Typing of methicillin-resistant Staphylococcus aureus in a university hospital setting by using novel software for spa repeat determination and database management

The spa gene of Staphylococcus aureus encodes protein A and is used for typing of methicillin-resistant Staphylococcus aureus (MRSA). We used sequence typing of the spa gene repeat region to study the epidemiology of MRSA at a German university hospital. One hundred seven and 84 strains were studied during two periods of 10 and 4 months, respectively. Repeats and spa types were determined by Ridom StaphType, a novel software tool allowing rapid repeat determination, data management and retrieval, and Internet-based assignment of new spa types following automatic quality control of DNA sequence chromatograms. Isolates representative of the most abundant spa types were subjected to multilocus sequence typing and pulsed-field gel electrophoresis. One of two predominant spa types was replaced by a clonally related variant in the second study period. Ten unique spa types, which were equally distributed in both study periods, were recovered. The data show a rapid dynamics of clone circulation in a university hospital setting. spa typing was valuable for tracking of epidemic isolates. The data show that disproval of epidemiologically suggested transmissions of MRSA is one of the main objectives of spa typing in departments with a high incidence of MRSA.

Evolution of Staphylococcus aureus by large chromosomal replacements

Conjugative transfer and replacement of hundreds of kilobases of a bacterial chromosome can occur in vitro, but replacements in nature are either an order of magnitude smaller or involve the movement of mobile genetic elements. We discovered that two lineages of Staphylococcus aureus, including a pandemic methicillin-resistant lineage, were founded by single chromosomal replacements of at least approximately 244 and approximately 557 kb representing approximately 10 and approximately 20% of the chromosome, respectively, without the obvious involvement of mobile genetic elements. The replacements are unprecedented in natural populations of bacteria because of their large size and unique structure and may have a dramatic impact on bacterial evolution.

Methicillin (Oxacillin)-resistant Staphylococcus aureus strains isolated from major food animals and their potential transmission to humans

From May 2001 to April 2003, various types of specimens from cattle, pigs, and chickens were collected and examined for the presence of methicillin (oxacillin)-resistant Staphylococcus aureus (MRSA). S. aureus was isolated and positively identified by using Gram staining, colony morphology, tests for coagulase and urease activities, and an API Staph Ident system. Among 1,913 specimens collected from the animals, 421 contained S. aureus; of these, 28 contained S. aureus resistant to concentrations of oxacillin higher than 2 micro g/ml. Isolates from 15 of the 28 specimens were positive by PCR for the mecA gene. Of the 15 mecA-positive MRSA isolates, 12 were from dairy cows and 3 were from chickens. Antimicrobial susceptibility tests of mecA-positive MRSA strains were performed by the disk diffusion method. All isolates were resistant to members of the penicillin family, such as ampicillin, oxacillin, and penicillin. All isolates were also susceptible to amikacin, vancomycin, and trimethoprim-sulfamethoxazole. To determine molecular epidemiological relatedness of these 15 animal MRSA isolates to isolates from humans, random amplified polymorphic DNA (RAPD) patterns were generated by arbitrarily primed PCR. The RAPD patterns of six of the isolates from animals were identical to the patterns of certain isolates from humans. The antibiotypes of the six animal isolates revealed types similar to those of the human isolates. These data suggested that the genomes of the six animal MRSA isolates were very closely related to those of some human MRSA isolates and were a possible source of human infections caused by consuming contaminated food products made from these animals.

Multilocus sequence typing and the evolution of methicillin-resistant Staphylococcus aureus

The prevalence of methicillin-resistant Staphylococcus aureus (MRSA) in many countries is increasing and, in hospitals in some areas, more than half of all S. aureus disease isolates are MRSA. MRSA strains are becoming increasingly multiresistant, and have recently developed resistance to vancomycin, used successfully to treat MRSA for more than 30 years. This review summarises recent studies that have elucidated the evolutionary history of MRSA. The first MRSA isolate evolved from a sensitive, epidemic strain prevalent in Europe, and its progeny-the first MRSA clone-quickly spread to other continents. Analyses of epidemic MRSA isolates from hospitals in different countries by molecular methods, including multilocus sequence typing (MLST) and DNA microarray analysis, reveal that MRSA strains have evolved separately within five distinct epidemic, sensitive lineages. However, resistance has been transferred to S. aureus on many more than five occasions, as some lineages have acquired different structural types of the element carrying the methicillin resistance gene. The emergence of MRSA as a community pathogen has been noted in several countries, and MLST and SCCmec typing have been used to demonstrate that community-acquired MRSA strains are typically related only distantly to hospital MRSA strains, and thus represent novel acquisitions of SCCmec.

Analysis on distribution and genomic diversity of high-level antiseptic resistance genes qacA and qacB in human clinical isolates of Staphylococcus aureus

High-level antiseptic resistance of Staphylococcus aureus is mediated by multidrug efflux pumps encoded by qacA and qacB genes. We investigated distribution and genomic diversity of these antiseptic resistance genes in a total of 522 clinical strains of S. aureus isolated recently in a Japanese hospital. The qacA/B gene was detected in 32.6% of methicillin-resistant S. aureus (MRSA) and 7.5% of methicillin-susceptible S. aureus (MSSA), whereas the low-level resistance gene smr, which was examined simultaneously, was detected at lower frequencies in both MRSA (3.3%) and MSSA (5.9%). Epidemiologic typing of S. aureus isolates suggested that higher prevalence of qacA/B in MRSA may be due to spread of a single predominant MRSA strain carrying qacA/B in the hospital. Restriction fragment length polymorphism (RFLP) analysis indicated higher prevalence of the qacB-type gene (59.3%) than the qacA-type gene (40.7%) among the qacA/B genes detected. Nucleotide sequencing analysis revealed the presence of two genetic variants in qacA (V1 and V2) and four variants in qacB (V1-V4) that differ from the qacA prototype in pSK1 by 1-5 nucleotides and 7-9 nucleotides, respectively. Although most strains with qacA-V1, qacA-V2, qacB-V3, and qacB-V4 showed high-level resistance to ethidium bromide (EB)(MIC > 100 microg/ml), all of the S. aureus isolates carrying qacB-V1 and qacB-V2 showed lower MICs of EB and some monovalent cationic antiseptic substances. By analysis of the genomic organization of the qacA/B downstream region, divergent forms of this region rearranged with an insertion of IS256 or IS257 were found primarily for qacB. The downstream region of qacA-V1 was suggested to be an evolutionary origin for other divergent forms. These findings indicated that both qacA and qacB are prevalent in recent clinical isolates, especially in MRSA, and these genes consist of variable genetic variants that may be responsible for different resistance levels against antiseptic substances.

Update on the major clonal types of methicillin-resistant Staphylococcus aureus in the Czech Republic

The purpose of our study was the molecular characterization of methicillin-resistant Staphylococcus aureus (MRSA) strains isolated in 21 hospitals in the Czech Republic in the period 2000-2002 and comparison with previous results from 1996-1997. Strains were analyzed by pulsed-field gel electrophoresis (PFGE) of SmaI digests and ribotyping of HindIII digests hybridized with a 16S-23S DNA probe. The prevalence of the most clinically important macrolide (ermA, ermB, ermC, and msrA) and aminoglycoside (aph3', ant4', and aac6'-aph2") resistance genes was evaluated as well. Selected isolates representative of each clonal type were analyzed by multilocus sequence typing and by a multiplex PCR method capable of identifying the structural type of the staphylococcal cassette chromosome mec (SCCmec) carried by the bacteria. Our results document the displacement of the Brazilian clone (ST239, SCCmec type IIIA, PFGE type B, ribotype H1) by a new clone that we named "Czech clone" (ST239, SCCmec type IIIA, PFGE type F, ribotype H6) and the maintenance of the Iberian clone (ST247, SCCmec type IA, PFGE type A, ribotype H2) exclusively in one hospital in the Czech Republic. In addition, we found a correlation between the distribution of aminoglycoside resistance genes and MRSA clonal types.

Evolutionary models of the emergence of methicillin-resistant Staphylococcus aureus

Five major lineages of methicillin-resistant Staphylococcus aureus (MRSA) have evolved since the introduction of methicillin for the treatment of infections caused by penicillin-resistant S. aureus in 1959. The clones of these lineages are responsible for the vast majority of hospital-acquired MRSA disease globally. We have constructed high-resolution evolutionary models for each lineage using a parsimony approach with 15 partial gene sequences from 147 geographically diverse isolates. On the basis of these models, we infer that MRSA has emerged at least 20 times upon acquisition of the methicillin resistance determinant, which is carried on a mobile genetic element called the staphylococcal cassette chromosome mec (SCCmec). The acquisition of SCCmec by sensitive clones was four times more common than the replacement of one SCCmec with another. Notably, SCCmec type IV was found in twice as many clones as any other SCCmec type, and it is this SCCmec type which is commonly found in clones from patients with community-acquired MRSA disease. Our findings suggest that most clones of MRSA arise by the acquisition of SCCmec type IV by methicillin-sensitive isolates.

Detection of Staphylococcus aureus enterotoxins A to D by real-time fluorescence PCR assay

Staphylococcus aureus is one of the most significant pathogens causing nosocomial and community-acquired infections. Among the secreted staphylococcal virulence factors, there is a growing list of enterotoxins which can induce gastroenteric syndrome and toxic shock syndrome. Here, we developed a real-time fluorescence PCR assay (TaqMan PCR) for the detection of genes encoding staphylococcal enterotoxins A, B, C1, and D (SEA, SEB, SEC1, and SED) of S. aureus as well as the mecA gene encoding methicillin resistance and the femB gene as a specific genomic marker for S. aureus. SEA to SED were selected because they are the four classically described enterotoxins of S. aureus and because they were detected by latex agglutination. In order to evaluate the reliability of TaqMan PCR, we investigated 93 isolates of S. aureus derived from patients at our hospital over 5 months and compared the results with data obtained by a commercially available reversed passive latex agglutination assay (SET-RPLA) for these isolates. Thirteen enterotoxin genes were detected by TaqMan PCR; however, no proteins expressed by these genes were detected by SET-RPLA. As a result, more isolates of S. aureus (n = 44) were found positive by TaqMan PCR for one or more enterotoxin genes than by SET-RPLA for the respective proteins expressed by these genes (n = 40). We conclude that TaqMan PCR is more sensitive because it offers the possibility for determining enterotoxins on a genotypic basis. Additionally, the assay allows the parallel detection of genes for SEA to SED and methicillin resistance in S. aureus. Furthermore, real-time PCR is well suited for screening large numbers of samples at the same time, allowing rapid, reliable, efficient, and cost-saving routine laboratory diagnosis.

Changes in drug susceptibility and toxin genes in Staphylococcus aureus isolated from blood cultures at a university hospital

We studied changes in toxin-producing genes and drug susceptibility in Staphylococcus aureus isolated from blood cultures at the University of Tokyo Hospital between 1980-1984 (six mecA gene-positive methicillin resistant S. aureus [MRSA] strains and 20 mecA gene-negative methicillin-susceptible S. aureus [MSSA] strains) and 1999 (11 MRSA and 20 MSSA strains). The prevalence of strains with toxin-producing genes increased from 66.7% to 90.9% in MRSA, and from 30.0% to 55.0% in MSSA during the interval. Among toxin-producing gene-positive S. aureus, the dominant strains shifted from those with the enterotoxin (ET) - A gene in 1980-1984 to those with both the toxic shock syndrome toxin-1 and the ET-C genes in 1999. All strains were susceptible to vancomycin and teicoplanin. Mupirocin and arbekacin inhibited all strains at concentrations of less than or equal to 0.5 micro g/ml and 4 micro g/ml, respectively. More than half of the MRSA strains in 1999 were considered to be nonsusceptible to flomoxef. Because almost all MRSA and more than half of MSSA among recently isolated strains possessed the toxin-producing genes, we should pay attention to whether toxin-related diseases caused by MRSA and MSSA are increasing.

Local variants of Staphylococcal cassette chromosome mec in sporadic methicillin-resistant Staphylococcus aureus and methicillin-resistant coagulase-negative Staphylococci: evidence of horizontal gene transfer?

The mecA gene in Staphylococcus aureus is located on the genetic element staphylococcal cassette chromosome (SCC). Different SCCmecs have been classified according to their putative recombinase genes (ccrA and ccrB) and overall genetic composition. Clinical isolates of coagulase-negative staphylococci (CoNS; n = 39) and S. aureus (n = 20) from Norway, India, Italy, Finland, the United States, and the United Kingdom were analyzed by pulsed-field gel electrophoresis, which showed that most isolates were genetically unrelated. Cluster analyses of 16S rRNA gene and pta sequences confirmed the traditional biochemical species identification. The mecI, mecR1, mecA, and ccrAB genes were detected by PCRs, identifying 19 out of 20 S. aureus and 17 out of 39 CoNS isolates as carriers of one of the three published ccrAB pairs. New variants of SCCmec were identified, as well as CoNS isolates containing ccrAB genes without the mec locus. ccrAB and mec PCRs were verified by hybridization. Sequence alignments of ccrAB genes showed a high level of diversity between the ccrAB alleles from different isolates, i.e., 94 to 100% and 95 to 100% homology for ccrAB1 and ccrAB2, respectively. All of the ccrAB3 genes identified were identical. Genetically unique and sporadic methicillin-resistant S. aureus (MRSA) contained local variants of ccrAB gene pairs identical to those found in MR-CoNS but different from those in MRSA from other regions. Allelic variants of ccrAB in isolates from the same geographic region showed sequence conservation independent of species. The species-independent sequence conservation found suggests that there is a closer genetic relationship between ccrAB2 in Norwegian staphylococci than between ccrAB2 sequences in international MRSA and Norwegian MRSA. This might indicate that different staphylococcal species acquire these genes locally by horizontal gene transfer.

[Methicillin resistance detection in Staphylococcus aureus: comparison between conventional methods and MRSA-Screen latex agglutination technique]

Methicillin-resistant Staphylococcus aureus (MRSA) is a significant pathogen that has emerged over the last four decades, causing both nosocomial and community-acquired infections. Rapid and accurate detection of methicillin resistance in S. aureus is important for the use of appropriate antimicrobial therapy and for the control of nosocomial spread of MRSA strains. We evaluated the efficiency of conventional methods for detection of methicillin resistance such as the disk diffusion, agar dilution, oxacillin agar screen test, and the latex agglutination test MRSA-Screen latex, in 100 isolates of S. aureus, 79 mecA positive and 21 mecA negative. The MRSA-Screen latex (Denka Seiken, Niigata, Japón), is a latex agglutination method that detects the presence of PLP-2a, product of mecA gene in S. aureus. The PCR of the mecA gene was used as the "gold standard" for the evaluation of the different methods tested. The percentages of sensitivity and specificity were as follows: disk difusión 97 and 100%, agar dilution 97 and 95%, oxacillin agar screen test 100 and 100%, and MRSA-Screen latex, 100 and 100 %. All methods presented high sensitivity and specificity, but MRSA-Screen latex had the advantage of giving a reliable result, equivalent to PCR, in only 15 minutes.

[Donor activity of methicillin resistant staphylococci in transduction experiments]

Antibiotic resistance gene transmission from methicillin resistant strains of Staphylococcus aureus (MRSA), isolated in a burn care unit, was studied in transduction experiments with type phages 29, 52, 52A and in experiments with prophage induction. The results of the experiments demonstrated high donor activity of MRSA. Recombinants with different antibiotic resistance phenotypes were revealed, but there were no methicillin resistant staphylococci among them. Stability of cloramphenicol resistance gene transmission in the experiments on specific transduction with the prophage induction could be indicative of the prophage localization near the chloramphenicol resistance genes. Variety of the antibiotic resistance combinations in the transductants from the clinical strains of MRSA could prove heterogeneity of the strains even under conditions of one hospital.

[Molecular analysis of methicillin-resistant staphylococci isolated from hospital patients and in the out-patient clinic]

In this study, a molecular analysis of the methicillin-resistant coagulase-negative staphylococci strains was performed. The obtained results of the biochemical and drug resistant pattern investigations were insufficient to assess the relationship between the strains. Therefore genotyping by the restriction fragment length polymorphism analysis (PCR-RFLP) method was performed. Analyzed strains characterized presence of the mecA gene-PCR products. The PCR products were digested with DraI and TasI, and the fragments separated by agarose gel electrophoresis. Typing of the methicillin-resistant gene using PCR-RFLP showed that all MRCNS strains possess an identical restriction pattern of the mecA gene. This identical restriction pattern of the mecA gene in investigated strains may suggest an easy transfer of this gene between different staphylococci species and lead to the spreading of methicillin-resistant among hospital strains. Furthermore performing the comparison of different phenotype and genotype methods has shown that the PCR-RFLP method is quick and reliable, enabling the detection and estimation of the relationship between MRCNS strains.

Ten years phage-typing of Belgian clinical methicillin-resistant Staphylococcus aureus isolates (1992–2001)

Gradual changes have been observed in the phage-types of methicillin-resistant Staphylococcus aureus (MRSA) isolates from Belgian hospitals. A total of 6551 isolates, collected in 93 Belgian hospitals over 10 years (1992-2001), was examined. A decreasing incidence of the main early Belgian epidemic phage-types ([A], [B], [H]*, Jo*) was observed. Since 1997 and 2000, a new series of phage-types ([Hv]*, [J]*, [O]*), which were likely related to the previous group [H]*, have been noted. The general trends were confirmed in two particular hospitals. Local epidemic and/or endemic phage-types were also encountered.

Assessment of three rapid methods for the detection of methicillin-resistant Staphylococcus aureus

We evaluated three rapid methods to detect methicillin-resistant Staphylococcus aureus (MRSA) and compared them with PCR amplification of mecA. A total of 103 S. aureus strains were studied by MRSA-Screen, BBL Crystal, Velogene Genomic and mecA PCR. All the methods detected the 61 MRSA strains having the mecA gene, showing 100% sensitivity and specificity. Despite the correlation between all the rapid methods and PCR, the ease of use and shorter turnaround time of MRSA-Screen were important factors leading to the selection of this method as the routine screening technique for MRSA.

Nucleotide Substitutions in Staphylococcus aureus Strains, Mu50, Mu3, and N315

A specific phenotype of Staphylococcus aureus strains Mu50 and Mu3 is characterized by thickened cell wall and moderate resistance to vancomycin. The N315 strain is a prototype of methicillin-resistant S. aureus (MRSA), but it is methicillin susceptible, despite carrying the mecA resistance gene. Here, we revised differences in the sequences of Mu50 and N315, referencing that of Mu3 which were assumed to be of one lineage. The 362 ORFs diverse between Mu50 and N315 were picked up, and the corresponding ones in three strains were re-sequenced. This defined 213 ORFs diverse between Mu50 and N315, and 9 between Mu50 and Mu3. The fixed diversities of 174 ORFs (except for 39 silent ORFs from 213), including nucleotide substitution (NSs), frame shift, and truncation were grouped into three major functional categories, which were transport (14.9% in the 174 diverse ORFs), metabolism of carbohydrates (5.7%), and RNA synthesis (9.6%). The other gene categories had small diversities. These gene categories seemed to be functionally decisive for the Mu50-specific characters, the thickened cell wall and moderate vancomycin resistance. All of the diverse genes and the high quality sequence of Mu50 can be viewed at the web site (http://133.5.48.239/VRSA/).

Obvious lack of association between dynamics of epidemic methicillin-resistant Staphylococcus aureus in central Europe and agr specificity groups

During the past 8 years, changes in the prevalence and spread of different epidemic methicillin-resistant Staphylococcus aureus (MRSA) have been observed in central Europe, with the emergence of new strains possessing fewer resistance characters. This has also been demonstrated at the level of particular hospitals. Since variation in agr specificity type has been proposed as a possible reason for population dynamics in Staphylococcus aureus, the agr specificity groups of different epidemic MRSA strains were investigated by PCR using agr group-specific primers. Four of the "old" as well as two "new" epidemic strains exhibited agr specificity group I. One group of epidemic MRSA strains, which has been observed since the beginning of the 1990s, exhibited the agr specificity group II. Sequencing the variable part ( agrB-D-C) of the agr locus revealed only six relevant nucleotide changes within this region, with three of them modifying the Shine-Dalgarno sequence region of agrC. On the basis of the results obtained, it is proposed that the dynamics observed in the population of MRSA in Germany is not due to different agr group specificities in "old" and "new" epidemic clones.

Emergence of community-acquired methicillin-resistant Staphylococcus aureus (MRSA) infection in Queensland, Australia

OBJECTIVES

To investigate the incidence and epidemiology of non-multiresistant methicillin-resistant Staphylococcus aureus (nmMRSA) infection in south-east Queensland, Australia.

STUDY DESIGN

A retrospective survey was done of hospital records of all patients who had non-multiresistant MRSA isolated at Ipswich Hospital (a 250-bed general hospital, 40 km south-west of Brisbane, Queensland, Australia) between March 2000 and June 2001. Laboratory typing of these isolates was done with antibiogram, pulsed-field gel electrophoresis, bacteriophage typing and coagulase gene typing.

RESULTS

There were 44 infections caused by nmMRSA. Seventeen infections (39%) occurred in patients from the south-west Pacific Islands (predominantly Samoa, Tonga and New Zealand). Laboratory typing showed that the isolates in Pacific Islanders were Pacific Island strains, and 16/17 of these infections were community acquired. Twenty-three infections (52%) occurred in Caucasians. Eleven of the isolates from Caucasians (48%) were a new predominantly community-acquired strain that we have termed the 'R' pulsotype, nine (39%) were Pacific Island strains, and three (13%) were health care institution-associated strains. Four infections occurred in patients who were not Caucasians or Pacific Islanders. Overall, 34 of all 44 infections (77%) were community acquired.

CONCLUSIONS

Non-multiresistant MRSA infection, relatively frequently observed in Pacific Islanders in south-east Queensland, is now a risk for Caucasians as well, and is usually community acquired. Clinicians should consider taking microbiological specimens for culture and antimicrobial susceptibility testing in patients with suspected staphylococcal infections who are not responding to empirical therapy with beta-lactam antibiotics.

Plasmid content of a vancomycin-resistant Enterococcus faecalis isolate from a patient also colonized by Staphylococcus aureus with a VanA phenotype

Vancomycin-resistant Enterococcus faecalis coisolated with vancomycin-resistant (VanA) Staphylococcus aureus was found to contain two plasmids, designated pAM830 (45 kb) and pAM831 (95 kb). pAM830, found to be conjugative and closely related to the Inc18 family of broad-host-range conjugative plasmids, encodes resistances to vancomycin (via a Tn1546-like element) and erythromycin; pAM831 encodes resistances to gentamicin, streptomycin, and erythromycin.

Genetic Analysis of a High-Level Vancomycin-Resistant Isolate of Staphylococcus aureus

Vancomycin is usually reserved for treatment of serious infections, including those caused by multidrug-resistant Staphylococcus aureus. A clinical isolate of S. aureus with high-level resistance to vancomycin (minimal inhibitory concentration = 1024 microg/ml) was isolated in June 2002. This isolate harbored a 57.9-kilobase multiresistance conjugative plasmid within which Tn1546 (vanA) was integrated. Additional elements on the plasmid encoded resistance to trimethoprim (dfrA), beta-lactams (blaZ), aminoglycosides (aacA-aphD), and disinfectants (qacC). Genetic analyses suggest that the long-anticipated transfer of vancomycin resistance to a methicillin-resistant S. aureus occurred in vivo by interspecies transfer of Tn1546 from a co-isolate of Enterococcus faecalis.

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.

[MRSA/MRSE-VISA/GISA/VRSA-PRP-VRE: current gram positive problem bacteria and mechanism of resistance, prevalence and clinical consequences]

The raising frequency of occurrence of multi-resistant Gram-positive pathogens has led to difficult to treat infections not only in hospitals but also in outpatient settings. Methicillin-resistant Staphylococcus aureus, the glycopeptide-susceptibility of which one cannot be sure of any more, vancomycin-resistant enterococci and penicillin-resistant pneumococci are on the top of the list of Gram-positive problem organisms nowadays. Clinical microbiology laboratories are faced with the challenge of accurately detecting emerging antibiotic resistance, which might be difficult and purpose of reporting data should be twofold: First to report adequately identified, relevant organisms as well as their susceptibility profiles to clinicians in context with adequate patient management and secondly to report continuously epidemiological data in form of statistics to the community in general and within a given setting as could be a specified hospital in particular. For reliable detection, laboratories may need to employ special screening- and susceptibility testing methods. Certain of these tests are highly specific, while others may require additional confirmatory testing for definite results. This article reports on current resistance mechanisms of gram-positive pathogens and subsequently resulting consequences for clinicians. Moreover, the frequency of occurrence in Austria in general as well as at the Vienna General Hospital in particular will be referred to.

Increasing prevalence of methicillin-resistant Staphylococcus aureus infection in California jails

Staphylococcus aureus clinical isolates obtained from patients who were inmates of the San Francisco County jail system showed an increase in the prevalence of methicillin-resistant Staphylococcus aureus (MRSA) from 29%, in 1997, to 74%, in 2002; 91% of the MRSA isolates carried staphylococcal chromosomal cassette mec (SCCmec) type IV. Pulsed field gel electrophoresis and multilocus sequence typing demonstrated 2 major clonal groups. One of these clonal groups is genetically indistinguishable from the strain responsible for an outbreak of MRSA in the Los Angeles County jail system in 2002.

Normally functioning murF is essential for the optimal expression of methicillin resistance in Staphylococcus aureus

A carboxy-terminal fragment of murF was used to construct and insert a suicide plasmid into the chromosomal copy of the gene in the highly and homogeneously methicillin-resistant Staphylococcus aureus (MRSA) strain COL by Campbell type integration. The plasmid insertion generated a mutant in which the MIC value for oxacillin was reduced from 400 microg/ml of the parental strain to 0.75 microg/ml in 90% of the cells of the mutant cultures that were heterogeneous: they contained subpopulations of bacteria with a frequency of 10(-3) that were capable of expressing resistance at nearly the parental level. The impact of the murF mutation on antibiotic resistance was selective for beta-lactam antibiotics: there was no change in the susceptibility of the mutant to D-cycloserine, fosfomycin, beta-D-chloro-alanine, moenomycin, bacitracin, or vancomycin. Analysis of the mutant peptidoglycan showed decrease in the percentage of oligomeric components in rough proportion to the accumulation of several abnormal muropeptide components, which were identified as structural variants of the disaccharide tripeptide monomer. An abnormal cell wall precursor identified as UDP MurNac tripeptide was also detected in the cytoplasmic pool of the mutant strain. A normal proportion of oligomers and a greatly reduced representation of the disaccharide tripeptide were demonstrated in the cell wall of the murF mutant's subpopulation that has retained the parental level of resistance. Northern analysis demonstrated a drastic reduction in the transcription rate of mecA in mutant F9 whereas mecA transcription increased in the subpopulation of bacteria that retained high-level resistance.

Three-dimensional structure of MecI. Molecular basis for transcriptional regulation of staphylococcal methicillin resistance

Methicillin-resistant Staphylococcus aureus is the main cause of nosocomial and community-onset infections that affect millions of people worldwide. Some methicillin-resistant Staphylococcus aureus infections have become essentially untreatable by beta-lactams because of acquired molecular machineries enabling antibiotic resistance. Evasion from methicillin challenge is mainly achieved by the synthesis of a penicillin-binding protein of low affinity for antibiotics, MecA, that replaces regular penicillin-binding proteins in cell wall turnover when these have been inactivated by antibiotics. MecA synthesis is regulated by a signal transduction system consisting of the sensor/transducer MecR1 and the 14-kDa transcriptional repressor MecI (also known as methicillin repressor) that constitutively blocks mecA transcription. The three-dimensional structure of MecI reveals a dimer of two independent winged helix domains, each of which binds a palindromic DNA-operator half site, and two intimately intertwining dimerization domains of novel spiral staircase architecture, held together by a hydrophobic core. Limited proteolytic cleavage by cognate MecR1 within the dimerization domains results in loss of dimer interaction surface, dissociation, and repressor release, which triggers MecA synthesis. Structural information on components of the MecA regulatory pathway, in particular on methicillin repressor, the ultimate transcriptional trigger of mecA-encoded methicillin resistance, is expected to lead to the development of new antimicrobial drugs.