Epidemiologic and clinical utility of typing systems for differentiating among strains of methicillin-resistant Staphylococcus aureus

Methicillin-Resistant Staphylococcus aureus: Molecular Characterization, Evolution, and Epidemiology

Staphylococcus aureus, a major human pathogen, has a collection of virulence factors and the ability to acquire resistance to most antibiotics. This ability is further augmented by constant emergence of new clones, making S. aureus a "superbug." Clinical use of methicillin has led to the appearance of methicillin-resistant S. aureus (MRSA). The past few decades have witnessed the existence of new MRSA clones. Unlike traditional MRSA residing in hospitals, the new clones can invade community settings and infect people without predisposing risk factors. This evolution continues with the buildup of the MRSA reservoir in companion and food animals. This review focuses on imparting a better understanding of MRSA evolution and its molecular characterization and epidemiology. We first describe the origin of MRSA, with emphasis on the diverse nature of staphylococcal cassette chromosome mec (SCCmec). mecA and its new homologues (mecB, mecC, and mecD), SCCmec types (13 SCCmec types have been discovered to date), and their classification criteria are discussed. The review then describes various typing methods applied to study the molecular epidemiology and evolutionary nature of MRSA. Starting with the historical methods and continuing to the advanced whole-genome approaches, typing of collections of MRSA has shed light on the origin, spread, and evolutionary pathways of MRSA clones.

Comparing pulsed-field gel electrophoresis with multilocus sequence typing, spa typing, staphylococcal cassette chromosome mec (SCCmec) typing, and PCR for panton-valentine leukocidin, arcA, and opp3 in methicillin-resistant Staphylococcus aureus isolates at a U.S. Medical Center

Methicillin-resistant Staphylococcus aureus (MRSA) has become a common cause of skin infections and invasive infections in community dwellers in the United States since the late 1990s. Isolates characterized as USA300 by pulsed-field gel electrophoresis (PFGE) are the predominant strain type in these infections. USA100 and USA500 strains commonly cause health care-associated infections. We compared PFGE with a number of other methods of genotyping in a sample of 149 clinical MRSA isolates from the University of Chicago Medical Center. The 5 USA500 isolates yielded 3 spa types and 2 multilocus sequence types (MLSTs). Among the 24 USA100 isolates, 21 (88%) were of spa type t002, 19 (79%) were of ST5, 2 carried arcA and opp3, and 1 was Panton-Valentine leukocidin positive (PVL(+)). Among the 102 USA300 isolates, 96 (94%) were of ST8 and 94 (92%) were of spa type t008. The combination of traits that provided the best sensitivity (98%), specificity (97%), positive predictive value (PPV) (99%), and negative predictive value (NPV) (95%) for identifying USA300 isolates were the presence of the arcA gene and the presence of the PVL genes (area under the curve, 0.980; 95% confidence interval [CI], 0.955 to 1.0). PFGE did not delineate a homogeneous group of MRSA genetic backgrounds, as documented for other typing methods, particularly for USA500 and USA100 pulsotypes. Documenting the presence of arcA and PVL genes by PCR was an efficient and accurate means of identifying USA300 in a collection of MRSA isolates in which USA300 is common. None of the tested genotyping methods provided an accurate means of identifying the next most common PFGE-based backgrounds, USA100 and USA500.

Changes in the clonal nature and antibiotic resistance profiles of methicillin-resistant Staphylococcus aureus isolates associated with spread of the EMRSA-15 clone in a tertiary care Portuguese hospital

Two hundred eighty methicillin-resistant Staphylococcus aureus (MRSA) clinical isolates recovered from a tertiary care hospital in Oporto, Portugal, between 2003 and 2005 were studied by a combination of molecular typing techniques in order to investigate the genetic backgrounds associated with the changes in the resistance phenotypes observed since 2001 and compare them to those previously found in the hospital. All MRSA isolates were grouped into resistance profiles for a panel of seven antibiotics and characterized by pulsed-field gel electrophoresis (PFGE) and SCCmec (staphylococcal cassette chromosome mec) typing. Representative isolates of PFGE types were further studied by spa typing and multilocus sequence typing. Our findings clearly document that the increasing isolation of nonmultiresistant MRSA strains was associated with the decline (from 69% in 1996 to 2000 to 12% in 2003 to 2005) and massive replacement of the multiresistant Brazilian clone (ST239-IIIA) by the epidemic EMRSA-15 clone (ST22-IV), in which resistance to antibiotics other than beta-lactams is very rare, as the major clone (80% of isolates). The Iberian clone (ST247-IA), a major clone in 1992 to 1993, was represented in the present study by just one isolate. Two other pandemic MRSA clones were detected, as sporadic isolates, for the first time in our hospital: the New York/Japan (ST5-II) and the EMRSA-16 (ST36-II) clones. Furthermore, the pattern of susceptibility of MRSA isolates both to gentamicin and to trimethoprim-sulfamethoxazole was shown to be an excellent phenotypic marker for the discrimination of the EMRSA-15 clone from other nonmultiresistant MRSA clones present in our hospital.

Clonal and antibiotic resistance profiles of methicillin-resistant Staphylococcus aureus (MRSA) from a Portuguese hospital over time

Methicillin-resistant Staphylococcus aureus (MRSA) isolates recovered from a general hospital in Oporto, Portugal, during two periods (1992-1993 and 1996-2000) were characterized by pulsed-field gel electrophoresis (PFGE) of SmaI fragments, and by hybridization of ClaI digests with mecA and Tn554 probes, discriminating the isolates in mecA::Tn554::PFGE genotypes. In addition, a representative sample of the defined genotypes was characterized by multilocus sequence typing (MLST) and SCCmec (staphylococcal cassette chromosome mec) typing, generating the corresponding ST-SCCmec types. In 1992-1993, 77% of MRSA belonged to the Iberian clone (genotype I::E::A or ST247-IA). In 1996-2000, the frequency of this clone decreased to 19% and the majority (69%) of the isolates belonged to another international clone, the Brazilian MRSA (genotype XI::B::B or ST239-IIIA). Trimethoprim/sulfamethoxazole (SXT) was confirmed to be an important phenotypic marker to distinguish the Iberian (SXT-susceptible) and the Brazilian (SXT-resistant) clones in MRSA isolates from Portugal. Our observations document major shifts in the dominant MRSA clonal types that occurred in this hospital since 1992, suggesting a selective advantage of the Brazilian relatively to the Iberian clone. In addition to these two MRSA clones that are the most frequent in Portuguese hospitals since the early 1990s, sporadic MRSA clones (representing 14% of the total) were identified and characterized.

[Methicillin-resistant Staphylococcus aureus: factors responsible for its incidence]

INTRODUCTION

How can we explain that the proportion of methicillin-resistant Staphylococcus aureus (MRSA) varies between the European countries, ranging from < 1% in Scandinavia to > 30% in Spain, France and Italy? This paper is aimed at attempting to determine factors at the origin of the spreading of endemic MRSA strains as of the early 1980s. Those strains are characterized by their ability to develop resistance to current antibiotics and make treatment of severe and deep infections more complex.

CURRENT KNOWLEDGE AND KEY POINTS

Differences in the virulence of MRSA strains and that of susceptible strains appear unlikely. MRSA prevalence seems to be a growing problem, especially in Southern Europe where rates of resistance to other anti-staphylococcal antibiotics are high. General policies for antibiotic therapy as well as the implementation of strategies for prevention and control of MRSA might be responsible for such rates. Indeed, once MRSA is introduced into a facility without control program, this multiresistant bacteria rapidly spreads within the hospital and becomes endemic, expanding its reservoir. FUTURE PROSPECTS ET PROJECTS: Due to the introduction of new methods in microbiology and communication, infection control measures including procedures for isolation and identification of MRSA reservoirs are still feasible; however, their implementation requires human and material resources. Areas requiring improvement in the detection of MRSA outbreaks are identified in this paper, with particular emphasis on the need for national surveillance of MRSA prevalence and reappraisal of MRSA control strategies in French hospitals.

Spread of a methicillin-resistant and multiresistant epidemic clone of Staphylococcus aureus in Argentina

One hundred forty-eight recent methicillin-resistant Staphylococcus aureus (MRSA) isolates collected from 13 hospitals in Argentina were examined for antibiotic susceptibility and clonal type, using hybridization with DNA probes specific for mecA and Tn554, and pulsed-field gel electrophoresis (PFGE) of chromosomal SmaI digests. The majority of the isolates (62.2%) shared the common PFGE B pattern and carried variants of mecA and Tn554 polymorphs characteristic of an MRSA clone widely spread in Brazilian hospitals. Similarly to the Brazilian isolates, the MRSA clone recovered in the Argentinian hospitals (XI::B::B) and its close relatives (XI::B'::B, XI::AA::B, XI::M::B, XI::omega omega::B, and III::W::B) showed susceptibility to spectinomycin and resistance to numerous antibacterial agents, including beta-lactams, tetracycline, aminoglycosides, macrolides, trimethoprim/sulfamethoxazole, ciprofloxacin, and fosfomycin, and more than 60% of the isolates were also resistant to chloramphenicol and rifampin. The XI::B::B MRSA clone represented the majority of isolates recovered in most of the hospitals, nine of which were located in the city of Buenos Aires, three in the province of Buenos Aires, and one in the province of Tucumán, 1,312 km northwest of the city of Buenos Aires. The observations document further geographic expansion of this South American MRSA clone across national boundaries.

Dissemination of two methicillin-resistant Staphylococcus aureus clones exhibiting negative staphylase reactions in intensive care units

From December 1997 to March 1998, 25 methicillin-resistant Staphylococcus aureus (MRSA) isolates exhibiting negative Staphylase (Oxoid Ltd., Basingstoke, England) reactions were identified from various clinical specimens from 13 patients in six intensive care units (ICUs) or in wards following a stay in an ICU at the National Taiwan University Hospital. The characteristics of these isolates have not been previously noted in other MRSA isolates from this hospital. Colonies of all these isolates were grown on Trypticase soy agar supplemented with 5% sheep blood and were nonhemolytic and unpigmented. Seven isolates, initially reported as Staphylococcus haemolyticus (5 isolates) and Staphylococcus epidermidis (2 isolates) by the routine identification scheme and with the Vitek GPI system (bioMerieux Vitek, Inc., Hazelwood, Mo.), were subsequently identified as S. aureus by positive tube coagulase tests, standard biochemical reactions, and characteristic cellular fatty acid chromatograms. The antibiotypes obtained by the E test, coagulase types, restriction fragment length polymorphism profiles of the staphylococcal coagulase gene, and random amplified polymorphic DNA patterns generated by arbitrarily primed PCR of the isolates disclosed that two major clones disseminated in the ICUs. Clone 1 (16 isolates) was resistant to clindamycin and was susceptible to trimethoprim-sulfamethoxazole (TMP-SMZ) and was coagulase type II. Clone 2 (eight isolates) was resistant to clindamycin and TMP-SMZ and was coagulase type IV. These two epidemic clones from ICUs are unique and underline the need for caution in identifying MRSA strains with colonial morphologies not of the typical type and with negative Staphylase reactions.

A profile of methicillin resistant Staphylococcus aureus infection in the burn center of the Sultanate of Oman

A retrospective study of patterns of infection in 168 patients admitted during 1995 and 1996 in the burns-unit of Khoula hospital at Muscat, Oman was performed. Out of 819 isolates positive for pathogenic bacterial culture, there were 326 (39.8%) isolates positive for methicillin resistant Staphylococcus aureus (MRSA) infection. Incidence of MRSA infection was marginally more than that of Pseudomonas aeroginosa. The proportion of patients developing MRSA infection sometime or the other during their burns-unit stay ranging from 1 to 112 days rose from 48% in 1995 to 52.7% in 1996. No sophisticated tests were done to identify the MRSA strain but study of the antibiograms of each MRSA positive isolate showed very similar patterns of sensitivity to different antibiotics. This suggests the source of infection to be common and in all probability 'noscomial', since all patients acquired MRSA infection in the hospital. The susceptibility of MRSA to ciprofloxacin, cotrimoxazole and fucidin was 76, 51 and 37% of isolates in 1995, and 59, 44 and 26% in 1996 were susceptible to these drugs. Vancomycin was the antibiotic to which most MRSA cultures were susceptible, but partial resistance was reported due to very low susceptibility observed in 1.4% of the isolates in 1995 and 1.1% of the isolates in 1996. The control measures being practiced in the burns-unit of Khoula Hospital, especially mechanical cleaning and chemical disinfection of all surfaces, are discussed in detail. This paper emphasizes the need for preventive measures against MRSA infection in the burns-unit.

Clonal identification of Aeromonas hydrophila strains using randomly amplified polymorphic DNA analysis

The suitability of arbitrary primer polymerase chain reaction (RAPD) as a typing technique was evaluated by comparing it with pulsed-field gel electrophoresis (PFGE) to characterize Aeromonas hydrophila strains isolated from a cluster of hospital-acquired infections. Five isolates from patients and 10 isolates from the water supply were compared to 10 epidemiologically unrelated strains isolated from patients and rivers. Two methods were used to prepare DNA and two primers (AP3 and AP5) were selected. The discriminatory power was better with the extractive DNA preparation than the boiling method. The discrimination of closely related from less related strains by PCR using AP3 was consistent with that by PFGE: water supply of Cholet hospital contaminated with Aeromonas species was not the source of the cluster of hospital infections and only two patients were infected with clonally-related strains. RAPD using primer AP3 was simpler, cheaper, and quicker to perform than pulsed-field gel electrophoresis and is well suited for the epidemiological study of A. hydrophila isolates.

Analysis of an outbreak of non-phage-typeable methicillin-resistant Staphylococcus aureus by using a randomly amplified polymorphic DNA assay

A cluster of methicillin-resistant Staphylococcus aureus (MRSA) infections among patients on an intensive care unit (ICU) was detected by routine infection control surveillance. In the period from 5 January to 22 June 1995, 10 patients on the ICU and a further 6 patients (5 on one ward that had received colonized patients transferred from the ICU) were affected by MRSA strains with the same antibiotic susceptibility patterns. Seven (44%) of these 16 colonized patients developed MRSA bacteremia. MRSA isolates with the same characteristics were also found on the hands of one member of the ICU staff. The isolates were untypeable by phage typing, but 15 of 17 outbreak strains analyzed genetically had identical randomly amplified polymorphic DNA (RAPD) and pulsed-field gel electrophoresis (PFGE) profiles. A single strain of MRSA that was nontypeable by phage typing and that was isolated on the ICU on 1 January and six nontypeable and epidemiologically unrelated MRSA isolates all had RAPD profiles distinct from that of the outbreak strain. Implementation of strict infection control measures stopped the further spread of MRSA on the ICU, the affected general ward, and seven other wards that received MRSA carriers from the ICU. Although nontypeable by phage typing and not previously recognized as an epidemic strain, this strain of MRSA was readily transmissible and highly virulent. RAPD typing was found to be a simple, rapid, and effective method for the epidemiological investigation of this outbreak, and performance of typing by this method was simpler and less time-consuming than that of typing by PFGE. RAPD typing may have more general application for the study of S. aureus infections in hospitals.

Comparison and application of ribosome spacer DNA amplicon polymorphisms and pulsed-field gel electrophoresis for differentiation of methicillin-resistant Staphylococcus aureus strains

Analysis of sequences in the fragments of the 16S-23S rRNA intergenic spacer region by the ribosome spacer PCR (RS-PCR) can differentiate strains of methicillin-resistant Staphylococcus aureus (MRSA). We compared this technique with pulsed-field gel electrophoresis (PFGE) for typing MRSA strains and its application during an investigation of an outbreak. A total of 180 isolates of MRSA collected from various hospital laboratories within the United Kingdom and elsewhere were typed by PFGE and RS-PCR. PFGE identified 17 different types among the 180 strains examined, and RS-PCR generated 13 different types. PFGE could detect minor genetic variations among the isolates and could identify the variants which were not discriminated by RS-PCR. Four unique strain types detected by PFGE were not detected by RS-PCR. When applied to typing the outbreak-related strains from the vascular surgery unit at the General Infirmary at Leeds, the results of RS-PCR were identical to those of PFGE. Our results have shown that RS-PCR is a rapid, inexpensive technique that is highly reproducible and almost as discriminatory as PFGE for typing MRSA isolates and should be useful in the local investigation of MRSA outbreaks.

Changes in major populations of methicillin-resistant Staphylococcus aureus in Belgium

A total of 102 epidemic methicillin-resistant Staphylococcus aureus (MRSA) isolates collected in 13 Belgian hospitals during two periods (1981-1985 and 1991-1992) were tested for phage-type, for the presence of aminoglycoside-modifying enzymes (AME), and examined by arbitrarily primed polymerase chain reaction (AP-PCR). All isolates, but five, belonged to a few distinct phage-types of group III. Most isolates expressed a combination of AAC(6')-APH(2") with APH(3')III, and ANT(4',4") or both. Both phage-typing and AME suggested a change in the MRSA population between the two periods but the AP-PCR method revealed only slight differences.

Comparison of DNA fingerprinting by PFGE and PCR-RFLP of the coagulase gene to distinguish MRSA isolates

Staphylococcus aureus isolates were collected from epidemiologically unrelated clinical sources in Japan between 1991 and 1993. A total of 40 isolates, five each of eight coagulase types, were analysed by polymerase chain reaction (PCR) of the coagulase gene, PCR-restriction fragment length polymorphism (RFLP) after AluI digestion, and pulsed-field gel electrophoresis (PFGE) of chromosomal DNA after SmaI digestion. The efficiency of discrimination among the isolates increased in the order of PCR < PCR-RFLP < PFGE, yielding five, 13 and 31 different types, respectively. To assess the clinical use of these methods, 42 additional methicillin-resistant S. aureus (MRSA) isolates collected from 27 inpatients in a hospital were analysed. PFGE and PCR-RFLP were able to discriminate 11 and four types, respectively. PFGE analysis detected cross-infection between four postoperative patients in an intensive-care unit, and in six neonates in intensive care. We conclude that of the three methods tested, PFGE analysis currently allows the most effective discrimination of MRSA strains.

Discriminatory power and usefulness of pulsed-field gel electrophoresis in epidemiological studies of Pseudomonas aeruginosa

We assessed the discriminatory power of pulsed-field gel electrophoresis (PFGE) for the analysis of DNA restriction fragment length polymorphism (RFLP) in Pseudomonas aeruginosa. We determined DraI PFGE-RFLP of DNA of unrelated clinical and environmental strains, and clinical strains isolated from two intensive care units of the Besançon University Hospital. The typeability and reproducibility was 100%. The discriminatory index was 0.998, and the DNA patterns were stable in vitro and in vivo. There was a very low correlation between PFGE-RFLP and traditional typing methods. The typeability, reproducibility, the high discriminatory power and the stability of PFGE-RFLP make this a valuable method to be used in conjunction with serotyping. Further standardization and quantitative interpretation are possible and should lead to this technique becoming a library typing system.

Nasal carriage of Staphylococcus aureus and cross-contamination in a surgical intensive care unit: efficacy of mupirocin ointment

A six month prospective study was carried out in a surgical intensive care unit (SICU) of a university hospital to assess the incidence and routes of exogenous colonization by Staphylococcus aureus. A total of 157 patients were included in the study. One thousand one hundred and eleven specimens (nasal, surgical wound swabs, tracheal secretions obtained on admission and once a week thereafter, and all clinical specimens) were collected over a four month period from patients without nasal decontamination (A). They were compared with 729 specimens collected over a two month period from patients treated with nasal mupirocin ointment (B). All S. aureus strains were typed by restriction fragment length polymorphism (RFLP) pulsed-field gel electrophoresis after SmaI macrorestriction. The nasal colonization rates on admission were 25.5 and 32.7% in groups A and B, respectively. Thirty-one untreated patients (31.3%) and three patients (5.1%) treated with nasal ointment, acquired the nasal S. aureus in the SICU (P = 0.00027). Nasal carriers were more frequently colonized in the bronchopulmonary tract (Bp) and surgical wound (Sw) (62%) than patients who were not nasal carriers (14%) (P < 0.00001). The patterns were identical for nasal, Bp and Sw strains from the same patient. RFLP analysis characterized seven epidemic strains of methicillin-resistant S. aureus (MRSA) which colonized 60% of group A and 9% of group B patients (P < 0.00001). The bronchopulmonary tract infection rate was reduced in group B (P = 0.032). In conclusion, in an SICU, nasal carriage of S. aureus appeared to be the source of endogenous and cross-colonization. The use of nasal mupirocin ointment reduced the incidence of Bp and Sw colonization, as well as the MRSA infection rate.

Molecular epidemiology of Corynebacterium diphtheriae from northwestern Russia and surrounding countries studied by using ribotyping and pulsed-field gel electrophoresis

A selection of 100 Corynebacterium diphtheriae isolates from asymptomatic carriers and clinical cases from five regions in northwestern Russia were examined. Six additional isolates from patients in Finland and Estonia with epidemiological links to Russia were also examined. All isolates were characterized by biotyping, toxigenicity testing, ribotyping, and pulsed-field gel electrophoresis (PFGE). Hybridization of genomic DNA digested with BstEII revealed five ribotype patterns among the biotype gravis isolates (G1 through G5) and two patterns among the biotype mitis isolates (M1 and M2). PFGE using SfiI was not able to distinguish between ribotypes G1, G2, and G4. The predominant ribotype pattern, G1, found in cases of disease in all the areas studied, appears to be disseminating, in view of the isolates received from imported cases in Finland and Estonia. Among the 106 isolates examined, 68 produced pattern G1 and 24 produced pattern M1. Most of the M1 isolates were from the Leningrad Oblast region. Distinct ribotypes such as G2, G3, G4, G5, and M2 could represent endemic disease.

Molecular epidemiology of recent United Kingdom isolates of Neisseria meningitidis serogroup C

The genomes of 34 recent United Kingdom isolates of Neisseria meningitidis serogroup C were examined by restriction enzyme digestion and by conventional and pulsed-field gel electrophoresis (PFGE). Strains were assigned to groups on the basis of the Dice similarity coefficient; strains with values of > 92% were considered to be clonally related. Twelve clones were identified by PFGE. Strains of two clonal groups predominated. Restriction endonuclease analyses using the 'high frequency cleavage' endonuclease Stu I and conventional electrophoresis gave 11 groups; in general it had lower resolving power than PFGE.

Molecular epidemiology for local outbreaks of methicillin resistant Staphylococcus aureus (MRSA). The need for several methods

Subtyping isolates may be useful for epidemiological studies of methicillin-resistant-Staphylococcus aureus (MRSA) outbreaks. Among subtyping methods, DNA-based techniques have been applied very effectively for this purpose. An outbreak of MRSA infections took place in one hospital in Barcelona early during 1991. From the beginning of the outbreak to December 92, 70 MRSA isolates from different patients and sources were collected. All strains were evaluated by restriction endonuclease analysis of plasmid DNA (REAP) and macrorestriction endonuclease analysis of genomic DNA using Sma I and pulsed-field-gel-electrophoresis (PFGE). Plasmid screening and REAP using Hind III demonstrated two plasmid subtypes: subtype A showing a large plasmid, and subtype B showing the same large plasmid plus a smaller one. Subtypes A and B corresponded to the more recent and older isolates, respectively, suggesting the loss of the small plasmid during the epidemic. PFGE using Sma I displayed two closely related profiles (PFGE subtype A and A'; CS = 0.90). These subtypes were different from those subtypes exhibited from 4 methicillin-susceptible-Staphylococcus aureus (MSSA) isolates from the same hospital and from 2 epidemiologically unrelated MRSA isolates. Almost all isolates showing PFGE subtype A preceded those isolates showing PFGE subtype A'. This fact and the similarity between both subtypes suggested minor chromosomal DNA rearrangement during the outbreak from a unique strain. While PFGE using Sma I is a useful tool in evaluation of clonal dissemination, our data suggest epidemic or local outbreaks may need several methods to best delineate the source and spread of MRSA strains. The reproducibility and discriminatory power of REAP makes it a useful adjunct in this context.

Outbreak of methicillin-resistant Staphylococcus aureus in a teaching hospital--epidemiological and microbiological surveillance

An outbreak of methicillin-resistant S. aureus (MRSA) in a large university teaching hospital occurred between December 1991 and May 1992, involving 7 different wards and more than 30 patients. Epidemiological typing was performed to control the epidemic and to identify the MRSA carriers. By a combination of various classical methods (antimicrobial susceptibility, phage typing) and molecular typing procedures (SDS-PAGE of extracellular proteins, plasmid DNA profile, restriction enzyme fragment pattern of chromosomal DNA), three different clones of MRSA could be discriminated. The epidemic clone A was recovered from 30 patients and from 3 staff members. By strict microbiological monitoring together with hygienic measures, the epidemic could be successfully controlled. It is concluded that a combination of phenotypic markers and DNA-based epidemiological markers is extremely useful in the microbiological surveillance of MRSA outbreaks.

Application of genomic DNA subtyping by pulsed field gel electrophoresis and restriction enzyme analysis of plasmid DNA to characterize methicillin-resistant Staphylococcus aureus from two nosocomial outbreaks

Pulsed-field gel electrophoresis (PFGE) and restriction enzyme analysis of plasmid DNA (REAP) were applied to study the epidemiologic relationship among methicillin-resistant Staphylococcus aureus (MRSA) isolates from outbreaks in two hospitals in São Paulo, Brazil: 82 MRSA isolates, 73 from a university hospital and nine from a general adult intensive care unit of a private hospital, were collected from 62 patients: 95% of the MRSAs were also resistant to gentamicin and ciprofloxacin. REAP subtyping of both collections identified six different subtypes: 55 (72.6%) MRSAs from the university hospital and nine isolates from the private hospital shared the same epidemic REAP subtype. Discrimination by restriction of genomic DNA with Sma I followed by PFGE enabled the identification of 14 DNA subtypes. Based on the combined REAP-genomic DNA subtype, the predominant subtype in the university hospital was A/A (44 isolates) whereas the epidemic subtype in the private hospital was A/M (seven isolates). The application of two typing methods showed better discrimination among MRSAs than did either method alone.

Concordant clonal delineation of methicillin-resistant Staphylococcus aureus by macrorestriction analysis and polymerase chain reaction genome fingerprinting

Pulsed-field gel electrophoresis of DNA macrorestriction fragments (macrorestriction analysis) allows epidemiologic typing and delineation of genetic relatedness of methicillin-resistant Staphylococcus aureus (MRSA) by indexing variations in the global chromosome architecture. Polymerase chain reaction (PCR)-mediated genome fingerprinting can also discriminate MRSA strains by detecting locally variable DNA motifs. To assess the correlation between these methods, 48 epidemic MRSA strains collected from 20 hospitals over a 10-year period were tested in a blind comparison by (i) macrorestriction analysis with SstII or SmaI endonuclease and (ii) PCR fingerprinting with four primer sets aimed at the mecA gene, enterobacterial repetitive intergenic consensus sequences, and arbitrary sequences. Isolates were discriminated into 22 macrorestriction patterns and 15 PCR fingerprints. MRSA strains belonging to 12 distinct clones by macrorestriction analysis showed 11 distinct PCR genotypes distinguished by multiple band differences. In contrast, 34 of 37 MRSA strains found to be clonally related by macrorestriction analysis clustered in two highly related PCR genotypes that differed by a single DNA fragment (P < 0.0001). These data demonstrate concordant clonal delineation of epidemic MRSA by macrorestriction analysis and PCR fingerprinting and thereby indicate that the rapid PCR assay may be an efficient epidemiologic typing system.

Pulsed-field gel electrophoresis for differentiation of hospital isolates of Klebsiella pneumoniae

Restriction enzyme analysis by pulsed-field gel electrophoresis (PFGE) was developed for differentiation of hospital isolates of Klebsiella pneumoniae. Restriction patterns generated by SpeI digestion of genomic DNAs of 36 isolates from patients in two major teaching hospitals established 34 PFGE types. All strains were typable by this technique and the SpeI restriction patterns were reproducible, stable and easy to interpret. As PFGE profiles generated were heterogenous, the incidence of cross-infection appeared to be low in each of the hospitals. The higher discriminatory power of PFGE when compared to conventional restriction endonuclease analysis (REA) suggests that this technique will be very useful for epidemiological investigations of nosocomial K. pneumoniae outbreaks.

Strains of methicillin-resistant Staphylococcus aureus isolated in Australian hospitals from 1986 to 1990. Australian Group for Antimicrobial Resistance

Major teaching hospitals in each state of Australia participated in five annual surveys (1986 to 1990) of clinically significant isolates of Staphylococcus aureus. All isolates of methicillin-resistant S. aureus (MRSA) were phage typed with the Basic International Set and an Australian experimental set of typing phages. One or two predominant strains were isolated in individual states during each of the survey periods. Less than 3% (33 of 1243) of MRSA isolates were not typable and more than 86% (1070 of 1243) belonged to strains that were isolated on at least five occasions during a single survey period. Strains of phage types 83A/85/95/90/88@47T/90A/87M/13M and 85/90/88@47T/90A/87A were the most prevalent, but each was identified in only four of the five surveys. Isolates of phage type (83A/85/95) weak/88@87M persisted throughout the survey period.

Exogenous or endogenous reservoirs of nosocomial Pseudomonas aeruginosa and Staphylococcus aureus infections in a surgical intensive care unit

OBJECTIVE

A 4 month prospective study was performed to assess the incidence and routes of endogenous or exogenous colonization and nosocomial infection caused by Staphylococcus aureus and Pseudomonas aeruginosa in surgical critically ill patients.

DESIGN

A total of 4634 specimens were obtained. Patient's nasal, scalp, and rectal swabs as well as tracheal secretion (TS) were cultured every second day beginning on the day of admission. Nasal swabs and hand cultures of the personnel as well as cultures from gowns were also taken. All isolates of S. aureus were phage typed and 116 of these isolates were also plasmid typed. P. aeruginosa isolates were sero- and pyocin typed. Resistance patterns were determined in all isolates.

SETTING

The study was carried out in the surgical intensive care unit (SICU) of an teaching hospital.

PATIENTS

During the study period each patient (a total of 153 patients) admitted to the SICU entered the study.

RESULTS

P. aeruginosa and S. aureus colonisation rate on admission were 5% and 36.5% respectively. Only 10 patients (6.5%) were colonized with P. aeruginosa during hospitalization, and only 7 patients (4.5%) acquired S. aureus in the surgical intensive care unit (SICU). The most common primary colonisation site of P. aeruginosa was the rectum, whereas S. aureus was predominantly found in nasal cultures. Horizontal transmission of S. aureus occurred in only 2 patients.

CONCLUSION

The study suggests that colonisation with P. aeruginosa and S. aureus occurs from endogenous rather than from exogenous sources and that the endogenous acquisition of both bacteria play a more important role in development of nosocomial infections than the exogenous route of transmission.

Evidence for a clonal origin of methicillin resistance in Staphylococcus aureus

Soon after methicillin was introduced into clinical practice in the early 1960s, resistant strains of Staphylococcus aureus (MRSA) appeared, bearing a newly acquired resistance gene, mecA, that encodes a penicillin binding protein, PBP2a. MRSA have spread throughout the world, and an investigation of the clonality of 472 isolates by DNA hybridization was performed. All 472 isolates could be divided into six temporally ordered mecA hybridization patterns, and three of these were subdivided by the chromomosomal transposon Tn554. Each Tn554 pattern occurred in association with one and only one mecA pattern, suggesting that mecA divergence preceded the acquisition of Tn554 in all cases and therefore that mecA may have been acquired just once by S. aureus.