EARSS activities and results: update

Assessment of band-based similarity coefficients for automatic type and subtype classification of microbial isolates analyzed by pulsed-field gel electrophoresis

Pulsed-field gel electrophoresis (PFGE) has been the typing method of choice for strain identification in epidemiological studies of several bacterial species of medical importance. The usual procedure for the comparison of strains and assignment of strain type and subtype relies on visual assessment of band difference number, followed by an incremental assignment to the group hosting the most similar type previously seen. Band-based similarity coefficients, such as the Dice or the Jaccard coefficient, are then used for dendrogram construction, which provides a quantitative assessment of strain similarity. PFGE type assignment is based on the definition of a threshold linkage value, below which strains are assigned to the same group. This is typically performed empirically by inspecting the hierarchical cluster analysis dendrogram containing the strains of interest. This approach has the problem that the threshold value selected is dependent on the linkage method used for dendrogram construction. Furthermore, the use of a linkage method skews the original similarity values between strains. In this paper we assess the goodness of classification of several band-based similarity coefficients by comparing it with the band difference number for PFGE type and subtype classification using receiver operating characteristic curves. The procedure described was applied to a collection of PFGE results for 1,798 isolates of Streptococcus pneumoniae, which documented 96 types and 396 subtypes. The band-based similarity coefficients were found to perform equally well for type classification, but with different proportions of false-positive and false-negative classifications in their minimal false discovery rate when they were used for subtype classification.

Epidemiology and typing of Staphylococcus aureus strains isolated from bloodstream infections

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

Isolation of patients in single rooms or cohorts to reduce spread of MRSA in intensive-care units: prospective two-centre study

BACKGROUND

Hospital-acquired infection due to meticillin-resistant Staphylococcus aureus (MRSA) is common within intensive-care units. Single room or cohort isolation of infected or colonised patients is used to reduce spread, but its benefit over and above other contact precautions is not known. We aimed to assess the effectiveness of moving versus not moving infected or colonised patients in intensive-care units to prevent transmission of MRSA.

METHODS

We undertook a prospective 1-year study in the intensive-care units of two teaching hospitals. Admission and weekly screens were used to ascertain the incidence of MRSA colonisation. In the middle 6 months, MRSA-positive patients were not moved to a single room or cohort nursed unless they were carrying other multiresistant or notifiable pathogens. Standard precautions were practised throughout. Hand hygiene was encouraged and compliance audited.

FINDINGS

Patients' characteristics and MRSA acquisition rates were similar in the periods when patients were moved and not moved. The crude (unadjusted) Cox proportional-hazards model showed no evidence of increased transmission during the non-move phase (0.73 [95% CI 0.49-1.10], p=0.94 one-sided). There were no changes in transmission of any particular strain of MRSA nor in handwashing frequency between management phases.

INTERPRETATION

Moving MRSA-positive patients into single rooms or cohorted bays does not reduce crossinfection. Because transfer and isolation of critically ill patients in single rooms carries potential risks, our findings suggest that re-evaluation of isolation policies is required in intensive-care units where MRSA is endemic, and that more effective means of preventing spread of MRSA in such settings need to be found.

Epidemiological interpretation of antibiotic resistance studies - what are we missing?

Antimicrobial resistance is an emerging public-health threat. Studies of the relationship between antibiotic use and resistance, as well as surveillance programmes, examine changes in the proportion of isolates that are resistant. Although proportions are helpful to the clinician prescribing empirical therapy, proportion-based analyses can be misleading to the public-health professional as they can yield biased estimates. Proportions do not adequately reflect the burden of resistance, a measure often of interest in public health. A more appropriate measure of this burden is the rate of isolation of resistant organisms, that is, the absolute number of resistant isolates in a population over time.

National surveillance of methicillin-sensitive and methicillin-resistant Staphylococcus aureus in Austrian hospitals: 1994-1998

In 1994, the Austrian Federal Ministry for Labour, Health and Social Affairs initiated a nationwide survey of the prevalence of methicillin-sensitive (MSSA) and methicillin-resistant Staphylococcus aureus (MRSA) in Austrian hospitals. A questionnaire had to be filled out by the medical directors of participating hospitals. During the five years of surveillance (1994-1998), a total of 9385 MRSA and 71510 MSSA episodes of infection/colonization were reported. The rate of MRSA decreased from 15.8% in 1994 to 8.2% in 1998. However, the frequency per 1000 hospital admissions increased from 0.85 to 1.29 with a maximum of 2.42 MRSA episodes per 1000 admissions in 1997. Overall, in 53.6% of 5020 S. aureus episodes, signs and symptoms of an infection were reported, of these 9.3% (251/2692) were due to MRSA. For large hospitals with more than 1000 beds, 20.9% of reported MRSA episodes were associated with infection, 54.9% of MRSA episodes from hospitals with 301-1000 beds, 60.1% from hospitals with 101-300 beds, and 80.0% from small hospitals with less than 100 beds. Our data do not support the hypothesis of higher virulence of MRSA compared with MSSA. Indeed, in this survey, MSSA episodes were more frequently associated with signs and symptoms of infection (OR=1.35, 95% CI: 1.12-1.62, P<0.001).

Contribution de la pression de sélection antibiotique dans l’acquisition de Staphylococcus aureus résistant à la méticilline (SARM) dans un centre hospitalier universitaire

Methicillin-resistant Staphylococcus aureus (MRSA) is a major nosocomial pathogen, which causes severe morbidity and mortality in hospitals. Antibiotic pressure is recognised as an individual risk factor in the acquisition of MRSA, however, some authors suggest that antibiotic use must be considered as an ecological and collective problem on a unit or hospital scale. The objective of our study was to determine whether antibiotic pressure affects the acquisition of MRSA. The study was conducted during 1 year (from October 2000 to September 2001) at Besançon hospital. The analysis was based on the research of a correlation between incidence of MRSA acquisition, the colonisation pressure exerted by imported MRSA and use of antimicrobials ineffective against MRSA at the unit level. Univariate analysis showed that acquired MRSA cases were significantly correlated with the use of all antimicrobials, as well as with the use of each class of antimicrobial and with colonisation pressure. In multivariate analysis, the stratification of the units into consumer classes allowed us to eliminate the scale effect of each antimicrobial class and to characterise the impact of high antimicrobial use compared to weak antimicrobial. The highest risks of MRSA acquisition were observed for beta-lactams and quinolones. This study shows that "MRSA acquisition" is correlated to antibiotic use and highlights the need to improve antibiotic usage in the control of MRSA.

Quinupristin/dalfopristin

Gram-positive pathogens are associated with both community- and hospital-acquired infections. These infections may be life-threatening in hospitalised patients, especially in those with significant underlying acute or chronic diseases. Prompt and appropriate antimicrobial therapy is essential for avoiding morbidity and mortality. The concept of appropriate therapy is being redefined by increasing antimicrobial resistance, especially amongst Gram-positive pathogens. This has been most dramatic with penicillin-resistant Streptococcus pneumoniae in the community, including cross-resistance to other classes of antimicrobial agents. In the US, the incidence of methicillin-resistant Staphylococcus aureus (MRSA) with community isolates is significant. For hospital-acquired Gram-positive pathogens, MRSA, vancomycin-resistant Enterococcus species and vancomycin-intermediate resistant and -resistant S. aureus are a great concern, particularly as the frequency of recovery of these pathogens from infected patients increases. The net result of these various resistance issues is a reduction in the number of appropriate antimicrobial agents for treating infected patients. Quinupristin/dalfopristin is a parental streptogramin with a spectrum of activity that includes Gram-positive pathogens, including those resistant to other classes of antimicrobial compounds. In this review, data summarising the frequency of recovered Gram-positive pathogens from various infectious diseases, the escalating prevalence of resistance amongst Gram-positive pathogens and the factors making quinupristin/dalfopristin a suitable agent for treating patients infected with Gram-positive organisms will be discussed.

Characterization of methicillin-resistant Staphylococcus aureus strains susceptible to tobramycin

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