudotuberculosis strain harbouring pPla potentiates the two proteolytic activities of Pla, this is not sufficient to confer to Y. pseudotuberculosis a higher pathogenicity potential. These results also suggest that acquisition of pPla may not have been sufficient to confer an immediate higher pathogenic potential to the ancestor Y. pestis strain.

Molecular evolution of the lysine decarboxylase-defective phenotype in Shigella sonnei

Shigella, the etiological agent of the bacillary dysentery, belongs to the extremely diverse species of Escherichia coli. In the evolutionary route of Shigella from commensal E. coli ancestors towards a pathogenic lifestyle, the critical events have been the acquisition of the pINV plasmid, through horizontal transfer and the inactivation of pre-existing genes. These so-called pathoadaptive mutations affect the expression of genes negatively interfering with the newly acquired functions necessary for the colonization of the host niche. Cadaverine, a small polyamine resulting from decarboxylation of lysine, has been shown to hamper the full expression of Shigella invasiveness mainly by altering the inflammatory response. Recent analysis of the evolution of the Shigella and enteroinvasive E. coli (EIEC ) cad region indicates that silencing of the cad locus has been attained with several strategies. The increasing relevance of S. sonnei in both, developing and industrial countries, prompted us to analyze the molecular origin of the LDC − phenotype in these strains. The results obtained on several S. sonnei strains reveal that despite the difference in geographic origin and antibiotic resistance patterns, all the strains have undergone the same modifications. Multiple IS insertions into the cadBA operon have interrupted gene continuity without inducing deletions or inversions of the cadA and cadB genes which are remained entirely conserved. Moreover, by functional analysis we show that all the strains carry a defective cadC gene, thus strengthening the hypothesis that inactivation of the regulatory cadC gene might have been the first step towards a complete lack of the cad locus.

Dissemination of nosocomial multiple-aminoglycoside-resistant Staphylococcus aureus caused by horizontal transfer of the resistance determinant ( aacA/aphD) and clonal spread of resistant strains

Background The multiple-aminoglycoside-resistant gene aacA/aphD exists as a transposable genetic element (Tn 4001) in gram-positive cocci. Here we describe our retrospective investigation of the mechanism responsible for the dissemination of Tn 4001 among staphylococci present in clinical isolates collected in our university hospital. At its peak, about 80% of the total isolates of methicillin-resistant Staphylococcus aureus showed multiple-aminoglycoside resistance, and all harbored aacA/aphD. Methods Clonal relatedness was analyzed by pulsed field gel electrophoresis after SmaI endonuclease digestion of the genomic DNA from isolates collected in 1991 and 1997. To detect Tn 4001 on the chromosome and conjugative plasmids, specific sequences from aacA/aphD and the insertion sequence IS 256, whose inverted sequence flanks aacA/aphD, were amplified by polymerase chain reaction. Results Pulsed field gel electrophoresis of genomic DNA, plasmid analysis, and polymerase chain reaction detection of the resistance determinant all indicated the presence of disseminated clones that had survived among hospitalized patients through acquisition of conjugative plasmids harboring aacA/aphD. Futhermore, aacA/aphD also disseminated among nosocomial strains other than S aureus as a consequence of the self-transferability of Tn 4001. Conclusions The nosocomial prevalence of multiple-aminoglycoside-resistant staphylococci is the result of both horizontal and interspecific transfer of aacA/aphD and the clonal spread and survival of resistant strains.

Molecular Investigation of Escherichia coli Strains Associated with Apparently Persistent Urinary Tract Infection in Dogs

Persistent Escherichia coli urinary tract infection (UTI) in dogs is a frustrating clinical problem. Affected dogs often appear to fail to respond to therapy or to reacquire infection shortly after therapy is completed. Urovirulence factors (UVFs) of the infecting E coli, antibiotic resistance, and tissue colonization may be contributory but have not been evaluated in dogs with persistent E coli UTI. In this study, the strain types of E coli in dogs with persistent UTI were evaluated with pulsed‐field gel electrophoresis (PFGE) to determine whether persistence was due to acquisition of new isolates or failure to eradicate existing isolates. UVFs in these isolates, assessed by polymerase chain reaction, and antibiograms were correlated with treatment outcome in these dogs. Results documented a mixed pattern: 9 dogs remained chronically infected with 1 or 2 strains, each with distinct reproducible UVFs, but 1 dog was infected with numerous unrelated E coli strains over time. Two dogs had a mixed pattern, consisting of 1 or more episodes of persistent E coli infection attributable to a single strain in addition to episodes caused by unrelated strains. Many isolates had no detectable UVFs, highlighting the likely importance of impaired colonization resistance in the affected dogs. Antibiotic resistance was common, often in response to previous treatments, especially with trimethoprim‐sulfamethoxazole. Antibiotic resistance patterns differed significantly within PFGE strain types, suggesting lateral acquisition of resistance plasmids or integrons. These results can be used to help guide testing for and management of persistent E coli UTI in dogs.

Molecular investigation of Escherichia coli strains associated with apparently persistent urinary tract infection in dogs.

Persistent Escherichia coli urinary tract infection (UTI) in dogs is a frustrating clinical problem. Affected dogs often appear to fail to respond to therapy or to reacquire infection shortly after therapy is completed. Urovirulence factors (UVFs) of the infecting E. coli, antibiotic resistance, and tissue colonization may be contributory but have not been evaluated in dogs with persistent E. coli UTI. In this study, the strain types of E. coli in dogs with persistent UTI were evaluated with pulsed-field gel electrophoresis (PFGE) to determine whether persistence was due to acquisition of new isolates or failure to eradicate existing isolates. UVFs in these isolates, assessed by polymerase chain reaction, and antibiograms were correlated with treatment outcome in these dogs. Results documented a mixed pattern: 9 dogs remained chronically infected with 1 or 2 strains, each with distinct reproducible UVFs, but 1 dog was infected with numerous unrelated E. coli strains over time. Two dogs had a mixed pattern, consisting of 1 or more episodes of persistent E. coli infection attributable to a single strain in addition to episodes caused by unrelated strains. Many isolates had no detectable UVFs, highlighting the likely importance of impaired colonization resistance in the affected dogs. Antibiotic resistance was common, often in response to previous treatments, especially with trimethoprim-sulfamethoxazole. Antibiotic resistance patterns differed significantly within PFGE strain types, suggesting lateral acquisition of resistance plasmids or integrons. These results can be used to help guide testing for and management of persistent E. coli UTI in dogs.

Unusual rise in mercury-resistant bacteria in coastal environs.

A sharp rise in mercury-resistant bacteria (MRB) capable of tolerating very high concentration of Hg was observed over the last 3-4 years in the coastal environs of India. While none or negligible colony-forming units (CFU) of bacteria were counted on seawater nutrient agar with 0.5 ppm ( 2.5 microM) Hg (II) as HgCl2 until 1997, from 13 to over 75% of the CFU grew on 20 times higher, 50 microM, Hg concentrations from almost every recently examined marine sample. Although exceptionally high counts of MRB (96% of CFU) were recorded from samples collected from the polluted zones off Mumbai, the MRB capable of growth on seawater nutrient agar with 50 microM Hg were quite abundant in most samples collected from many locations with few or no pollution effects. We noticed for the first time the occurrence of aerobic heterotrophic bacterial isolates capable of growth with 250 microM Hg. Such MRB grew with higher concentrations of many other toxic xenobiotics than the Hg sensitive ones. Based on the unusually high populations of viable MRB and some simple experiments, we propose that many marine bacterial species are selected, possibly through acquisition of plasmids and/or transposable elements and modifying Hg, whose concentration, according to recent studies, is on the rise in marine habitats.

Expected Returns on Stocks and Bonds

<h3>Abstract</h3> Multidrug resistance (MDR) plasmids drive the spread of antibiotic resistance between bacterial lineages. The immediate impact of MDR plasmid acquisition on fitness and cellular processes varies among bacterial lineages, but how the evolutionary processes enabling the genomic integration of MDR plasmids vary is less well understood, particularly in clinical pathogens. Using diverse <i>Escherichia coli</i> lineages experimentally evolved for ∼700 generations, we show that the evolutionary response to gaining the MDR plasmid pLL35 was dominated by chromosomal mutations affecting metabolic and regulatory functions, with both strain-specific and shared mutational targets. The expression of several of these functions, such as anaerobic metabolism, is known to be altered upon acquisition of pLL35. Interactions with resident mobile genetic elements, notably several IS-elements, potentiated parallel mutations, including insertions upstream of <i>hns</i> that were associated with its upregulation and the downregulation of the plasmid-encoded extended-spectrum beta-lactamase gene. Plasmid parallel mutations targeted conjugation-related genes, whose expression was also commonly downregulated in evolved clones. Beyond their role in horizontal gene transfer, plasmids can be an important selective force shaping the evolution of bacterial chromosomes and core cellular functions.

Acquisition of Antibiotic Resistance Plasmids by Enterohemorrhagic Escherichia coli O157:H7 within Rumen Fluid

The emergence of antibiotic resistance among important foodborne pathogens like Escherichia coli O157:H7 has become an important issue with regard to food safety. In contrast to the case for Salmonella, antibiotic resistance has been slow in its development in E. coli O157:H7 despite the presence of mobile antibiotic resistance genes in other E. coli organisms that inhabit the same animal host. We set out to determine if rumen fluid influences the transfer of plasmid-mediated, antibiotic resistance to E. coli O157:H7. A commensal E. coli strain from a dairy cow was transformed with conjugative R plasmids and served as the donor in matings with naladixic acid–resistant E. coli O157:H7. R plasmids were transferred from the donor E. coli strain to E. coli O157:H7 in both Luria-Bertani (LB) broth and rumen fluid. R plasmids were transferred at a higher frequency to E. coli O157:H7 during 6 h of incubation in rumen fluid at rates comparable to those in LB broth, indicating that conditions in rumen fluid favor the transfer of the plasmids to E. coli O157. This finding suggests that the cow's rumen is a favorable environment for the genetic exchange of plasmids between microflora and resident E. coli O157:H7 in the bovine host.

Characterization of the spv locus in Salmonella enterica serovar Arizona.

Salmonella enterica serovar Arizona (S. enterica subspecies IIIa) is a common Salmonella isolate from reptiles and can cause serious systemic disease in humans. The spv virulence locus, found on large plasmids in Salmonella subspecies I serovars associated with severe infections, was confirmed to be located on the chromosome of serovar Arizona. Sequence analysis revealed that the serovar Arizona spv locus contains homologues of spvRABC but lacks the spvD gene and contains a frameshift in spvA, resulting in a different C terminus. The SpvR protein functions as a transcriptional activator for the spvA promoter, and SpvB and SpvC are highly conserved. The analysis supports the proposal that the chromosomal spv sequence more closely corresponds to the ancestral locus acquired during evolution of S. enterica, with plasmid acquisition of spv genes in the subspecies I strains involving addition of spvD and polymorphisms in spvA.