Methicillin-resistant Staphylococcus aureus (MRSA) detected at four U.S. wastewater treatment plants

BACKGROUND

The incidence of community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) infections is increasing in the United States, and it is possible that municipal wastewater could be a reservoir of this microorganism. To date, no U.S. studies have evaluated the occurrence of MRSA in wastewater.

OBJECTIVE

We examined the occurrence of MRSA and methicillin-susceptible S. aureus (MSSA) at U.S. wastewater treatment plants.

METHODS

We collected wastewater samples from two Mid-Atlantic and two Midwest wastewater treatment plants between October 2009 and October 2010. Samples were analyzed for MRSA and MSSA using membrane filtration. Isolates were confirmed using biochemical tests and PCR (polymerase chain reaction). Antimicrobial susceptibility testing was performed by Sensititre® microbroth dilution. Staphylococcal cassette chromosome mec (SCCmec) typing, Panton-Valentine leucocidin (PVL) screening, and pulsed field gel electrophoresis (PFGE) were performed to further characterize the strains. Data were analyzed by two-sample proportion tests and analysis of variance.

RESULTS

We detected MRSA (n = 240) and MSSA (n = 119) in 22 of 44 (50%) and 24 of 44 (55%) wastewater samples, respectively. The odds of samples being MRSA-positive decreased as treatment progressed: 10 of 12 (83%) influent samples were MRSA-positive, while only one of 12 (8%) effluent samples was MRSA-positive. Ninety-three percent and 29% of unique MRSA and MSSA isolates, respectively, were multidrug resistant. SCCmec types II and IV, the pvl gene, and USA types 100, 300, and 700 (PFGE strain types commonly found in the United States) were identified among the MRSA isolates.

CONCLUSIONS

Our findings raise potential public health concerns for wastewater treatment plant workers and individuals exposed to reclaimed wastewater. Because of increasing use of reclaimed wastewater, further study is needed to evaluate the risk of exposure to antibiotic-resistant bacteria in treated wastewater.

Cost analysis of a hospital-wide selective screening programme for methicillin-resistant Staphylococcus aureus (MRSA) carriers in the context of diagnosis related groups (DRG) payment

The costs of a hospital-wide selective screening programme were analysed for a period of 19 months. During this time, 539 inpatients were screened, of whom 111 were MRSA-positive. Based on microbiological costs (staff and materials) and the costs of preventive contact isolation for 2 days until microbiological results were available (including material costs for medical consumable goods and the costs of additional nursing time), a total of 26,241.51 Euro was spent for the 539 patients screened. Based on cost units, the costs were 39.96 Euro for a patient found to be MRSA-negative and 82.33 Euro for a patient found to be MRSA-positive. Under the prospective diagnosis related groups (DRG) payment system in Germany, the costs of a prolonged hospital stay resulting from a hospital-acquired MRSA infection (HA-MRSA-I) are not reimbursed adequately by revenues, with a calculated average cost-revenue loss/patient with HA-MRSA-I of 5705.75 Euro. The screening programme was able to prevent 48% of predicted HA-MRSA-Is (35.2 patients with infection), thereby saving a predicted 200,782.73 Euro. After subtracting the screening costs, there was a net saving of 110,236.56 Euro annually. A sensitivity analysis of the break-even points for different screening frequencies and different MRSA incidence rates indicated that the screening programme became cost-effective at a low MRSA incidence rate, meaning that it can be recommended for most hospitals with an MRSA problem.

Detection of gyrA gene mutations associated with ciprofloxacin resistance in methicillin-resistant Staphylococcus aureus: analysis by polymerase chain reaction and automated direct DNA sequencing

A portion of the gyrA gene from amino acid codons 67 to 129 was sequenced in 34 methicillin-resistant Staphylococcus aureus strains (14 isolated in Minnesota, 10 isolated in Indiana, and 10 isolated in Tennessee). Twenty-eight of these strains were ciprofloxacin resistant. Sixteen of the strains contained a Ser----Leu mutation at codon 84; 3 contained strains a Ser----Ala mutation at codon 84; 3 strains contained two mutations, Ser----Leu at codon 84 and Ser----Pro at codon 85; and 6 strains contained a Glu----Lys mutation at codon 88. Six strains were wild type and ciprofloxacin susceptible. Several mutations from amino acid codons 84 through 88 can be associated with high-level quinolone resistance.

Epidemiologic distribution of the arginine catabolic mobile element among selected methicillin-resistant and methicillin-susceptible Staphylococcus aureus isolates

We tested 214 Staphylococcus aureus isolates for the arcA locus of the arginine catabolic mobile element (ACME). All USA300 SCCmec IVa isolates, but no isolates containing other SCCmec subtypes, were arcA positive. arcA was also detected in selected methicillin-susceptible USA300 and methicillin-resistant USA100 isolates. DNA sequence analysis confirmed the integration of ACME in orfX.

Novel SCCmec type XIII (9A) identified in an ST152 methicillin-resistant Staphylococcus aureus

We identified a novel staphylococcal cassette chromosome mec (SCCmec) element in an ST152 methicillin-resistant Staphylococcus aureus (MRSA) isolate by combining Illumina and MinION sequencing. The element contains a new ccrC allotype designated ccrC2. The mec complex resembles mec class A, but with an altered organization of the genes. The element was acknowledged as novel by the International Working Group on the Classification of Staphylococcal Cassette Chromosome Elements (IWG-SCC) and designated SCCmec type XIII (9A).

Effectiveness of a hospital-wide selective screening programme for methicillin-resistant Staphylococcus aureus (MRSA) carriers at hospital admission to prevent hospital-acquired MRSA infections

Screening of potential MRSA-positive patients at hospital admission is recommended in German and international guidelines. This policy has been shown to be effective in reducing the frequency of nosocomial MRSA transmissions in the event of an outbreak, but the influence of screening on reducing hospital-acquired MRSA infections in a hospital setting where MRSA is endemic is not yet well-documented. This study describes the effect of hospital-wide screening of defined risk groups in a 700-bed acute care hospital during a period of 19 months. In a cohort study with a 19-month control period, the frequencies of hospital-acquired MRSA infections were compared with and without screening. In the control period, there were 119 MRSA-positive patients, of whom 48 had a hospital-acquired MRSA infection. On the basis of this frequency, a predicted total of 73.2 hospital-acquired MRSA infections was calculated for the screening period, but only 52% of the expected number (38 hospital-acquired MRSA infections) were observed, i.e., 48% of the predicted number of hospital-acquired MRSA infections were prevented by the screening programme. The screening programme was performed with minimal effort and can therefore be recommended as an effective measure to help prevent hospital-acquired MRSA infections.

Community- and health care-associated methicillin-resistant Staphylococcus aureus: a comparison of molecular epidemiology and antimicrobial activities of various agents

The epidemiology of methicillin-resistant Staphylococcus aureus (MRSA) is changing. We determined the inhibitory and bactericidal activity of select antimicrobial agents utilizing a well-characterized group of 200 staphylococcal cassette chromosome mec (SCCmec) type IV community-associated MRSA (CAMRSA) and 50 SCCmec type II health care-associated MRSA (HAMRSA). Differences in carriage of the Panton-Valentine leukocidin (PVL) genes, agr group, and agr function in CAMRSA and HAMRSA were also examined. Pulsed-field gel electrophoresis (PFGE) patterns were determined for a subset of study strains. CAMRSA typically belonged to the USA 300 PFGE profile, were associated with high rates of PVL carriage (78%), and primarily were agr group I. Susceptibility to daptomycin, linezolid, teicoplanin, and vancomycin was 100%. In contrast, HAMRSA isolates typically belonged to the USA 100 PFGE profile, were associated with low rates of PVL carriage (8%), and primarily were agr group II. Comparing susceptibilities between the 2 types of MRSA strains, there was a 2-fold increase in MIC for daptomycin, doxycycline, teicoplanin, trimethoprim-sulfamethoxazole (TMP/SMX), and vancomycin in HAMRSA versus CAMRSA. Levofloxacin and clindamycin susceptibly decreased dramatically by 66% and 54%, respectively, against HAMRSA versus CAMRSA. With respect to agr function, 3.5% of CAMRSA and 48% of HAMRSA displayed a down-regulated agr gene cluster. The comparative bactericidal activities of daptomycin were similar to those of vancomycin and clindamycin, but were significantly greater than those of linezolid, teicoplanin, and TMP/SMX against CAMRSA at 24-h terminal end points. Further studies are warranted against a larger number of molecularly defined, geographically diverse CAMRSA to confirm these findings.

Resistance of staphylococci in The Netherlands: surveillance by an electronic network during 1989-1995

An electronic surveillance network for monitoring antibiotic resistance in The Netherlands has been in operation since 1989. Seven public health laboratories participate and the system covers about 25% of all bacteriological determinations in The Netherlands. This paper reports the results of staphylococci isolated in the period 1989-1995. About 0.3% of the Staphylococcus aureus isolates in the study period were resistant to methicillin. This low percentage may be due to the restrictive use of antibiotics and to strict isolation measures aimed at eradicating methicillin-resistant S. aureus. Low frequencies of resistance among methicillin-resistant S. aureus were found for vancomycin (0%), chloramphenicol (11%), cotrimoxazole (11%), mupirocin (3% low-level resistance) and fusidic acid (7%). Twenty-one percent of the coagulase-negative staphylococci were resistant to methicillin. Low frequencies of resistance among these methicillin-resistant coagulase-negative staphylococci were those to vancomycin (0.4%), nitrofurantoin (2%), doxycycline (20%) and amikacin (20%). Coagulase-negative staphylococci from cerebrospinal fluid, blood and skin were less often resistant to quinolones than isolates from respiratory tract, faeces and urine. A significant increase in resistance of coagulase-negative staphylococci to methicillin, erythromycin, gentamicin and ciprofloxacin was observed in the investigated period but the resistance to doxycycline and co-trimoxazole decreased in the last few years. To confirm the determination of methicillin resistance and coagulase production, a PCR method was developed which detects both the mecA and the coagulase gene. The results of the PCR method correlated well with the methicillin MIC as determined by an agar-dilution method.

Typing of Staphylococcus aureus strains by PCR-amplification of variable-length 16S-23S rDNA spacer regions: characterization of spacer sequences

To develop a rapid and accurate method of typing large numbers of clinical isolates of Staphylococcus aureus, the spacer region C of the rRNA operon [1391-507 (16S-23S)] was enzymically amplified from 322 strains. When the products were separated by denaturing PAGE, 15 variable-length rrn alleles were demonstrated, ranging in size from 906 to 1223 bp. The variable-length HpaII-digested region C [(region E; 1446-196 (16S-23S)] amplification products were cloned into M13mp18RF to sequence separate variable-length alleles. A total of 17 region E inserts were sequenced, aligned and divided into nine alleles by length (938-1174) and sequence properties. The 16S-23S spacer rDNA varied in length (303-551 bp) and in properties; three alleles contained a tRNAIle gene alone, two alleles contained a tRNAIle and a tRNAAla gene, and four alleles lacked tRNA genes. The sequences of two alleles showed less than 1% variation when isolated from two or three S. aureus strains. The 48 penicillin- and methicillin-sensitive strains were divided into 26 ribotypes; in contrast, the 274 methicillin-resistant S. aureus (MRSA) strains were divided into nine ribotypes (A-I) with 97% typing as either ribotype A or B (rrnL was missing in B). The sequence conservation of the rrn operons argues for the use of the 16S-23S spacer region as a stable and direct indicator of the evolutionary divergence of S. aureus strains.

Characterization of oxacillin-susceptible mecA-positive Staphylococcus aureus: a new type of MRSA

Methicillin-resistant Staphylococcus aureus (MRSA) has been defined as S. aureus having the mecA gene or showing a minimum inhibitory concentration (MIC) of oxacillin higher than 4 mg/l. However, some clinical isolates are mecA-positive and oxacillin-susceptible. Therefore, we surveyed the occurrence of S. aureus having the mecA gene and an MIC of oxacillin of less than 2 mg/l (oxacillin-susceptible MRSA; OS-MRSA) in a total of 480 strains of S. aureus collected from 11 hospitals in different location in Japan isolated from 2003 through 2005. We found 6 strains matching the criteria for OS-MRSA. All 6 strains were staphylococcal cassette chromosome (SCC) mec-positive, without exception, and 4 strains showed the SCCmec type III-variant, which is unique in Japan. These OS-MRSAs were least resistant to oxacillin among the MRSAs tested and they were within the susceptible range to seven other beta-lactam antibiotics tested. Thus, OS-MRSA may become a high-resistant MRSA upon the treatment of patients with beta-lactam antibiotics. To characterize whether these OS-MRSAs were hospital-acquired or community-acquired MRSAs, we tested for the presence of the genes encoding toxins. Genes encoding hemolysin, exfoliative toxin, enterotoxin, toxic shock syndrome toxin-1, and Panton-Valentine leukocidin were found in 6, 4, 0, 0, and 0 strains, respectively. These results revealed that OS-MRSAs could be classified as a new type of MRSA that exhibits properties distinguishable from either hospital- or community-acquired MRSA. Coagulase typing of the OS-MRSAs supported the above conclusion. In this study, the occurrence of OS-MRSA at a certain frequency was noted; precautions are called for in the classification of oxacillin-resistant S. aureus and in the treatment of OS-MRSA infection.

Rapid detection of the mecA gene in methicillin-resistant staphylococci by enzymatic detection of polymerase chain reaction products

In order to identify methicillin-resistant staphylococci from clinical sources with ease and reliability, enzymatic detection of polymerase chain reaction (ED-PCR) was applied. ED-PCR is based on the capture of amplified products via biotin-streptavidin affinity and the detection of an incorporated hapten in amplified products with an enzyme-linked antibody. In order to identify methicillin-resistant staphylococci of all species, a 150-bp fragment of the mecA gene was targeted for ED-PCR. After PCR was performed with a pair of biotin and dinitrophenol 5'-labeled primers, the reaction mixture was applied to a microtiter well precoated with streptavidin. Thereafter, bound PCR products were detected colorimetrically with alkaline phosphatase-conjugated anti-dinitrophenol antibody. The extraction of DNA from staphylococcal cells for PCR was simplified so that it could be performed within one tube. The total assay, including PCR, took less than 3 h. The sensitivity of mecA gene detection ranged from greater than 5 x 10(2) CFU per tube for Staphylococcus aureus to greater than 5 x 10(3) CFU per tube for Staphylococcus epidermidis. Genotyping results obtained by ED-PCR of 161 tested strains from the colonies (97 strains of S. aureus and 64 strains of coagulase-negative staphylococci) were compared with the phenotypic susceptibilities of the strains to oxacillin. The results of ED-PCR showed excellent agreement with the MICs of oxacillin with very few exceptions; only one strain of S. aureus and two strains of coagulase-negative staphylococci were found to possess the mecA gene, which was discrepant with their phenotypes. Fifty-five blood culture samples were also tested by ED-PCR. For staphylococcal isolates in 33 of the cultures, oxacillin MICs were >4 microgram/ml; 31 of the 33 staphylococcal isolates were determined by ED-PCR to be mecA gene positive. These results suggest that ED-PCR can be used with reasonable confidence in the clinical microbiological laboratory.

Multiplex polymerase chain reaction for detection of genes for Staphylococcus aureus thermonuclease and methicillin resistance and correlation with oxacillin resistance

A multiplex polymerase chain reaction (mPCR) was used for simultaneous amplification of the staphylococcal nuc gene, encoding the thermostable nuclease (TNase), and the mecA gene, encoding the penicillin-binding protein 2a which is associated with staphylococcal methicillin resistance. A total of 219 staphylococcal strains were tested and the mPCR data were compared with coagulase production and in vitro oxacillin susceptibility. The agreement was 100% for coagulase production and nuc amplification, and 97.7%, 96.8 and 97.3% for mecA amplification and oxacillin resistance tested with MIC determination, disk diffusion and agar screen methods, respectively. Discrepant results were due to non-S. aureus isolates with borderline MICs of oxacillin (1-8 micrograms/ml). In a pilot test the mPCR simultaneously amplified both genes of staphylococci in blood cultures. This mPCR is a rapid and reliable method for single-step identification of cultures of MRSA and may prove to be useful for direct application on clinical specimens.

Comparison of PCR detection of mecA with standard susceptibility testing methods to determine methicillin resistance in coagulase-negative staphylococci

Approximately 75% of coagulase-negative staphylococci are resistant to methicillin, but it is suspected that even more resistance exists that is not detected by standard susceptibility assays. To determine the most accurate assay for measuring resistance, we compared the detection of mecA by PCR with detection by National Committee for Clinical Laboratory Standards methods using oxacillin as the class drug. Strains from 11 species of coagulase-negative staphylococci were selected such that 84% were susceptible by the broth microdilution method. Of 45 mecA-positive strains, 1 strain was unable to express the mecA gene product after induction and was not included in further analyses. For microdilution with 2% NaCl, the disk test without salt, and agar screen containing 4% NaCl plus-6 micrograms of oxacillin per ml, the sensitivities in detecting the 44 mecA-positive strains were 50, 84, and 70%, respectively, at 24 h and 77, 82, and 100%, respectively, at 48 h. The specificities of microdilution, disk, and agar screen in detecting the 97 strains lacking mecA were 100, 89, and 100%, respectively, at 24 h. Only the disk test proved to be less specific at 48 h (81%). Furthermore, for 10 of the mecA-positive strains plus an additional 8 strains subsequently added to the analyses, the MICs were 2 micrograms/ml at 24 h by the broth microdilution method; all 18 strains were positive for mecA by PCR. Thus, an oxacillin MIC of > or = 2 micrograms/ml indicated resistance and is probably a more appropriate breakpoint than the current National Committee for Clinical Laboratory Standards breakpoint of 4 micrograms/ml for coagulase-negative staphylococci. Strains for which MICs are < 2 micrograms/ml may be methicillin resistant and should be verified as susceptible by oxacillin agar screening with incubation for 48 h.

Replacement of methicillin-resistant Staphylococcus aureus clones in Hungary over time: a 10-year surveillance study

The prevalence of methicillin-resistant Staphylococcus aureus (MRSA) in Hungary has been increasing and is now close to 20% among invasive isolates of S. aureus. In order to understand the evolution of MRSA in Hungary, two collections of isolates were studied: 22 representatives of a collection of 238 MRSA isolates recovered between 1994 and 1998, and a collection of 299 MRSA isolates recovered between 2001 and 2004. The isolates were first characterised by pulsed-field gel electrophoresis (PFGE) and were distributed into 19 different PFGE patterns. Representatives of each pattern were further characterised by spa typing, multilocus sequence typing (MLST) and staphylococcal cassette chromosome mec (SCCmec) typing. The Hungarian clone that was predominant in 1994-1998 (PFGE E, ST239-III) had almost disappeared in 2003-2004, being replaced by the Southern German clone (PFGE B, ST228-I) and the New York/Japan epidemic clone (PFGE A, ST5-II), which represented c. 85% of the 2001-2004 isolates. Thus, this study describes, for the first time, the co-dominance and extensive spread of the New York/Japan clone in a European country.

Role of mecA transcriptional regulation in the phenotypic expression of methicillin resistance in Staphylococcus aureus

The gene required for methicillin resistance in staphylococci, mecA, encodes the low-affinity penicillin-binding protein 2a (PBP2a). Transcriptional regulation of mecA is accomplished in some isolates by mecR1 and mecI, cotranscribed chromosomal genes that encode a putative signal transducer and a transcriptional repressor, respectively. Two Staphylococcus aureus strains that have identical mecR1-mecI nucleotide sequences, BMS1 and N315P, both exhibit low-level, heterotypic expression of methicillin resistance and contain no beta-lactamase coregulatory sequences. mecR1-mecI was amplified from BMS1 by PCR and was shown to be functional on a high-copy-number plasmid when introduced into an S. aureus strain with a deleted mecR1-mecI locus. Cloned mecR1-mecI repressed phenotypic expression of methicillin resistance, mecA transcription and PBP2a production and mediated PBP2a induction in response to certain beta-lactam antibiotics. However, mecR1-mecI had different regulatory activities in its native chromosomal location in N315P compared with those in BMS1. Uninduced mecA transcription was markedly repressed in N315P, and mecI inactivation increased mecA transcription and PBP2a production 5- and 40-fold, respectively. Furthermore, the N315P phenotype changed from low-level, heterotypic resistance with intact mecI to high-level, homotypic resistance in strains with disrupted mecI. In contrast, uninduced BMS1 produced abundant mecA transcript and PBP2a, while the disruption of mecI had no effect on phenotype and little effect on mecA transcription or PBP2a production. Thus, mecI-mediated repression of mecA appears to be dysfunctional in BMS1 because of the presence or absence of additional regulatory cofactors. Furthermore, heterotypic resistance expression in this strain is independent of mecA transcriptional regulation.

The femC locus of Staphylococcus aureus required for methicillin resistance includes the glutamine synthetase operon

Tn551 insertional inactivation of femC is known to reduce methicillin resistance levels in methicillin-resistant and -susceptible Staphylococcus aureus. By use of cotransductional crosses, femC was mapped close to thrB on the SmaI-A fragment of the S. aureus NCTC 8325 chromosome. The Tn551 insertion femC::omega 2005 was found to interrupt an open reading frame coding for a putative protein of 121 amino acids which is highly similar to the glutamine synthetase repressors (GlnR) of Bacillus spp. Downstream of femC, an open reading frame highly similar to Bacillus sp. glutamine synthetases (GlnA) was found. Northern (RNA) blots probed with putative glnR or glnA fragments revealed that 1.7- and 1.9-kb transcripts characteristic of wild-type cells were replaced by less abundant 7.0- and 7.2-kb transcripts in the femC::omega 2005 mutant. Total glutamine synthetase activity was also decreased in the mutant strain; the addition of glutamine to defined media restored the wild-type methicillin resistance phenotype of the femC mutant. This result suggests that the omega 2005 insertion in glnR has a polar effect on glnA and that glnR and glnA are transcribed together as an operon. These results suggest that the loss of wild-type levels of glutamine synthetase and the consequent decrease in glutamine availability cause a decreased level of methicillin resistance.

Fitness cost of staphylococcal cassette chromosome mec in methicillin-resistant Staphylococcus aureus by way of continuous culture

We examined the effect of introducing type I or IV staphylococcal cassette chromosome mec (SCCmec) elements on the growth yield of Staphylococcus aureus in glucose-limited continuous culture. Type I showed increased glucose consumption and ATP demand per gram of cells synthesized and decreased cell yield compared to those of the parent strain. In contrast, type IV SCCmec elements had no adverse energetic effect.

Laboratory characterization of methicillin-resistant Staphylococcus aureus in Canadian hospitals: results of 5 years of National Surveillance, 1995-1999

Two thousand seven hundred eighty single-patient, methicillin-resistant Staphylococcus aureus (MRSA) isolates collected between January 1995 and December 1999 at 17 tertiary care hospital sites across Canada were characterized by phenotypic and genotypic techniques. Six clonal types, as defined by pulsed-field gel electrophoresis, comprised 87% of all isolates and were labeled Canadian (C) MRSA-1 through -6. CMRSA-1 was the most prevalent clonal type, representing 45% of all MRSA. CMRSA-2 was indistinguishable from the New York clone and was more likely to be associated with community acquisition. CMRSA-3 was more likely to cause an infection, compared with the other CMRSA types. CMRSA-4 was indistinguishable from epidemic (E) MRSA-16 from the United Kingdom. Both CMRSA-5 and -6 occurred primarily in single-site, multiyear outbreaks. This study confirms that the epidemiology of MRSA in Canada is evolving, but most isolates at this time appear to belong to one of a small number of epidemic clones.

Longitudinal study on methicillin-resistant Staphylococcus pseudintermedius in households

Methicillin-resistant Staphylococcus pseudintermedius (MRSP) is an emerging pathogen in dogs and has been found in Europe, Asia and North America. To date most studies are one-point prevalence studies and therefore little is known about the dynamics of MRSP in dogs and their surrounding. In this longitudinal study MRSP colonization in dogs and the transmission of MRSP to humans, contact animals and the environment was investigated. Sixteen dogs with a recent clinical MRSP infection were included. The index dogs, contact animals, owners and environments were sampled once a month for six months. Samples taken from the nose, perineum and infection site (if present) of the index cases and contact animals, and the nares of the owners were cultured using pre-enrichment. Index cases were found positive for prolonged periods of time, in two cases during all six samplings. In five of the 12 households that were sampled during six months, the index case was intermittently found MRSP-positive. Contact animals and the environment were also found MRSP-positive, most often in combination with a MRSP-positive index dog. In four households positive environmental samples were found while no animals or humans were MRSP-positive, indicating survival of MRSP in the environment for prolonged periods of time. Genotyping revealed that generally similar or indistinguishable MRSP isolates were found in patients, contact animals and environmental samples within the same household. Within two households, however, genetically distinct MRSP isolates were found. These results show that veterinarians should stay alert with (former) MRSP patients, even after repeated MRSP-negative cultures or after the disappearance of the clinical infection. There is a considerable risk of transmission of MRSP to animals in close contact with MRSP patients. Humans were rarely MRSP-positive and never tested MRSP-positive more than once suggesting occasional contamination or rapid elimination of colonization of the owners.

Detection of methicillin-resistant Staphylococcus aureus strains resistant to multiple antibiotics and carrying the Panton-Valentine leukocidin genes in an Algiers hospital

Forty-five Panton-Valentine leukocidin (PVL)-positive, methicillin-resistant Staphylococcus aureus strains were isolated in Algeria between 2003 and 2004; 18 isolates were isolated in the community and 27 in a hospital. Five PVL-positive hospital isolates were resistant to multiple antibiotics, including ofloxacin and gentamicin for three isolates.

Detection of methicillin-resistant Staphylococcus aureus and simultaneous confirmation by automated nucleic acid extraction and real-time PCR

A molecular assay for the simultaneous detection of a Staphylococcus aureus-specific gene and the mecA gene, responsible for the resistance to methicillin in staphylococci, was evaluated. The assay included an automated DNA extraction protocol conducted with a MagNA Pure instrument and real-time PCR conducted with a LightCycler instrument. The performance and robustness of the assay were evaluated for a suspension of methicillin-resistant S. aureus (MRSA) strain with a turbidity equivalent to a McFarland standard of 0.5, which was found to be the ideal working concentration. The specificity of the new molecular assay was tested with a panel of 30 gram-negative and gram-positive bacterial strains other than MRSA. No cross-reactivity was observed. In a clinical study, 109 isolates of MRSA were investigated. All clinical MRSA isolates gave positive results for the S. aureus-specific genomic target, and all but one were positive for the mecA gene. In conclusion, the new molecular assay was found to be quick, robust, and laborsaving, and it proved to be suitable for a routine molecular diagnostic laboratory.

Interference-based detection of nucleic acid targets on optically coated silicon

Sequence-specific detection of polynucleotides typically requires modified reporter probes that are labeled with radioactive, fluorescent, or luminescent moieties. Although these detection methods are capable of high sensitivity, they require instrumentation for signal detection. In certain settings, such as clinical point of care, instrumentation might be impractical or unavailable. Here we describe a detection approach in which formation of a nucleic acid hybrid is enzymatically transduced into a molecular thin film that can be visually detected in white light. The system exploits a flat, optically coated silicon-based surface to which capture oligonucleotides are covalently attached. The optimized system is capable of detection of nucleic acid targets present at sub-attomole levels. To supplement visual detection, signals can be quantitated by a charge-coupled device. The design and composition of the optical surface, optimization of immobilization chemistry for attachment of capture probes, and characterization of the efficiency of the hybridization process are presented. We describe the application of this system to detection of a clinically relevant target, the mecA gene present in methicillin-resistant Staphylococcus aureus.

Origins and evolution of methicillin-resistant Staphylococcus aureus clonal lineages

Most methicillin-resistant Staphylococcus aureus (MRSA) isolates identified among blood isolates collected in Denmark between 1957 and 1970 belonged to either phage group III or the closely related 83A complex and had a PSTM antibiotype (resistance to penicillin [P], streptomycin [S], tetracycline [T], and methicillin [M]). Recently, some of these isolates were shown to have the same genetic backgrounds as contemporary epidemic MRSA isolates, and Danish methicillin-susceptible S. aureus (MSSA) isolates from the 1960s with a PST antibiotype were proposed to have been the recipients of the mecA gene in those lineages. In this study, we investigated the genetic backgrounds of isolates from the 83A complex that were fully susceptible or resistant to penicillin only in order to try to trace the evolutionary trajectory of contemporary MRSA lineages. We also studied MSSA and MRSA isolates from other phage groups in order to investigate if they had the potential to develop into contemporary MRSA clones. Most susceptible or penicillin-resistant isolates from phage group III or the 83A complex belonged to sequence type 8 (ST8) or ST5, while four isolates were ST254. STs 30, 45 and 25 were represented by MSSA isolates from other phage groups, which also included several singletons. Representatives of most of the current major epidemic MRSA lineages were identified among fully susceptible isolates collected in the 1960s, suggesting that these were MSSA lineages which carried genetic traits important for superior epidemicity before the acquisition of methicillin resistance.

Similarity of antibiotic resistance patterns and molecular typing properties of methicillin-resistant Staphylococcus aureus isolates widely spread in hospitals in New York City and in a hospital in Tokyo, Japan

One hundred and forty-three single-patient methicillin-resistant Staphylococcus aureus (MRSA) isolates collected during April-June, 1997, and February, 1998, in a hospital in Tokyo, Japan, were characterized by molecular typing techniques that involved hybridization of ClaI restriction digests with the mecA- and Tn554-specific DNA probes and determination of macrorestriction patterns of SmaI-digested chromosomal DNA by pulsed-field gel electrophoresis (PFGE). A large proportion (76%) of the isolates carried the mecA polymorph I, Tn554 pattern A, and PFGE pattern A (clonal type I:A:A), which was the same as the clonal type of an MRSA widely spread in hospitals in New York City and hospitals in neighboring New Jersey, Connecticut, and Pennsylvania. Also similarly to the New York clone, most of the MRSA isolates from the Japanese hospital were resistant to penicillin, ciprofloxacin, erythromycin, tetracycline, and high concentrations (500 microg/ml) of spectinomycin, but were susceptible to chloramphenicol, sulfamethoxazole-trimethoprim, and rifampin. All of the 143 MRSA isolates had vancomycin MICs < or = 2 mg/L.

Expression of resistance to methicillin

Methicillin-resistant staphylococci have an additional low-affinity penicillin-binding protein, PBP2a (PBP2'), encoded by the mecA gene. The typical heterogeneity seen in the expression of resistance to methicillin and in levels of resistance depends on the concerted action of chromosomally encoded genes, including fem and aux, that are also present in the genome of susceptible staphylococci.