Epidemiology of methicillin-resistant staphylococci in Europe

Methicillin-resistant staphylococci are mostly resistant not only to all β-lactams but also to a wide range of other antibiotics, and have emerged as major nosocomial pathogens during the past two decades. Considerable variations in the prevalence of methicillin-resistant Staphylococcus aureus (MRSA) exist between institutions and between geographic areas. In Europe, in general, a north–south gradient is observed, MRSA strains being rare in Scandinavian hospitals (<2%) and far more prevalent in Mediterranean hospitals (>40%). Whether low or high, the rates of MRSA prevalence in European countries have remained approximately the same during the last decade. Recent findings suggest that MRSA might also be emerging as a community-acquired pathogen. The first stage in the emergence of MRSA is its acquisition by methicillin-susceptible S. aureus, and the integration into its chromosome, of the mecA gene, which, together with the other mec genes, is carried on a mobile genetic element, the staphylococcal chromosomal cassette mec (SCCmec). The origin of SCCmec elements as well as the mechanisms of their acquisition remain unknown. Molecular epidemiology studies using different techniques clearly indicate that the massive geographic spread of MRSA results from the dissemination of relatively few highly epidemic clones. Five major lineages (the so-called Iberian, Brazilian, Hungarian, New York/Japan and pediatric pandemic MRSA clones) have been defined. In Europe, the Iberian clone has been reported in several countries; the Brazilian, pediatric and Hungarian clones have also been detected, but less frequently. A unique Italian clone is predominant in Italy. As with S. aureus, coagulase-negative staphylococci (CNS) represent a serious concern in hospital-acquired infections. Despite marked geographic variations, in some areas of Europe high proportions (60–70%) of CNS are methicillin resistant. The formation of biofilm is a key virulence factor of S. epidermidis, the prominent CNS pathogen, which is the most common cause of bacteremia in device-related infections. Another emerging nosocomial pathogen, S. hemolyticus, is characterized by a tendency to develop multiple antibiotic resistances, with a unique predisposition to glycopeptide resistance. Methicillin-resistant staphylococci are mostly resistant not only to all β-lactams but also to a wide range of other antibiotics, and have emerged as major nosocomial pathogens during the past two decades. Considerable variations in the prevalence of methicillin-resistant Staphylococcus aureus (MRSA) exist between institutions and between geographic areas. In Europe, in general, a north–south gradient is observed, MRSA strains being rare in Scandinavian hospitals (<2%) and far more prevalent in Mediterranean hospitals (>40%). Whether low or high, the rates of MRSA prevalence in European countries have remained approximately the same during the last decade. Recent findings suggest that MRSA might also be emerging as a community-acquired pathogen. The first stage in the emergence of MRSA is its acquisition by methicillin-susceptible S. aureus, and the integration into its chromosome, of the mecA gene, which, together with the other mec genes, is carried on a mobile genetic element, the staphylococcal chromosomal cassette mec (SCCmec). The origin of SCCmec elements as well as the mechanisms of their acquisition remain unknown. Molecular epidemiology studies using different techniques clearly indicate that the massive geographic spread of MRSA results from the dissemination of relatively few highly epidemic clones. Five major lineages (the so-called Iberian, Brazilian, Hungarian, New York/Japan and pediatric pandemic MRSA clones) have been defined. In Europe, the Iberian clone has been reported in several countries; the Brazilian, pediatric and Hungarian clones have also been detected, but less frequently. A unique Italian clone is predominant in Italy. As with S. aureus, coagulase-negative staphylococci (CNS) represent a serious concern in hospital-acquired infections. Despite marked geographic variations, in some areas of Europe high proportions (60–70%) of CNS are methicillin resistant. The formation of biofilm is a key virulence factor of S. epidermidis, the prominent CNS pathogen, which is the most common cause of bacteremia in device-related infections. Another emerging nosocomial pathogen, S. hemolyticus, is characterized by a tendency to develop multiple antibiotic resistances, with a unique predisposition to glycopeptide resistance. Among pathogens causing hospital infections, Gram-positive cocci have become predominant over the past two decades. To a large extent, this results from their ability to accumulate antibiotic resistance determinants. An example is that of methicillin-resistant staphylococci, which are mostly resistant not only to all β-lactams, but also to a wide range of other antibiotics. Their susceptibility is sometimes limited to glycopeptides and to a few very recent or experimental drugs. No antibiotic resistance marker has distinguished a species as much as methicillin resistance has distinguished Staphylococcus aureus [1Mulligan ME Murray-Leisure KA Ribner BS et al.Methicillin-resistant Staphylococcus aureus: a consensus review of the microbiology, pathogenesis, and epidemiology with implications for prevention and management.Am J Med. 1993; 94: 313-328Abstract Full Text PDF PubMed Scopus (457) Google Scholar]. Methicillin-resistant S. aureus (MRSA), first reported in the UK in 1961 [2Jevons MP ‘Celbenin’-resistant staphylococci.BMJ. 1961; i: 124-125Crossref Scopus (1354) Google Scholar], soon after the introduction of methicillin into clinical practice, spread to European hospitals in the early 1960s [3Stewart JT Holt RJ Evolution of natural resistance to the newer penicillins.BMJ. 1963; i: 308-311Crossref Scopus (82) Google Scholar,4Benner EJ Kayser FH Growing clinical significance of methcillin-resistant Staphylococcus aureus.Lancet. 1968; 2: 741-751Abstract PubMed Google Scholar], and subsequently to healthcare institutions around the world, emerging as an agent of serious, often lethal, infections, especially in high-risk departments such as surgery, hematology, intensive care and burn units [5Wenzel RP The emergence of methicillin-resistant Staphylococcus aureus.Ann Intern Med. 1982; 97: 440-442Crossref PubMed Scopus (75) Google Scholar]. A distinctive feature of MRSA strains is their resistance not only to all β-lactam antibiotics, but also to a wide range of other antimicrobials, which makes MRSA infections difficult to manage and costly to treat, susceptibility being sometimes recorded only to glycopeptides and to a few very recent or experimental drugs [6Projan SJ Antibiotic resistance in the staphylococci.in: Fischetti VA Novick RP Ferretti JJ Portnoy DA Rood JI Gram-positive pathogens. ASM Press, Washington, DC2000: 463-470Google Scholar,7Carbon C. MRSA and MRSE: is there an answer? Clin Microbiol Infect 6(suppl 2): 17-22.Google Scholar]. In the late 1990s, however, MRSA strains with intermediate or reduced heterogeneous susceptibility to vancomycin were reported, first in Japan and then in other countries [8Hiramatsu K Vancomycin-resistant Staphylococcus aureus: a new model of antibiotic resistance.Lancet Infect Dis. 2001; 1: 147-155Abstract Full Text Full Text PDF PubMed Scopus (610) Google Scholar]. Very recently, and most alarmingly, a highly vancomycin-resistant MRSA strain, which had acquired the enterococcal vanA gene, has been described in the USA [9Centers for Disease Control and Prevention Staphylococcus aureus resistant to vancomycin-United States.MMWR. 2002; 51: 565-567PubMed Google Scholar], confirming our worst fears about vanA acquisition [10Moellering Jr, RC The specter of glycopeptide resistance: current trends and future considerations.Am J Med. 1998; 104: 3-6Abstract Full Text Full Text PDF Scopus (29) Google Scholar]. On the other hand, several recent findings suggest that MRSA might be emerging as a community pathogen, perhaps following the historic precedent of penicillin-resistant S. aureus [11Chambers HF The changing epidemiology of Staphylococcus aureus?.Emerg Infect Dis. 2001; 7: 178-182Crossref PubMed Scopus (1193) Google Scholar]. Community-acquired strains of MRSA tend to be more susceptible to several antibiotics, and are often resistant only to β-lactams [12Boyce JM Are the epidemiology and microbiology of methicillin-resistant Staphylococcus aureus changing.JAMA. 1998; 279: 623-624Crossref PubMed Scopus (120) Google Scholar]; typing by pulsed-field gel electrophoresis (PFGE) also suggests that these are distinct strains [13Adcock PM Pastor P Medley F Patterson JE Murphy TV Methicillin-resistant Staphylococcus aureus in two child care centers.J Infect Dis. 1998; 178: 577-580Crossref PubMed Scopus (223) Google Scholar]. Only a few countries around the world have not seen the emergence and spread of MRSA [14Fluckiger U Widmer AF Epidemiology of methicillin-resistant Staphylococcus aureus.Chemotherapy. 1999; 45: 121-134Crossref PubMed Scopus (46) Google Scholar]; however, considerable variation in its prevalence exists between institutions (e.g. intensive care units (ICUs) versus outpatient departments) and geographic areas. In Europe, in general, there is a north–south gradient, MRSA strains being rare in Scandinavia and far more prevalent in southern Europe [7Carbon C. MRSA and MRSE: is there an answer? Clin Microbiol Infect 6(suppl 2): 17-22.Google Scholar,15Voss A Milatovic D Wallrauch-Schwartz C Ros-dahl VT Braveny I Methicillin-resistant Staphylococcus aureus in Europe.Eur J Clin Microbiol Infect Dis. 1994; 13: 50-55Crossref PubMed Scopus (583) Google Scholar, 16Diekema DJ Pfaller MA Schmitz FJ et al.Survey of infections due to Staphylococcus species: frequency of occurrence and antimicrobial susceptibility of isolates collected in the United States, Canada, Latin America, Europe, and the Western Pacific region for the SENTRY antimicrobial surveillance program, 1997-1999.Clin Infect Dis. 2001; 32: 114-132Crossref Scopus (1109) Google Scholar, 17Fluit AC Wielders CLC Verhoef J Schmitz FJ Epidemiology and susceptibility of 3,051 Staphylococcus aureus isolates from 25 university hospitals participating in the European SENTRY Study.J Clin Microbiol. 2001; 39: 3727-3732Crossref PubMed Scopus (248) Google Scholar]. In a recent survey of 3051 S. aureus isolates from 25 university hospitals (participating in the SENTRY study) distributed among 15 countries of central and southern Europe [17Fluit AC Wielders CLC Verhoef J Schmitz FJ Epidemiology and susceptibility of 3,051 Staphylococcus aureus isolates from 25 university hospitals participating in the European SENTRY Study.J Clin Microbiol. 2001; 39: 3727-3732Crossref PubMed Scopus (248) Google Scholar], MRSA isolates constituted 25% of all isolates; the highest prevalence was seen in hospitals in Portugal (54%) and Italy (from 43% to 58%), whereas the lowest prevalence was observed in hospitals in Switzerland and The Netherlands (2%). In the European Prevalence of Infection in Intensive Care (EPIC) study, the highest prevalence of MRSA strains was found in Italy (81%) and in France (78%) [18Vincent JL Microbial resistance: lessons from EPIC study. European prevalence of infection.Intensive Care Med. 2000; 26: S3-S8Crossref PubMed Google Scholar]. In Table 1, a European picture of the incidence of MRSA is shown from three different studies from 1994 to 2001. In most countries, the proportion of MRSA seems to be relatively stable, whereas in some it has increased [50EARSS Annual report 2001. ESCMID, Bilthoren2002Google Scholar].Table 1Percentages of isolation of MRSA in different European countriesCountriesPeriodsPercentagesGeneral trendReferencesU.K.1999–200133.4, 39.6, 45.4General increase5020012817Denmark19940.1Steady151999–20010.3, 0.2, 0.850France199433.6Steady15200120,2517200133.450Sweden19940.3General increase151999–20019,1450Netherlands19941.5Steady151999–20010.3, 0.4, 0.5502001217Germany19945.5General increase151999–20019.6, 14.4, 17.55020014,517Austria199421.6General decrease152000–015.1, 7.6502001917Belgium199425.1Steady151999–200123.1, 20.9, 21.65020012517Spain199430.3Not classifiable152000–200128.1, 23.15020019, 12, 3417Italy199434.4General increase151999–200140.8, 43.9, 41.050200143,5817Greece1999–200136.9, 25.3, 31.9Steady5020013417Portugal1999–200136.9, 25.3, 31.9General increase5020015417 Open table in a new tab In Italy, an overall incidence of 48% has recently been found in a 2-year study involving three ICUs in the north, center and south of the country [19Nicoletti G Bonfiglio G Bartoloni A et al.Distribution and antibiotic resistance of isolates from lower respiratory tract and blood cultures from patients in three Italian intensive care units: a 2-year comparison.Int J Antimicrob Agents. 2000; 15: 265-269Abstract Full Text Full Text PDF PubMed Scopus (20) Google Scholar]. Comparing European data from 1994 [15Voss A Milatovic D Wallrauch-Schwartz C Ros-dahl VT Braveny I Methicillin-resistant Staphylococcus aureus in Europe.Eur J Clin Microbiol Infect Dis. 1994; 13: 50-55Crossref PubMed Scopus (583) Google Scholar] with data from the last few years [16Diekema DJ Pfaller MA Schmitz FJ et al.Survey of infections due to Staphylococcus species: frequency of occurrence and antimicrobial susceptibility of isolates collected in the United States, Canada, Latin America, Europe, and the Western Pacific region for the SENTRY antimicrobial surveillance program, 1997-1999.Clin Infect Dis. 2001; 32: 114-132Crossref Scopus (1109) Google Scholar, 17Fluit AC Wielders CLC Verhoef J Schmitz FJ Epidemiology and susceptibility of 3,051 Staphylococcus aureus isolates from 25 university hospitals participating in the European SENTRY Study.J Clin Microbiol. 2001; 39: 3727-3732Crossref PubMed Scopus (248) Google Scholar, 18Vincent JL Microbial resistance: lessons from EPIC study. European prevalence of infection.Intensive Care Med. 2000; 26: S3-S8Crossref PubMed Google Scholar, 19Nicoletti G Bonfiglio G Bartoloni A et al.Distribution and antibiotic resistance of isolates from lower respiratory tract and blood cultures from patients in three Italian intensive care units: a 2-year comparison.Int J Antimicrob Agents. 2000; 15: 265-269Abstract Full Text Full Text PDF PubMed Scopus (20) Google Scholar,50EARSS Annual report 2001. ESCMID, Bilthoren2002Google Scholar], we can conclude that countries that suffered from high rates of MRSA prevalence in the early studies maintained approximately the same rate of prevalence over time; the same also seems to be true of lower rates. This variability is difficult to explain; it might be due to geographic variations of MRSA strains with different virulence or colonization properties, or it might reflect differences in the use of antibiotics or in hospital infection control practices. The emergence and dissemination of MRSA has been regarded as a case of accelerated evolution propelled by selective pressure due to the introduction of massive quantities of antibacterial agents being introduced into the environment [20Tomasz A Accelerated evolution: emergence of multidrug resistant gram-positive bacterial pathogens in the 1990s.Neth J Med. 1998; 52: 219-227Crossref PubMed Scopus (10) Google Scholar]. The first stage in the emergence of MRSA is the acquisition of the mecA gene and associated mec DNA by methicillin-susceptible S. aureus and their integration into its chromosome. There are four genetic classes of the mec gene complex, of which only class A (the original one) and class B have been identified in S. aureus (classes C and D have been identified in coagulase-negative staphylococci (CNS)) [21Hiramatsu K Cui L Kuroda M Ito T The emergence and evolution of methicillin-resistant Staphylococcus aureus.Trends Microbiol. 2001; 10: 486-493Abstract Full Text Full Text PDF Scopus (620) Google Scholar]. The mec gene complex is carried on a mobile genetic element, the staphylococcal chromosomal cassette mec (SCCmec), of which four forms have been described that differ in size and genetic composition [21Hiramatsu K Cui L Kuroda M Ito T The emergence and evolution of methicillin-resistant Staphylococcus aureus.Trends Microbiol. 2001; 10: 486-493Abstract Full Text Full Text PDF Scopus (620) Google Scholar,22Oliveira D Tomasz A de Lencastre H Secrets of success of a human pathogen: molecular evolution of pandemic clones of methicillin-resistant Staphylococcus aureus.Lancet Infect Dis. 2002; 2: 180-189Abstract Full Text Full Text PDF PubMed Scopus (420) Google Scholar]. However, the origin of these elements as well as the number of times that these foreign pieces of DNA have entered the species S. aureus and the mechanisms of their acquisition remain unknown. The assembly of the several SCCmec elements may have involved multiple hosts, possibly among CNS [23Archer GL Niemeyer DM Original evolution of DNA associated with resistance to methicillin in staphylococci.Antimicrob Agents Chemother. 1994; 2: 343-347Google Scholar]. Although methicillin resistance is present in many S. aureus lineages, MRSA populations have a clonal distribution that may reflect limited horizontal transfer of mec DNA among isolates [24Musser JM Kapur V Clonal analysis of methicillin-resistant Staphylococcus aureus strains from intercontinental sources: association of the mec gene with divergent phylogenetic lineages implies dissemination by horizontal transfer and recombination.J Clin Microbiol. 1992; 30: 2058-2063PubMed Google Scholar]. Recent studies of the evolutionary history of MRSA suggest multiple introduction of the four SCCmec elements into methicillin-susceptible S. aureus strains with the same sequence type, indicating that horizontal transfer of mec genes is relatively frequent within S. aureus [25Enright M Robinson DA Randle G Feil EJ Grundmann H Spratt BG The evolutionary history of methicillin-resistant Staphylococcus aureus (MRSA).Proc Natl Acad Sci USA. 2002; 99: 7687-7692Crossref PubMed Scopus (1213) Google Scholar]. Traditionally, infections due to MRSA are acquired almost exclusively in hospitals, long-term-care facilities or similar settings; these strains, generally resistant to multiple antibiotics, can be detected by typing schemes and appear to be closely related genetically, and also related to other hospital strains [26Agodi A Campanile F Basile G Viglianisi F Stefani S Phylogenetic analysis as a measure of genetic change in Staphylococcus aureus: the epidemiological impact of methicillin resistance.Eur J Epidemiol. 1999; 15: 637-712Crossref PubMed Scopus (11) Google Scholar,27van Belkum A Molecular epidemiology of methicillin-resistant Staphylococcus aureus strains: state of affairs and tomorrow's possibilities.Microb Drug Resist. 2000; 6: 173-188Crossref PubMed Scopus (27) Google Scholar]. Molecular epidemiology studies using different techniques clearly indicate that the massive geographic spread of MRSA results from the dissemination of relatively few highly epidemic clones [27van Belkum A Molecular epidemiology of methicillin-resistant Staphylococcus aureus strains: state of affairs and tomorrow's possibilities.Microb Drug Resist. 2000; 6: 173-188Crossref PubMed Scopus (27) Google Scholar, 28Enright MC Day NP Davies CE Peacock SJ Spratt BG Multilocus sequence typing for characterization of methicillin-resistant and methicillin-susceptible clones of Staphylococcus aureus.J Clin Microbiol. 2000; 38: 1008-1015Crossref PubMed Google Scholar, 29Roberts RB de Lencastre H Eisner W et al.Molecular epidemiology of methicillin-resistant Staphylococcus aureus (MRSA) in 12 New York hospitals. MRSA Collaborative Study Group.J Infect Dis. 1998; 178: 164-171Crossref PubMed Scopus (153) Google Scholar, 30Sá-Leão R Santos Sanches I Dias D Peres I Barros RM de Lencastre H Detection of an archaic clone of Staphylococcus aureus with low-level resistance to methicillin in a pediatric hospital in Portugal and in international samples: relics of a formerly widely disseminated strain.J Clin Microbiol. 1999; 37: 1913-1920PubMed Google Scholar]. These epidemic strains can be best identified on the basis of their rate of nosocomial spread; however, the ‘epidemic potential’ depends on a multifactorial spectrum of bacterial genetic determinants and the role the environment (selective usage of antibiotics, hygiene measures in hospitals, etc.) plays in their expression is still unclear. Over 3000 MRSA isolates collected throughout the world between 1994 and 2000 and genotyped by a common protocol (mecA:: Tn554:PFGE) through the international CEMNET initiative [31Tomasz A de Lencastre H Molecular microbiology and epidemiology: coexistence or alliance?.in: Wenzel RP Prevention and control of nosocomial infections. 3rd edn. Williams & Wilkins, Baltimore1997: 309-321Google Scholar] provided a rich database to study the recent spread of S. aureus and MRSA clones. Five major clonal lineages—the so-called Iberian, Brazilian, Hungarian, New York/Japan, and pediatric pandemic MRSA clones—were defined [22Oliveira D Tomasz A de Lencastre H Secrets of success of a human pathogen: molecular evolution of pandemic clones of methicillin-resistant Staphylococcus aureus.Lancet Infect Dis. 2002; 2: 180-189Abstract Full Text Full Text PDF PubMed Scopus (420) Google Scholar]. In Europe, the Iberian clone has been reported in several countries, the Brazilian clone in Portugal, the pediatric clone in Portugal and Poland, and the Hungarian clone in several hospitals in Hungary [22Oliveira D Tomasz A de Lencastre H Secrets of success of a human pathogen: molecular evolution of pandemic clones of methicillin-resistant Staphylococcus aureus.Lancet Infect Dis. 2002; 2: 180-189Abstract Full Text Full Text PDF PubMed Scopus (420) Google Scholar]. In Italy, a scenario dissimilar to the common European trend [32Sanches IS Ramirez M Troni H et al.Evidence for the geographic spread of a methicillin-resistant Staphylococcus aureus clone between Portugal and Spain.J Clin Microbiol. 1995; 33: 1243-1246PubMed Google Scholar, 33Oliveira D Santos Sanches I Mato R et al.Virtually all methicillin-resistant Staphylococcus aureus (MRSA) infections in the largest Portuguese teaching hospital are caused by two internationally spread multiresistant strains: the ‘Iberian’ and the ‘Brazilian’ clones of MRSA.Clin Microbiol Infect. 1998; 4: 373-384Crossref PubMed Scopus (67) Google Scholar, 34Melter O Santos Sanches I Schindler J et al.Methicillin-resistant Staphylococcus aureus clonal types in the Czech Republic.J Clin Microbiol. 1999; 37: 2798-2803PubMed Google Scholar] is emerging from recent studies [35Campanile F Mato R Stefani S Messina C Santos Sanchez I De Lencastre H Molecular epidemiology of MRSA in Italy: clonal diffusion and evolution of polymorphisms in mecA and Tn554 regions [abstract T205].in: Program and Abstracts of the 3rd European Congress of Chemotherapy, Madrid. ESCMID, Milan2000: 85Google Scholar,36Campanile F Cafiso V Cascone C Gianninò V Di Marco O Stefani S Clonal diffusion and evolution of mecA and Tn554 polymorphisms in methicillin-resistant Staphylococcus aureus in Italy.Infez Med. 2001; 9: 30-38PubMed Google Scholar]. They have shown the clonal evolution of the MRSA population in two different periods, with the presence of a unique Italian clone (uniformly susceptible to tetracycline and rifampin) alongside the two widespread and multiresistant Iberian and Brazilian clones (Figure 1). The whole sequences of the genomes of two MRSA strains, including the prototype vancomycin-intermediate S. aureus, have recently been published [37Kuroda M Ohta T Uchiyama I et al.Whole genome sequencing of methicillin-resistant Staphylococcus aureus.Lancet. 2001; 357: 1225-1240Abstract Full Text Full Text PDF PubMed Scopus (1593) Google Scholar]. As more genome sequences become available, comparative genomics will allow structural approaches to fundamental questions, such as why some strains are better able to colonize, spread and infect than others [38Woodford N Livermore DM Can we beat MRSA now we know its genome sequence.Lancet Infect Dis. 2001; 1: 9-10Abstract Full Text Full Text PDF PubMed Scopus (12) Google Scholar]. Like S. aureus, CNS represent a serious concern in hospital-acquired infections. While, until recently, it was assumed that most infections caused by CNS were endogenous, i.e. derived from the patient's indigenous flora, many recent publications suggest that most of these infections are hospital acquired, due to strain transmission among hospitalized patients [39Huebner J Goldmann DA Coagulase-negative staphylococci: role as pathogens.Annu Rev Med. 1999; 50: 223-236Crossref PubMed Scopus (337) Google Scholar]. Soon after the emergence of MRSA, it was realized that the issue of methicillin resistance involved not only S. aureus, but also CNS, among which the resistance was often even more common [40Wenzel RP Methicillin-resistant S. aureus and S. epidermidis strains: modern hospital pathogens.Infect Control. 1986; 7: 118-119PubMed Google Scholar]. Although there are marked geographic variations, in some areas of Europe high proportions (60–70%) of CNS are methicillin resistant [7Carbon C. MRSA and MRSE: is there an answer? Clin Microbiol Infect 6(suppl 2): 17-22.Google Scholar]. In a recent Italian study [19Nicoletti G Bonfiglio G Bartoloni A et al.Distribution and antibiotic resistance of isolates from lower respiratory tract and blood cultures from patients in three Italian intensive care units: a 2-year comparison.Int J Antimicrob Agents. 2000; 15: 265-269Abstract Full Text Full Text PDF PubMed Scopus (20) Google Scholar], CNS accounted for 41% of all isolates from blood cultures in ICU patients; of these, 64% were methicillin resistant and demonstrated high rates of resistance to many other antibiotics (Table 2).Table 2Antibacterial activity of some commonly used antibiotics against S. aureus, S. epidermidis and CNS [51Stefani S Mezzatesta ML Tempeta G et al.Comparative activity of Linezolid against Staphylococci and Enterococci isolated in Itlay.Clin Microbiol Infect. 2002; 8: 368-372Abstract Full Text Full Text PDF PubMed Scopus (20) Google Scholar]S. aureusResistance in % S. epidermidisCNSMRMSMRMSMRMSPenicillin99.360.210086.3100100Imipenem89.4078.713.664.70Amoxicillin-clavulanate99.5088.7070.60Vancomycin000000Teicoplanin00005.80Gentamicin97.413.280.036.394.128.6Erythromycin90.322.995.063.694.128.6Clindamycin98.126.595.036.364.728.6Ciprofloxacin93.910.857.036.374.157.1Chloramphenicol68.528.957.550.041.242.8Rifampin57.32.452.527.341.228.6Co-trimoxazole11.72.445.036.352.928.6MR, methicillin-resistant strains; MS, methicillin-susceptible strains. Open table in a new tab MR, methicillin-resistant strains; MS, methicillin-susceptible strains. Among CNS, the prominent pathogen is S. epidermidis, which, on the one hand, is a prevalent member of the skin flora, and, on the other, is a major infective agent in compromised patients. Even more importantly, it is the most common cause of bacteremia related to foreign bodies and indwelling medical devices [39Huebner J Goldmann DA Coagulase-negative staphylococci: role as pathogens.Annu Rev Med. 1999; 50: 223-236Crossref PubMed Scopus (337) Google Scholar]. Device-related S. epidermidis infections are complicated by the formation of biofilm, which is a key virulence factor of this species and has a huge impact on both the pathogenesis and therapy of these infections. Bio-films can form on many medical implants, such as catheters, artificial hips, and contact lenses; since biofilms care refractory to antimicrobial agents, the removal of the infected device is often the only possible solution, thus increasing the trauma to the patient and the cost of treatment [41O'Gara JP Humphreys H Staphylococcus epidermidis biofilms: importance and implications.J Med Microbiol. 2001; 50: 582-587Crossref PubMed Scopus (315) Google Scholar]. It has been estimated that biofilms are associated with 65% of nosocomial infections [42Licking E Getting a grip on bacterial slime.Business Week. 13 September 1999; : 98-100Google Scholar]. Recently, Frebourg et al. [43Frebourg NB Lefebvre S Baert S Lemeland JF PCR-based assay for discrimination between invasive and contaminating Staphylococcus epidermidis strains.J Clin Microbiol. 2000; 38: 877-880PubMed Google Scholar] compared the results of polymerase chain reaction (PCR)-based detection of putative virulence genes in contaminating strains, sepsis-related strains, catheter strains, and saprophytic strains, in order to discriminate between invasive and contaminating S. epidermidis. While several genes were almost ubiquitously amplified, the ica (intercellular adhesion gene cluster) gene and the mecA gene were detected significantly more often in infecting strains than in contaminating strains. In a recent study [44Cafiso V Campanile F Santagati M Di Bassiano F Nicoletti G Stefani S L'espressione contemporanea di icaA e icaD determina la produzione di biofilm in S. epidermidis indipendentemente dalla loro fonte di isolamento.in: Program and Abstract Del 30° Congresso Nazionale Della Società Italiana Di Microbiologia. Societa Italiana di Microbiologia (SIM), Catania. Catania2002: 114Google Scholar], the presence and expression of the ica operon was investigated in a group of S. epidermidis strains (including both methicillin-resistant and methicillin-susceptible isolates) from catheter-related infections, and the results were compared with those obtained from a group of non-device-related nosocomial S. epidermidis strains; all isolates were also characterized by genome macrorestriction analysis. As shown in Table 3, the presence of four ica genes (icaA, icaB, icaC, and icaD) by PCR and the coexpression of icaA and icaD correlated with biofilm production in all strains from both catheter-related and nosocomial infections. Cluster analysis of strains after genome macrorestriction showed that catheter-related strains formed a homogeneous group of organisms with a coefficient of similarity of 0.12 (Figure 2).Table 3Amplification and transcription of the four ica operon genes from S. epidermidis isolated from catheters and other nosocomial infections and correlation with biofilm productionPCR (bp)RT–PCR (bp)StrainSourceicaAicaBicaCicaDicaAicaBicaCicaDBiofilm production1CTCatheter81452698922514665261787225Yes2CTCatheter8145269892251466––225Yes3CTCatheter814526989225–––225No4CTCatheter81452698922514665261787225Yes5CTCatheter81452698922514665261787225Yes6CTCatheter81452698922514665261787225Yes7CTCatheter81452698922514665261787225Yes8CTCatheter––––––––No9CTCatheter––––––––No10CTCatheter––––––––No11CTCatheter––––––––No12CTNosocomial81452698922514665261787225Yes13CTNosocomial81452698922514665261787225Yes14CTNosocomial81452698922514664001787225Yes15CTNosocomial––––––––No16CTNosocomial––––––––No Open table in a new tab Besides S. epidermidis, another CNS species, also emerging as a nosocomial pathogen is S. hemolyticus, which has been well known for several years, due to its tendency to develop multiple antibiotic resistance [45Froggatt JW Johnston JL Galetto DW Archer GL Antimicrobial resistance in nosocomial isolates of Staphylococcus haemolyticus.Antimicrob Agents Chemother. 1989; 33: 460-466Crossref PubMed Scopus (135) Google Scholar], and in particular for its unique predisposition among CNS to acquire glycopeptide resistance [46Schwalbe RS Stapleton JT Gilligan PH Vancomycin-resistant staphylococcus.N Engl J Med. 1987; 317: 766-768PubMed Google Scholar,47Biavasco F Vignaroli C Varaldo PE Glycopeptide resistance in coagulase-negative staphylococci.Eur J Clin Microbiol Infect Dis. 2000; 19: 403-417Crossref PubMed Scopus (129) Google Scholar]. Recent studies [48Sieradzki K Villari P Tomasz A Decreased susceptibilities to teicoplanin and vancomycin among coagulase-negative methicillin-resistant clinical isolates of staphylococci.Antimicrob Agents Chemother. 1998; 42: 100-107Crossref PubMed Google Scholar,49Biavasco F Vignaroli C Lazzarini R Varaldo PE Glycopeptide susceptibility profiles of Staphylococcus haemolyticus bloodstream isolates.Antimicrob Agents Chemother. 2000; 44: 3122-3126Crossref PubMed Scopus (27) Google Scholar] indicate that heterogeneous expression of teicoplanin resistance is prevalent among S. hemolyticus strains, and may be associated with heterogeneous resistance to vancomycin; therefore, susceptibility to glycopeptides, as determined by standard dilution methods, is unlikely to represent a reliable basis for glycopeptide treatment of S. hemolyticus infections. S. hemolyticus strains with heterogeneous glycopeptide resistance profiles are mostly methicillin resistant, but methicillin-susceptible strains of the same species with fully comparable heterogeneous profiles for both teicoplanin and vancomycin have been described [49Biavasco F Vignaroli C Lazzarini R Varaldo PE Glycopeptide susceptibility profiles of Staphylococcus haemolyticus bloodstream isolates.Antimicrob Agents Chemother. 2000; 44: 3122-3126Crossref PubMed Scopus (27) Google Scholar]. In conclusion, we can say that knowledge of how frequently new methicillin-resistant clones arise in the population is not merely a theoretical problem. If mec transfer is a rare event, then measures for infection control alone should prevent the dissemination of resistant clones. But if it is a common event, then the control of antibiotic usage to limit selection pressure favoring multiresistant organisms is of paramount importance. Increased attention to microbial population genetics and dynamics is needed to track down the origin of these multiresistant staphylococcal strains.