Reemergence of gentamicin-susceptible strains of methicillin-resistant Staphylococcus aureus in France: a phylogenetic approach

High prevalence of multiple-antibiotic-resistant (MAR) Escherichia coli in river bed sediments of the Apies River, South Africa

This study aimed at investigating the presence of antibiotic-resistant Escherichia coli in river bed sediments of the Apies River, Gauteng, South Africa, in order to better inform health management decisions designed to protect users of the river. Overall, 180 water and sediment samples were collected at 10 sites along the Apies River from January to February 2014. E. coli was enumerated using the Colilert® 18/Quanti-Tray® 2000 (IDEXX). Isolates were purified by streaking on eosin methylene blue agar followed by the indole test. Pure E. coli isolates were tested for resistance to nine antibiotics by the Kirby-Bauer disc diffusion method. Over 98% of the isolates were resistant to at least one of the antibiotics tested. The highest resistance was observed against nitrofurantoin (sediments) and ampicillin (water). Over 80% of all resistant isolates showed multiple antibiotic resistance (resistance to ≥3 antibiotics). The abundance of E. coli in the sediments not only adds to the evidence that sediments are a reservoir for bacteria and possibly other pathogens including antibiotic-resistant bacteria but also suggests that antibiotic-resistant genes could be transferred to pathogens due to the high prevalence of multiple-antibiotic-resistant (MAR) strains of E. coli observed in the sediment. Using untreated water from the Apies River following resuspension for drinking and other household purposes could pose serious health risks for users. Our results suggest that river bed sediments could serve as reservoirs for MAR bacteria including pathogens under different climatic conditions and their analysis could provide information of public health concerns.

Short term evolution of a highly transmissible methicillin-resistant Staphylococcus aureus clone (ST228) in a tertiary care hospital

Staphylococcus aureus is recognized as one of the major human pathogens and is by far one of the most common nosocomial organisms. The genetic basis for the emergence of highly epidemic strains remains mysterious. Studying the microevolution of the different clones of S. aureus is essential for identifying the forces driving pathogen emergence and spread. The aim of the present study was to determine the genetic changes characterizing a lineage belonging to the South German clone (ST228) that spread over ten years in a tertiary care hospital in Switzerland. For this reason, we compared the whole genome of eight isolates recovered between 2001 and 2008 at the Lausanne hospital. The genetic comparison of these isolates revealed that their genomes are extremely closely related. Yet, a few more important genetic changes, such as the replacement of a plasmid, the loss of large fragments of DNA, or the insertion of transposases, were observed. These transfers of mobile genetic elements shaped the evolution of the ST228 lineage that spread within the Lausanne hospital. Nevertheless, although the strains analyzed differed in their dynamics, we have not been able to link a particular genetic element with spreading success. Finally, the present study showed that new sequencing technologies improve considerably the quality and quantity of information obtained for a single strain; but this information is still difficult to interpret and important investments are required for the technology to become accessible for routine investigations.

Comparative genomics of epidemic versus sporadic Staphylococcus aureus strains does not reveal molecular markers for epidemicity

Staphylococcus aureus, especially when it is methicillin resistant, has been recognised as a major cause of nosocomial and community-acquired infections. It has also been shown that certain strains were able to cause clonal epidemics whereas others showed a more incidental occurrence. On the basis of this behavioural distinction, a genetic feature underlying this difference in epidemicity can be assumed. Understanding the difference will not only contribute to the development of markers for the identification of epidemic strains but will also shed light on the evolution of clones. Genomes of strains from two independent collections (n=18 and n=10 strains) were analysed. Both collections were composed of carefully selected, genetically diverse strains with clinically well-defined epidemic and sporadic behaviour. Comparative genome hybridisation (CGH) was performed using an Agilent array for one collection (up to 11 probes per open reading frame - ORF), and an Affymetrix array for the other (up to 30 probes per ORF). Presence and absence information of probe homologues and ORFs was taken for analysis of molecular variance (AMOVA) at the strain and behaviour levels. Not a single probe showed 100% concordant differences between epidemic and sporadic strains. Moreover, probe differences between groups were always smaller than those within groups. This was also true, when the analysis was focussed on presence versus absence of ORF's or when probe information was transformed into allelic profiles. These findings present strong evidence against the presence or absence of a single common specific genetic factor differentiating epidemic from sporadic S. aureus clones.

Genomic analysis of an emerging multiresistant Staphylococcus aureus strain rapidly spreading in cystic fibrosis patients revealed the presence of an antibiotic inducible bacteriophage

Background

Staphylococcus aureus is a major human pathogen responsible for a variety of nosocomial and community-acquired infections. Recent reports show that the prevalence of Methicillin-Resistant S. aureus (MRSA) infections in cystic fibrosis (CF) patients is increasing. In 2006 in Marseille, France, we have detected an atypical MRSA strain with a specific antibiotic susceptibility profile and a unique growth phenotype. Because of the clinical importance of the spread of such strain among CF patients we decided to sequence the genome of one representative isolate (strain CF-Marseille) to compare this to the published genome sequences. We also conducted a retrospective epidemiological analysis on all S. aureus isolated from 2002 to 2007 in CF patients from our institution.

Results

CF-Marseille is multidrug resistant, has a hetero-Glycopeptide-Intermediate resistance S. aureus phenotype, grows on Cepacia agar with intense orange pigmentation and has a thickened cell wall. Phylogenetic analyses using Complete Genome Hybridization and Multi Locus VNTR Assay showed that CF-Marseille was closely related to strain Mu50, representing vancomycin-resistant S. aureus. Analysis of CF-Marseille shows a similar core genome to that of previously sequenced MRSA strains but with a different genomic organization due to the presence of specific mobile genetic elements i.e. a new SCCmec type IV mosaic cassette that has integrated the pUB110 plasmid, and a new phage closely related to phiETA3. Moreover this phage could be seen by electron microscopy when mobilized with several antibiotics commonly used in CF patients including, tobramycin, ciprofloxacin, cotrimoxazole, or imipenem. Phylogenetic analysis of phenotypically similar h-GISA in our study also suggests that CF patients are colonized by polyclonal populations of MRSA that represents an incredible reservoir for lateral gene transfer.

Conclusion

In conclusion, we demonstrated the emergence and spreading of a new isolate of MRSA in CF patients in Marseille, France, that has probably been selected in the airways by antibiotic pressure. Antibiotic-mediated phage induction may result in high-frequency transfer and the unintended consequence of promoting the spread of virulence and/or antibiotic resistance determinants. The emergence of well-adapted MRSA is worrying in such population chronically colonized and receiving many antibiotics and represents a model for emergence of uncontrollable super bugs in a specific niche.

Reviewers

This article was reviewed by Eric Bapteste, Pierre Pontarotti, and Igor Zhulin. For the full reviews, please go to the Reviewers' comments section.

Changing molecular epidemiology of methicillin-resistant Staphylococcus aureus in a small geographic area over an eight-year period

The epidemiology of methicillin-resistant Staphylococcus aureus (MRSA) at an international level shows that most MRSA strains belong to a few pandemic clones. At the local level, a predominance of one or two clones was generally reported. However, the situation is evolving and new clones are emerging worldwide, some of them with specific biological characteristics, such as the presence of Panton-Valentine leucocidin (PVL). Understanding these changes at the local and international levels is of great importance. Our objective was to analyze the evolution of MRSA epidemiology at multiple sites on a local level (Western Switzerland) over a period of 8 years. Data were based on MRSA reports from seven sentinel laboratories and infection control programs covering different areas. Pulsed-field gel electrophoresis was used to type MRSA isolates. From 1997 to 2004, a total of 2,256 patients with MRSA were reported. Results showed the presence of four predominant clones (accounting for 86% of patients), which could be related to known international clones (Berlin, New York/Japan, Southern Germany, and Iberian clones). Within the small geographic region, the 8-year follow-up period in the different areas showed spacio-temporal differences in the relative proportions of the four clones. Other international MRSA clones, as well as clones showing genetic characteristics identical to those of community-acquired MRSA (SCCmec type IV and the presence of PVL genes), were also identified but presumably did not disseminate. Despite the worldwide predominance of a few MRSA clones, our data showed that at a local level, the epidemiology of MRSA might be different from one hospital to another. Moreover, MRSA clones were replaced by other emerging clones, suggesting a rapid change.

Bacteremia due to clonally derived methicillin-resistant, gentamicin-susceptible isolates and methicillin-susceptible, gentamicin-resistant isolates of Staphylococcus aureus

We report recurrent bacteremia due to mixed infection with two clonally derived isolates of Staphylococcus aureus in a patient with Sezary syndrome. The two isolates, one gentamicin resistant and methicillin susceptible and the other gentamicin susceptible and methicillin resistant, developed by the deletion of the mecA, ant(4')Ia, and aacA-aphD genes from a common gentamicin-resistant and methicillin-susceptible ancestor.

Gentamicin-susceptible or gentamicin-resistant methicillin-resistant Staphylococcus aureus: a case-case study

Gentamicin-susceptible methicillin-resistant Staphylococcus aureus (GS-MRSA) strains are replacing gentamicin-resistant MRSA (GR-MRSA) strains. We showed that exposure to other patients harboring GS-MRSA was the major risk factor for GS-MRSA acquisition, and that antimicrobial exposure, especially to beta-lactams, was strongly associated with acquisition of GR-MRSA but not GS-MRSA. Gentamicin use was significantly associated with GR-MRSA acquisition in univariate analysis only.

Prevalence and evolution of methicillin-resistant Staphylococcus aureus in Spanish hospitals between 1996 and 2002

Pulsed-field gel electrophoretic analysis of 2,144 methicillin-resistant Staphylococcus aureus (MRSA) strains isolated from patients in Spanish hospitals over a 7-year period revealed 17 predominant profiles. Typing showed the replacement of Iberian clone E1 (ST247-MRSA-I) by two prevalent clones, E7 and E8, that are closely related to each other and have the same genetic background as ST125-MRSA-IV.

Evidence for clonal evolution among highly polymorphic genes in methicillin-resistant Staphylococcus aureus

The evolution of Staphylococcus aureus has been described as predominantly clonal, based on evidence from seven housekeeping genes. We aimed to test if this was also true for more polymorphic genes. In a collection of 60 isolates including major European epidemic methicillin-resistant S. aureus (MRSA) and sporadic MRSA strains, we compared the partial gene sequences of seven housekeeping genes (arcC, aroE, glpF, gmk, pta, tpi, and yqiL), six core adhesion genes (present in all strains) (clfA, clfB, fnbA, map, sdrC, and spa), and four accessory adhesion genes (not present in all strains) (ebpS, fnbB, sdrD, and sdrE). Nucleotide diversity of adhesion genes was 2- to 10-fold higher than genes used for multilocus sequence typing. All genes showed evidence for purifying selection with a weakly reduced level among accessory adhesion genes. Among these highly variable genes, there was no evidence for a difference in molecular evolution between epidemic and sporadic strains. Gene trees constructed from concatenated sequences of housekeeping, core adhesion, and accessory adhesion genes were highly congruent, indicating clonality, despite some evidence for homologous exchange. Further evidence for clonality was found with an overall positive correlation of allelic and nucleotidic divergence for both seven housekeeping genes and six core adhesion genes. However, for small allelic differences that fit the demarcations of clonal complexes (CCs) there was no such correlation, suggesting that recombination occurred. Therefore, despite an overall clonal population structure, recombination between related isolates within CCs might have contributed to S. aureus evolution.

Genetic differentiation of methicillin-resistant Staphylococcus aureus strains from Korea and Japan

In this study, we evaluated genetic differentiation between methicillin-resistant Staphylococcus aureus (MRSA) strains from Korea and Japan. Seventy-five MRSA strains, including 25 h VISA strains, were analyzed by molecular typing methods, including multilocus sequence typing (MLST), SCC mec typing, and spa typing. The most prevalent genotype of MRSA strains, in both Korea and Japan, was ST 5-MRSA-II with the DMGMK spa motif, characteristic of the New York/Japan MRSA clone. In spite of these common features in MRSA strains from Korea and Japan, we also observed some genotypic divergence in MRSA from the two countries. Several spa types might be differentiated from a prevalent prototype (TJMBMDMGMK) that is shared by the two countries, revealing a unique geographic distribution. SCC mec type II lacking pUB110, designated type IIA, was found more frequently in Korea than in Japan. The rate of gentamicin resistance was also dramatically different between the two countries: 87.2% (Korea) vs. 28.6% (Japan). These preliminary findings suggested that MRSA strains from Korea and Japan might have originated from a common ancestor, but then clearly differentiated according to locality. A further comprehensive study should be performed to document the hypotheses from this study.

Heterogeneity of methicillin-resistant Staphylococcus aureus strains at a German university hospital during a 1-year period

Heterogeneous methicillin-resistant Staphylococcus aureus (MRSA) strains, including community-acquired MRSA strains, have been observed in Central Europe. The purpose of this study was to characterize by molecular methods MRSA isolated during the period 2002-2003 at the Otto-von-Guericke University Hospital in Magdeburg, Germany, and at a nearby chronic care facility. Strains were analyzed for their resistance phenotype. Selected isolates were typed by multilocus sequence typing (MLST), by a multiplex polymerase chain reaction (PCR) for the staphylococcal cassette chromosome mec (SCCmec), by an allele-specific PCR for the staphylococcal accessory gene regulator (agr), and by PCR for the presence of toxin genes (sea-sej, tsst-1, hlgA, C, and B, lukE/D, and luk-pvl). Of the 2,731 S. aureus isolates studied, 199 (7.3%) were MRSA, with a prevalence of 21.6%, 19.6%, and 12% in the department of dermatology, the chronic care facility, and the intensive care units. Six different sequence types (ST247, ST228, ST22, ST22a, ST225, and ST45) were observed. Of these, ST22, ST22a, and ST45 dominated (>50%) in the department of dermatology and the chronic care facility. Strains with these sequence types were usually not resistant to gentamicin and were associated with agr group I, the SCCmec type IV element, and the presence of the sec and sed toxin genes. ST228 strains were found mainly in the intensive care units and had a broader resistance phenotype and were associated with agr group II and the SCCmec type I element. All luk-pvl-positive MRSA isolates (n=8) belonged to agr group I and were typed as ST22 or ST45 and contained the SCCmec type I (n=1), type III (n=1), or type IV (n=6) element. The main observations of this study are in concordance with previously reported findings showing dissemination of MRSA in Central Europe. Through the multitude of applied methods, the data from this study contribute to a more precise knowledge about the heterogeneity of MRSA in a clinical setting. Rapid dissemination of MRSA clones at a university hospital was demonstrated, indicating that dissemination may depend on the environmental conditions within the individual departments.

Transmission and epidemiology of MRSA: current perspectives

Staphylococcus aureus is a gram-positive bacterium that developed resistance to the penicillin derivative methicillin. Subsequently, methicillin-resistant S. aureus (MRSA) emerged as a bacterium that became less susceptible to the actions of methicillin and thus developed the ability to colonize and cause life-threatening infections. Globally, MRSA continues to cause hospital-acquired infections which are becoming difficult to treat owing to increasing glycopeptide resistance and the increasing development of community-associated MRSA. Nurses caring for patients in both hospital and community settings should be able to acknowledge the importance of MRSA, the difficulties of treating the bacterium and the need to comprehend and adhere to universal precautions that are important in the prevention of transmission of MRSA.

Epidemiology and typing of Staphylococcus aureus strains isolated from bloodstream infections

We carried out an epidemiological study covering 2,365,067 patient days of hospitalization between 2000 and 2003. During this time, 413 Staphylococcus aureus bloodstream infections occurred. This corresponds to 15% of the 2,676 bloodstream infections observed during this period in the 31 hospitals in our region of France, which has 2.5 million inhabitants. The incidence of nosocomial S. aureus bloodstream infections was 0.11 per 1,000 days of hospitalization. The prevalence of methicillin-resistant S. aureus (MRSA) strains, of which 13% were nonmultiresistant MRSA (NORSA), was 33%, and this percentage was stable over the 4 years. In contrast, the prevalence of S. aureus strains susceptible to methicillin but resistant to quinolones or susceptible to methicillin but multiresistant to antibiotics (EMSSA strains) increased from 4% in 2000 to 23% in 2003. As previously reported, MRSA strains were mostly recovered from nosocomial bloodstream infections, whereas NORSA strains-generally considered to be responsible for community-acquired infections-were always isolated from nosocomial bloodstream infections. Pulsed-field gel electrophoresis (PFGE) analysis of 109 MRSA strains and 15 EMSSA strains demonstrated clonal diffusion of the three major French MRSA clones and revealed considerable genetic heterogeneity among EMSSA strains. Although no epidemiologically related NORSA strains clustered in particular PFGE groups, the distribution of MRSA strains isolated from bloodstream infections according to the portal of entry (vascular devices, pulmonary, and urinary) was not random for the major PFGE clones, suggesting that each MRSA lineage displays particular virulence features.

Retour vers la sensibilité aux antibiotiques des souches de Staphylococcus aureus résistant à la méticilline (SARM) : relation entre génotype et antibiotype

The objective of our study was to investigate the relationship between molecular epidemiology and antibiotic susceptibility of MRSA during a four-year period. In this aim, we determined the antibiotype of all MRSA identified during a yearly period of 3 months and typed 50 consecutive non-replicate MRSA isolates of each year. We also recorded antibiotic use. Susceptibility rates to gentamicin, tobramycin and ofloxacin remained stable, respectively, 95, 16 and 4%. In contrast, the proportion of MRSA susceptible to erythromycin progressively increased from 7.0% to 32.5% (P < 0.001). PFGE analysis of genomic DNA from the 200 isolates revealed 15 different clones. We identified two epidemic clones, which contained 150 (clone A) and 28 isolates (clone C), respectively. The proportion of isolates belonging to clone A decreased during the study from 86% to 66%. Conversely, clone C increased from 4% to 22%. The increase of erythromycin-susceptibility within MRSA was caused by the emergence of clone C. Non-epidemic strains were more frequently susceptible to ofloxacin (31.8% vs. 1.1%) and tobramycin (45.4% vs. 16.8%) than epidemic strains. Antimicrobial use had not significantly varied during the study. The proportion of beta-lactams, fluoroquinolones, macrolides and aminosides was 71.8, 13.9, 5.0 and 3.8% of the total antibiotic use, respectively. In our hospital, MRSA isolates became more susceptible to antimicrobial of minor use. The selection pressure exerted by beta-lactams and fluoroquinolones was in favor of the spread of strains resistant to these both major antibiotic classes.

Prevalence, molecular epidemiology, and clinical significance of heterogeneous glycopeptide-intermediate Staphylococcus aureus in liver transplant recipients

We investigated the prevalence, molecular epidemiology, and clinical significance of heterogeneous glycopeptide-intermediate Staphylococcus aureus (hGISA) isolates in 48 liver transplant recipients infected or colonized with methicillin-resistant S. aureus over a 5-year period. Strains were screened for hGISA on Mueller-Hinton agar containing 5 mg of teicoplanin per liter. Heterogeneous glycopeptide resistance was confirmed by the E-test method with a dense inoculum and a simplified method of population analysis. hGISA strains were found in 13 (27%) of the 48 patients studied. Eleven of the 13 strains shared a common multiresistant phenotype with homogeneous methicillin resistance and gentamicin resistance, and they were closely related according to the results of pulsed-field gel electrophoresis. Only 2 of the 13 patients infected or colonized with hGISA strains had previously received glycopeptide therapy. Most patients were successfully treated with vancomycin, but one patient who failed to respond to vancomycin subsequently died. These results suggest that the high prevalence of hGISA among our patients was due to the clonal spread of a multiresistant strain.

[Hospital infection surveillance and multiresistance monitoring: role of an external quality control]

The "Relais Régional d'Hygiène Hospitalière du Centre" (RHC) promotes the hospital infection prevention at a regional level in France, including 80 healthcare institutions. The accuracy of antimicrobial susceptibility data submitted by laboratories to surveillance is essential. Since 2001, RHC imposed an external quality control to validate the accuracy of the data submitted by the laboratories that are involved in survey programs. Most laboratories are able to detect homogenous methicillin resistance in S. aureus, and high-level vancomycin resistance in E. faecalis. Nevertheless, the ability of laboratories to detect organisms with emerging antimicrobial resistance patterns is not optimal for (i) detection of heterogeneous methicillin resistance, (ii) reduced susceptibility to teicoplanin in a non-multiresistant S. aureus and (iii) detection of resistance to extended-spectrum cephalosporins. Educational program to optimize the testing methods has been programmed and perennially of quality control testing prior to accepting data from laboratory participating in surveillance system is decided.

Emergence and spread of gentamicin-susceptible strains of methicillin-resistant Staphylococcus aureus in Belgian hospitals

The aim of this study was to follow the evolution of the clonal distribution and antimicrobial susceptibility of clinical strains of methicillin-resistant Staphylococcus aureus (MRSA) recovered from Belgian hospitals between 1995 and 1997-1998. MRSA strains were genotyped by inter-IS256 spacer length polymorphism PCR and SmaI macrorestriction analysis. MICs of 18 antimicrobials were determined by the agar dilution method. MRSA strains from the 1997-1998 survey were further tested by vancomycin screen agar, E-test, broth microdilution methods, and population analysis. Between 1995 and 1997-1998, epidemic group A strains decreased in proportion from 73% to 44%, whereas MRSA Group B and C strains increased from 17% to 38% and from 5% to 8%. The proportion of strains susceptible to gentamicin increased between the surveys from 22% to 48%. This was associated with a higher proportion of group B and C strains in the last survey. Heterogeneous vancomycin-intermediate S. aureus (hetero-VISA) strains were found in 2% isolates from 1997 to 1998. These hetero-VISA isolates were genotypically related to the MRSA group A strains and were resistant to gentamicin. In conclusion, two emerging epidemic MRSA genotypes, susceptible to gentamicin, have spread among Belgian hospitals during the 1990s. Hetero-VISA were present at low frequency among MRSA strains belonging to a widespread endemic genotype.

Relationship between molecular epidemiology and antibiotic susceptibility of methicillin-resistant Staphylococcus aureus (MRSA) in a French teaching hospital

The objective of this study was to investigate the relationship between molecular epidemiology and antibiotic susceptibility of methicillin-resistant Staphylococcus aureus (MRSA) over a period of 4 years. The antibiotype of all MRSA isolates that were identified during a yearly period of 3 months was determined; 50 consecutive non-replicate MRSA isolates were typed each year. Susceptibility rates to gentamicin, tobramycin and ofloxacin remained stable (95, 16 and 4 %, respectively). In contrast, the proportion of MRSA isolates susceptible to erythromycin increased progressively from 10.5 to 32.5 % (P < 0.001). PFGE analysis of genomic DNA from 200 isolates revealed the presence of 15 different clones. Two epidemic clones were identified, which contained 150 (clone A) and 28 (clone C) isolates. Non-epidemic strains were more frequently susceptible to ofloxacin (31.8 versus 1.1 %) and tobramycin (45.4 versus 16.8 %) than epidemic strains; those isolates that were susceptible to all antibiotics tested belonged to sporadic clones. The increase of erythromycin susceptibility within MRSA isolates was caused by the emergence of clone C. This study suggests that when selection pressure exerted by an antibiotic is insufficient (i.e. below a threshold level), fitness advantages play a predominant role in the dissemination of MRSA clones. The balance between the selection pressure exerted by antibiotics and the disadvantage of lower replication rates of resistant strains in the absence of antibiotics complicates the biological model of clonal dissemination of epidemic MRSA strains.

Comparative analysis of multidrug-resistant, non-multidrug-resistant, and archaic methicillin-resistant Staphylococcus aureus isolates from Central Sydney, Australia

In this study, the phenotypic and genotypic characteristics of 50 methicillin-resistant Staphylococcus aureus (MRSA) isolates (43 contemporary and 7 archaic strains from the mid-1960s) from four Sydney hospitals in the central Sydney area were compared. Phenotypic analysis based on antibiotic profiles and phage typing patterns categorized the MRSA isolates into three major groups: multidrug resistant (mMRSA), non-multidrug resistant (nmMRSA), and archaic. The nmMRSA isolates could be further subdivided into nmMRSA group 1, which was phage typeable and similar to the archaic group; nmMRSA group 2, which was non-phage typeable and only resistant to ciprofloxacin; and nmMRSA group 3, which was also nontypeable and generally resistant to other antibiotics. The characterization of all five phenotypic groups was then extended by genetic analysis. Restriction fragment length polymorphism (RFLP) analysis showed the 50 isolates could be sorted into 20 group-specific pulsotypes. mecI gene deletions and mutations at various percentages among the five MRSA groups were detected by sequencing. Several mec promoter mutations were also found. The overall findings indicated that nmMRSA strains may have independently acquired mec DNA and are more likely to be newly emergent strains than nmMRSA variants.

Poor value of pulsed-field gel electrophoresis to investigate long-term scale epidemiology of methicillin-resistant Staphylococcus aureus

Pulsed-field gel electrophoresis (PFGE) is widely used for epidemic investigations of methicillin-resistant Staphylococcus aureus (MRSA). In the present study, we evaluated its use in a long-term epidemiological setting (years to few decades, country to continent level). The clustering obtained from PFGE patterns after SmaI digestion of the DNA of 20 strains was compared to that obtained using a phylogenetic typing method (multiprimer RAPD). The results showed that the analysis of small PFGE bands (10-85kb) correlates better with multiprimer RAPD than the analysis of large PFGE bands (>85-700kb), suggesting that the analysis of small bands would be more suitable for the investigation of long-term epidemiological setting. However, given the technical difficulties to obtain a good resolution of these bands and the putative presence of plasmids among them, PFGE does not appear to be a method of choice for the long-term epidemiology analysis of MRSA.

Characterization of methicillin-resistant Staphylococcus aureus strains susceptible to tobramycin

Gentamicin-susceptible, methicillin-resistant, Stapylococcus aureus strains (GS-MRSA) emerged in 1992 in various Parisian hospitals and have subsequently been isolated from all French hospitals. This new GS-MRSA epidemic clone accounted for 50% of MRSA strains in 1996 and for 85% in 2000 in our hospital. We have observed a parallel increase in the prevalence of tobramycin and amikacin-susceptible GS-MRSA (TKS-MRSA). The number of TKS-MRSA strains per 100 MRSA strains has steadily increased from 3.1 in 1996 to 24.0 in 2000. Genotypic characterization of TKS-MRSA strains showed that these strains are a phenotypic variant of the dominant clone of GS-MRSA. To improve our understanding of the changes in methicillin-resistant Staphylococcus aureus (MRSA) susceptibility to non-beta-lactam antibiotics, gentamicin- and amikacin-susceptible MRSA (TKS-MRSA) from our acute-care hospital were compared with TKS-MRSA isolated from a long-term care hospital located in another region of France. The nature of the care facility did not seem to play a major role in the hospital dissemination of TKS-MRSA. We also found that changes in antibiotic use alone do not account for the emergence of these strains.

Comparative pulsed-field gel electrophoresis typing of gentamicin-resistant and -susceptible methicillin-resistant Staphylococcus aureus strains isolated in France between 1991 and 1998. Changes in antibiotic susceptibility

Using macrorestriction of genomic DNA and pulsed-field gel electrophoresis, we examined 504 non-redundant, infection-causing human isolates of methicillin-resistant Staphylococcus aureus susceptible (G(S): 238 isolates) or resistant to gentamicin (G(R): 266 isolates). The strains were isolated at Albert Chenevier Hospital (Créteil, France) between 1 January 1991 and 31 December 1998. Their susceptibility to erythromycin, lincomycin, tetracycline, rifampicin, fusidic acid and fosfomycin was also studied. Seventy-six genotypes were identified (percentage similarity<80). Ten types, each containing at least eight strains, predominated. G(R) strains showed higher genetic polymorphism than G(S) strains: the 266 G(R) isolates belonged to 67 genotypes, five of which predominated (44, 42, 38, 30 and 15 isolates); the 238 G(S) isolates belonged to only 18 types, four of which predominated (112, 83, 11 and 10 isolates). Fifty-six percent of G(R) strains (34 Gt) were resistant to erythromycin, lincomycin, tetracycline and rifampicin, and were isolated at relatively stable frequencies. Resistance to five antibiotics studied (susceptible to fusidic acid) was observed among 16.5% of G(R) strains. The frequency of strains with this profile diminished from 30% in the early 1990s to 10% in 1998. One hundred and twenty-six G(S) isolates were susceptible to all six antibiotics; this profile was only found from 1993 onwards, and was increasingly frequent (60% of G(S) strains in 1996). Resistance to erythromycin and lincomycin only was found in 70 G(S) isolates; this profile accounted for approximately half the isolates in 1992/1993 and only one-third in 1998. These results, obtained over an eight-year period, show an overall increase in antibiotic susceptibility. They confirm the spread of two major clones of MRSA-G(S) and support the hypotheses that G(S) strains derive from G(R) strains that have lost the aac6'-aph2" gene; and that G(S) strains are genetically related to those that were present before the use of gentamicin and persisted at a low frequency until 1992-1993.

[Staphylococci resistant to antibiotic therapy]

Increasing frequency in antibiotic resistance in Staphylococcus aureus is worrying. In this review, we briefly describe the mechanisms of resistance to antibiotics in staphylococci and we report the current incidence of antibiotic resistance in France. Methicillin-resistance is mostly confined to hospitals. However, spread of multiply resistant strains of staphylococci to the community is a putative threat. Strains with diminished susceptibility to glycopeptides are rare but possibly underestimated because of the in vitro difficulties to detect glycopeptide resistance. A continuous resistance survey is necessary. The problem of multiresistance is different for hospital and community staphylococci. The later strains still remain susceptible to many antibiotics, apart from some exceptions. By contrast, incidence of multiply resistance in hospitals remains high. After a recent decrease, incidence of oxacillin resistance seems to be stabilized. A new and recent feature is the great variety of resistance phenotypes which can be found in methicillin-resistant strains, in particular with gentamicin susceptibility.

Nine-year surveillance of methicillin-resistant Staphylococcus aureus in a hospital suggests instability of mecA DNA region in an epidemic strain

The distributions of the antibiotic resistance patterns in a population of Staphylococcus aureus isolates from a teaching hospital were studied over a 9-year period. The results indicate the existence of successive major epidemic methicillin-resistant strains and the emergence of a methicillin-susceptible strain with an unusual resistance pattern. Our findings suggest that this methicillin-susceptible S. aureus strain could be derived from the dominant gentamicin-susceptible methicillin-resistant S. aureus strain with the loss of a 40-kb DNA fragment.