Methicillin-resistant Staphylococcus aureus in U.S. hospitals, 1975-1991

Infection versus colonization in the critical care unit

Serious infections in the critical care unit are commonplace. However, distinguishing true infection from mere colonization is a difficult and often uncertain process that has been shown to result in both over- and under-treatment of patients. Antimicrobial agents used in the CCU setting are expensive and not without toxicities. This article discusses methods to differentiate colonization from infection.

Innovative approaches to novel antibacterial drug discovery

Increasing antibiotic resistance in microorganisms and new emerging pathogens have become a major problem in our society. Rising to satisfy this urgent medical need is a recent confluence of powerful new drug discovery technologies: combinatorial chemistry; sequence and functional genomic analysis; and novel methods of high-throughput screening. The combination of these technologies will bring to bear untapped power in the search for new antimicrobials.

Surveillance for the emergence and dissemination of antimicrobial resistance in bacteria

Effective surveillance of antimicrobial-resistant bacteria is important for developing rational empiric therapy guidelines and for guiding public health efforts to control and prevent the spread of infective agents. Surveillance must include a timely and thorough review of the test results generated in clinical microbiology laboratories because this data serves as the core of surveillance activities. Besides ensuring data accuracy and optimizing detection of emerging resistance, the role of clinical microbiology also includes supporting the production of informative surveillance reports, providing laboratory resources for outbreak investigations, and monitoring the performance of commonly used susceptibility testing methods. Once the accuracy of susceptibility results has been validated, the data are used by public health agencies and professional societies to monitor resistance trends on a local, state, national, and international level. This information is also used to develop policies for prudent antimicrobial use locally and nationally.

Antimicrobial resistance in staphylococci. Epidemiology, molecular mechanisms, and clinical relevance

Staphylococcal infections continue to pose important clinical problems in children and adults. Antibiotic resistance among the staphylococci has rendered therapy of these infections a therapeutic challenge. Despite early, uniform susceptibility to penicillin, staphylococci acquired a gene elaborating beta-lactamase that rendered penicillin inactive and that is borne by nearly all clinical isolates. "Penicillinase-resistant beta-lactams," such as methicillin, were introduced in the early 1960s, but resistance to them has become an increasing concern. The mechanism of the so-called "methicillin resistance" is complex. Moreover, once confined to the ecology of hospitals and other institutions, a recent increase in community-acquired methicillin-resistant S. aureus infections has been observed. Glycopeptides, until now the only uniformly reliable therapeutic modality, have been increasingly used for therapy of staphylococcal infections. The recent recognition of clinical isolates with reduced susceptibility to glycopeptides is of concern.

The impact of antimicrobial use on the emergence of antimicrobial-resistant bacteria in hospitals

Abundant evidence suggests a relationship between antimicrobial resistance and use, including models, consistent associations between resistance and antimicrobial use in hospitals, concomitant variation in resistance as antimicrobial use varies, and a dose-response relationship for many pathogen/antimicrobial combinations. Much of the evidence is from studies performed in single hospitals. Most multicenter studies on resistance have not included data on antimicrobial usage. Despite this substantial body of evidence, some studies have not demonstrated an association between antimicrobial resistance and use, suggesting other contributing factors such as cross transmission, interhospital transfer of resistance, a community contribution to resistance, or a complex relationship between resistance and the use of a variety of antimicrobials. Understanding the problem of antimicrobial resistance in a hospital cannot be achieved without knowledge of the hospital's pattern of antimicrobial use.

Comparative in vitro assessment of sparfloxacin activity and spectrum using results from over 14,000 pathogens isolated at 190 medical centers in the USA. SPAR Study Group

Sparfloxacin, a new orally administered fluoroquinolone, was tested against 14,182 clinical strains isolated (generally blood stream and respiratory tract cultures) at nearly 200 hospitals in the United States (USA) and Canada. Sparfloxacin activity was compared with 13 other compounds by Etest (AB BIODISK, Solna, Sweden), broth microdilution, or a standardized disk diffusion method. Using the Food and Drug Administration/product package insert MIC breakpoint for sparfloxacin susceptibility (< or = 0.5 microgram/ml), 94% of Streptococcus pneumoniae (2666 isolates) and 89% of the other streptococci (554 isolates) were susceptible. However, at < or = 1 microgram/ml (the breakpoint for all nonstreptococcal species) sparfloxacin susceptibility rates increased to 100% and 98%, respectively, for the two groups of streptococci. Only 50% and 65% of pneumococci were susceptible to ciprofloxacin (MIC90, 3 micrograms/ml) and penicillin (MIC90, 1.5 micrograms/ml), respectively. Although there were significant differences between regions in the USA in the frequency of penicillin-resistant pneumococcal strains, results indicate that the overall sparfloxacin MIC90 was uniformly at 0.5 microgram/ml. Nearly all (> or = 99%) Haemophilus species and Moraxella catarrhalis, including those harboring beta-lactamases, were susceptible to sparfloxacin, ciprofloxacin, and amoxicillin/clavulanic acid. Only cefprozil and macrolides demonstrated lower potency and spectrum against these two species. Sparfloxacin was active against oxacillin-susceptible Staphylococcus aureus (96 to 97%), Klebsiella spp. (95%), and other tested enteric bacilli (93%). Comparison between broth microdilution MIC and disk diffusion interpretive results for M. catarrhalis, Staphylococcus aureus, and the Enterobacteriaceae showed an absolute intermethod categorical agreement of > 95% using current sparfloxacin breakpoints, in contrast to those of cefpodoxime for S. aureus where a conspicuous discord (98% versus 59%) between methods was discovered. These results demonstrate that sparfloxacin possesses sufficient in vitro activity and spectrum versus pathogens that cause respiratory tract infections (indications), especially strains resistant to other drug classes such as the earlier fluoroquinolones, oral cephalosporins, macrolides, and amoxicillin/clavulanic acid. The sparfloxacin susceptibility breakpoint for streptococci may require modification (< or = 1 microgram/ml) based on the MIC population analysis presented here. A modal MIC (0.38 to 0.5 microgram/ml) was observed at the current breakpoint. Regardless, sparfloxacin inhibited 89% (nonpneumococcal Streptococcus spp.) to 100% (Haemophilus spp., M. catarrhalis) of the isolates tested with a median activity of 97% against indicated species.

A long-term survey of methicillin-resistant Staphylococcus aureus in the oral cavity of children

Methicillin-resistant Staphylococcus aureus (MRSA), an indigenous bacteria in healthy people, often causes nosocomial infections. If the host human becomes compromised, MRSA can cause a serious infection. The long-term colonization of MRSA increases this risk. The purpose of this study was to demonstrate the incidence of S. aureus and MRSA colonization in the oral cavities of healthy children, and to examine the stability of identical strains of MRSA over a long-term period. Fourteen children were examined in two stages (first stage: 1987-88, second stage: 1992-93). Five of the 14 children were negative for S. aureus in both stages, seven children were positive in both stages and two children were positive in only the second stage. The children who were colonized with S. aureus in the first stage always harbored the bacteria in the second stage. Of the seven children that were positive for S. aureus in both stages, three persisted in carrying MRSA. We compared two MRSA strains isolated from the same children in both stages by coagulase typing, antibiogram typing and DNA fingerprinting. In two children, the strains showed the same coagulase types, similar antibiograms and similar DNA fragment profiles. These data strongly suggest that identical strains of MRSA persisted in the oral cavities for more than five years, and that the oral cavity can serve as a reservoir for MRSA with the potential to cause nosocomial infections.

Enterotoxin and toxic shock syndrome toxin-1 production of methicillin resistant and methicillin sensitive Staphylococcus aureus strains

In this study the production of enterotoxin A-D and toxic shock syndrome toxin-1 (TSST-1) of 181 methicillin resistant (MRSA) and 100 methicillin sensitive (MSSA) Staphylococcus aureus first isolates from different patients was investigated. All the MRSA- and MSSA isolates in the study were collected in a period between 1993 and 1995 from specimens sent from 11 different acute care hospitals in the greater Düsseldorf area. As far as possible the isolates were matched according to ward and hospital. The isolates were collected in the same time period and matched for specimen from which isolated. Furthermore, only first isolates were analysed in both groups. No significant difference in the production of toxin of any type between MRSA and MSSA could be detected (51 and 40% respectively). When the individual toxins were analysed, again no significant difference between MRSA and MSSA was demonstrable (enterotoxin production by MRSA 40% and MSSA 36%, and TSST-1 16% and 8% respectively). Despite this, a slight tendency for MRSA to produce enterotoxin A and B and for MSSA to produce enterotoxin C was observed. In addition, generation of TSST-1 by both groups was independent of enterotoxin A-D production. Interestingly, no increase in the proportion of TSST-1- or enterotoxin-producing MRSA and MSSA isolates was observed in strains isolated from blood cultures from patients with a clinical diagnosis of sepsis. Genotypical pulsed-field-gel-electrophoresis (PFGE) and phenotypical (bacteriophage typing, lysotyping) characterization of the 181 MRSA isolates resulted in 28 different PFGE patterns (of which 19 were toxin producers) and 22 lysotyping groups (18 of which produced toxin). In summary, the investigated clinical S. aureus isolates showed no difference in their ability to produce toxin and this was independent of their sensitivity to methicillin.

Methicillin resistant Staphylococcus aureus strains in the greater Düsseldorf area

Over a period of three years the incidence of methicillin resistant Staphylococcus aureus (MRSA) isolates in 11 hospitals in the greater Düsseldorf area was observed. From a total of 7,814 S. aureus isolates, 489 (6.3%) were methicillin resistant. From 198 different patients, MRSA first isolates and 291 second isolates could be cultured. Methicillin resistance among all S. aureus isolates from 11 hospitals in the greater Düsseldorf area, ranged from 0.5 to 7.8% dependant on the size of the hospital. The highest incidence (7.8%) was found in a 1,500 bed hospital and the lowest incidence in a smaller 200 bed hospital (0.5%). With respect to the distribution among clinical departments the highest incidence of MRSA isolates was found on intensive care units and surgical wards, 25.5% and 13.0% respectively. The commonest specimen from which the MRSA isolates were cultured were respiratory secretions (17.6%) followed by central venous catheter tips (12.8%). In terms of the drug resistance pattern: all isolates were resistant to the aminoglycosides and gyrase inhibitors, whereas between 80% and 90% were sensitive to fusidic acid, chloramphenicol and pyrimethamine-sulfamethoxazole. All the strains were sensitive to the glycopeptide antibiotics, vancomycin and teicoplanin. Strain typing of 181 available first isolates (from a total of 198 first isolates) by PFGE and phage lysotyping produced identical results in more than 90% of all cases. Twenty-eight different MRSA strain types were identified by PFGE and in total 23 lysotypes could be determined. During the period of investigation an increased incidence of MRSA on an intensive care unit was observed, in which a total of 204 MRSA (42% of the total number) were isolated. The strain typing using both methods showed that on that ICU eight different MRSA types were involved in this outbreak. A hygiene plan was implemented on the unit with considerable success in reducing the incidence and spread of MRSA.

Regulation of beta-lactamase synthesis as a novel site of action for suppression of methicillin resistance in Staphylococcus aureus

Nearly all clinical isolates of methicillin resistant Staphylococcus aureus (MRSA) produce beta-lactamase as well as an additional low-affinity penicillin-binding protein called PBP2a or PBP2', the main factor for mediating methicillin resistance. Polidocanol (PDO), a dodecyl polyethyleneoxide ether, resensitizes clinical isolates of MRSA to methicillin; in addition, their resistance to benzylpenicillin (BP) is reduced. The action of PDO is based on the inhibition of the induced syntheses of PBP2a and beta-lactamase. Induction in our study was performed with 2-(2'-carboxyphenyl)benzoyl-6-aminopenicillanic acid (CBAP). Inducible PBP2a production in MRSA strains is under the control of the same regulatory system which is responsible for the induction of beta-lactamase synthesis. BlaR1, a membrane-spanning protein with a penicillin sensor and a signal transducer domain represents the starting point of this induction cascade. Based on its amphiphilic properties, it is likely that the action of PDO is located in the bacterial membrane. Therefore we investigated the possibility that BlaR1 might be the main target for PDO action. We were able to detect the BlaR1 sensor domain in resistant staphylococcal cells even in the noninduced state by fluorography. In a competition assay, CBAP was bound specifically, with a high affinity to the penicillin sensor. Moreover, the binding of CBAP was very stable. As concerns PDO, no significant interaction with the penicillin binding site of BlaR1 was detectable. This is why the BlaR1 transducer domain is thought to be the actual target area of PDO. In this case, PDO would interfere with the transmission of the signal, generated by the receptor binding of CBAP, through the membrane via BlaR1 into the staphylococcal cell. This assumption could be confirmed by the analysis of the concentration-effect relationship, whereafter PDO does not work as a competitive, but as a noncompetitive antagonist of CBAP. Our results demonstrate that BlaR1 could be an attractive new target for the development of new drugs to overcome methicillin resistance.

Current perspectives on multidrug-resistant bacteria. Epidemiology and control

Antimicrobial resistance in bacteria has diminished the availability of effective antimicrobial agents. Knowledge of epidemiology, mechanisms of resistance, and new diagnostic modalities can help to identify and treat patients at risk for infection by these organisms. Limited or nonexistent effective microbial therapy underscores the importance of effective preventive and containment measures.

The emergent needs for basic research, education, and surveillance of antimicrobial resistance. Problems facing the report from the American Society for Microbiology Task Force on Antibiotic Resistance

The American Society for Microbiology (ASM) convened a task force to study the current prevelance of antibiotic resistance and the problems associated with it. The task force produced a series of recommendations centered around three key elements: (a) education of the physician and the public; (b) encouragement of more basic research directed to the development of new antimicrobials and vaccines; and (c) the setting-up of a national surveillance system to both confirm and monitor the extent of the problem. Since the publication of this report in 1995, progress has been slow. No "consortium-style" funding has been set aside and any initiatives, such as the first steps in an education program by the ASM, are still in their planning phases. The spirit of cooperation and trust needed to deal with this problem appears to be lacking.

Treatment of urinary tract infections: selecting an appropriate broad-spectrum antibiotic for nosocomial infections

Clinical and in vitro data indicate that cefepime, a fourth-generation cephalosporin, may be a valuable addition in the treatment of serious infections. In this study, hospitalized patients with complicated and uncomplicated urinary tract infection (UTI), for which parenteral therapy was appropriate, were enrolled in a 2:1 ratio open, randomized trial comparing the efficacy and safety of cefepime and ceftazidime. A total of 180 patients, including 6 with concurrent bacteremia, were evaluated for their response to cefepime (n = 118) or ceftazidime (n = 62), both of which were administered by intravenous infusion or intramuscular injection in doses of 500 mg every 12 hours. In cases of complicated UTI, cefepime produced a satisfactory clinical response in 83 of 93 (89%) patients and eradicated 83 of 98 (85%) pathogens. A satisfactory clinical response to ceftazidime was experienced by 43 of 50 (86%) patients; and in 39 of 50 (78%) cases pathogens were eradicated. In uncomplicated cases, the clinical response and bacterial eradication rates for cefepime were 23 of 25 (92%) and 22 of 26 (85%), respectively, and for ceftazidime 12 of 12 (100%) and 11 of 12 (92%). Of the 6 patients with concomitant bacteremia, 5 received cefepime and 1, ceftazidime. The infecting organisms, Escherichia coli and Proteus mirabilis, were eradicated in all cases, although one cefepime-treated patient had an unsatisfactory clinical response. The most common adverse events in both groups were headache, diarrhea, and vomiting; most events were unrelated to therapy. Adverse events forced only a 2% withdrawal of patients in either group. There was local tolerance to both agents, and abnormalities in laboratory values were judged to be clinically insignificant. The results of this study indicate that cefepime can be used safely and successfully to treat both complicated and uncomplicated nosocomial infection of the urinary tract, including cases associated with concurrent bacteremia. Moreover, its safety profile appears comparable to those of other cephalosporins, and local tolerance is similar to that of ceftazidime. No patient in either group required discontinuation of therapy because of local intolerance at the infusion or injection site.

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

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

Antimicrobial resistance patterns in long term geriatric care. Implications for drug therapy

There is a high prevalence of bacterial infections in long term care facilities (4.4 to 16.2%). This, together with the fact that antimicrobial resistance is a big concern in current medical practice, makes infection control so important in nursing home care. This article covers the mechanisms of antibacterial resistance and focuses on 4 major antibacterial-resistant bacteria. Vancomycin is the treatment of choice for methicillin-resistant Staphylococcus aureus (MRSA). Colonisation with MRSA is not uncommon in nursing homes and eradication is probably not necessary. Any clinically important enterococcal infection should be tested for high-level resistance. An infectious disease consultation should be sought for vancomycin-resistant enterococcal infections. Gram-negative bacilli have developed multi-resistance. Susceptibility testing can identify the most appropriate therapy. Multiresistance should also be considered when treating Streptococcus pneumoniae. Overall, handwashing is highly recommended. Barrier precautions, minimising hospitalisations and avoiding unnecessary personnel rotation can reduce the chance of resistance spread.

Antimicrobial-resistant pathogens in the 1990s

Streptococcus pneumoniae, Enterococcus faecalis, Enterococcus faecium, Staphylococcus aureus, and Klebsiella pneumoniae have become increasingly resistant to antimicrobial agents. This chapter reviews the epidemiology of this resistance, its detection in the laboratory, the mechanisms of resistance, and the options for therapy and infection control.

Surgical patients with multiantibiotic-resistant bacteria

Although antibiotics can cure most bacterial infections, there is an increasing number of bacteria that are resistant to antibiotics. Methicillin-resistant Staphylococcus aureus (MRSA) is becoming increasingly prevalent in US health care facilities. The majority of these infections are found in patients who have extensive burns or surgical wounds. As a result, perioperative nurses must be knowledgeable about MRSA and its implications for the OR. There are many theories on how to control the spread of MRSA but not one definitive set of control measures. Perioperative nurses, in cooperation with infection control practitioners, must develop policies that detail how patients with MRSA will be treated.

Methicillin-resistant Staphylococcus aureus in Western Australia, 1983-1992

A statewide screening programme has prevented imported strains of methicillin-resistant Staphylococcus aureus (MRSA) from becoming established in any hospital in Western Australia (WA). Recently, notifications of MRSA in WA have increased, prompting a review of surveillance data for the period 1983-1992. Our aims were to determine: (i) the distribution by age and sex of persons with MRSA; (ii) changes in notification rates over time and by location in WA; and (iii) temporal changes in antimicrobial resistance patterns. There were 631 notifications of MRSA for the 10 year period 1983-1992, ranging from a low of 36 in 1988 to a high of 117 in 1992. When the distribution by age and sex was examined, three age group peaks were apparent: 0-9 years, 20-39 years and 60-79 years. There was a predominance of females in the 20-39 years age group, reflecting a greater proportion of hospital nursing staff carrying MRSA. In those aged 50 years or more, there was a marked predominance of males. The highest notification rates overall occurred in the remote Kimberley region of WA, however, rates increased significantly in all regions of the state in 1992. Based on antimicrobial resistance patterns, MRSA was classified into two groups: multiresistant imported strains which often caused outbreaks in hospitals; and a less resistant MRSA (WA MRSA). WA MRSA appears to have originated in the Kimberley region and then spread widely in the community to other regions of the state, and the proportion of WA MRSA has increased significantly since 1989.

Comparison of mupirocin susceptibility of nasal and nonnasal Staphylococcus aureus isolates

Susceptibilities of 414 nasal and 586 nonnasal Staphylococcus aureus isolates, both methicillin resistant and methicillin susceptible, to the topical antimicrobial agent mupirocin were compared. A susceptibility of 99.1% was observed for the 1000 isolates. Nasal and nonnasal isolates showed a similar 90% minimum inhibitory concentration (MIC90) and statistically equivalent percent susceptibilities.

Infection control and prevention strategies in the ICU

Although only 5-10% of all hospitalized patients are treated in ICUs, they account for approximately 25% of all nosocomial infections, and the incidence of nosocomial infections in ICUs is 5-10 times higher than that observed in general hospital wards. Systemic and respiratory infections are far more common than in general wards, and most epidemics originate in ICUs. Nosocomial infections are the primary focus of most infection control programmes because they are the cause of high mortality rates in ICUs. Effective programmes are usually based on the cooperation of the intensive care physician, the infectious disease specialist, the microbiologist and the clinical epidemiologist. The infectious disease specialist develops specific guidelines for the antimicrobial therapy of typical infections which minimize the selective pressure for microorganisms within the ICU. The microbiologist provides rapid and accurate diagnosis of the pathogens involved. The clinical epidemiologist identifies epidemics at early stages, using epidemiological tools and molecular typing methods, as well as summarizing trends of antimicrobial susceptibility patterns and setting standards for isolation practices. A simple and inexpensive way to reduce nosocomial infections in ICUs is to ensure that staff disinfect their hands after dealing with a patient. Intravascular devices, mechanical ventilation and urinary catheterization are major risk factors for nosocomial infections, and their use should be evaluated daily and discontinued as soon as clinically possible. Selective decontamination of the digestive tract and the use of standard immunoglobulin for prophylaxis are still controversial and need further investigation.(ABSTRACT TRUNCATED AT 250 WORDS)

Antimicrobial resistance: prognosis for public health

Antimicrobial resistance poses a significant threat to public health worldwide, with certain infections already being untreatable with antibiotics. Increasing resistance is resulting from antimicrobial use coupled with various epidemiological factors that enhance transmission of drug-resistant organisms, and the problem is likely to worsen. Control of antimicrobial resistance is feasible, but will be difficult.

Comparative prophylactic efficacies of ciprofloxacin, ofloxacin, cefazolin, and vancomycin in experimental model of staphylococcal wound infection

Recent shifts in the species and antibiotic resistance patterns of bacteria causing nosocomial infections present new challenges for providing effective prophylaxis in surgery. Traditional regimens lack activity against methicillin-resistant staphylococci and many gram-negative species causing nosocomial infections. The new fluoroquinolones exhibit in vitro activity against many emerging surgical wound pathogens. To determine the potential of this class of antimicrobial agents for use in surgery, we compared the prophylactic efficacies of ciprofloxacin and ofloxacin with those of cefazolin and vancomycin in a guinea pig model of abscess formation. Four Staphylococcus aureus strains, one Staphylococcus epidermidis strain, and one Staphylococcus haemolyticus strain were evaluated. Vancomycin was the most effective prophylactic agent, exhibiting in vivo activity against all strains which was superior or equivalent to those of all other agents tested. Cefazolin was the least effective agent and surpassed the two quinolones in prophylactic efficacy against only one organism, a quinolone- and methicillin-resistant strain of S. aureus. The two quinolones provided excellent protection against infection with all but the quinolone-resistant isolate. The in vivo emergence of quinolone resistance among quinolone-susceptible isolates was not detected. The methicillin-resistant, quinolone-susceptible S. epidermidis and S. haemolyticus isolates were extremely susceptible to prophylaxis, exhibiting 50% infective doses above 4 x 10(6) CFU for seven of the eight antibiotic-strain combinations. We conclude that ciprofloxacin and ofloxacin may be effective antistaphylococcal agents in surgery. The role of these agents remains to be defined, and the definition should include consideration of an adverse effect upon antibiotic resistance patterns of organisms causing nosocomial infections.

Hospital-acquired infections: diseases with increasingly limited therapies

About 5% of patients admitted to acute-care hospitals acquire nosocomial infections. A variety of factors contribute: increasing age of patients; availability, for treatment of formerly untreatable diseases, of extensive surgical and intensive medical therapies; and frequent use of antimicrobial drugs capable of selecting a resistant microbial flora. Nosocomial infections due to resistant organisms have been a problem ever since infections due to penicillinase-producing Staphylococcus aureus were noted within a few years of the introduction of penicillin. By the 1960s aerobic Gram-negative bacilli had assumed increasing importance as nosocomial pathogens, and many strains were resistant to available antimicrobials. During the 1980s the principal organisms causing nosocomial bloodstream infections were coagulase-negative staphylococci, aerobic Gram-negative bacilli, S. aureus, Candida spp., and Enterococcus spp. Coagulase-negative staphylococci and S. aureus are often methicillin-resistant, requiring parenteral use of vancomycin. Prevalence of vancomycin resistance among enterococcal isolates from patients in intensive care units has increased, likely due to increased use of this drug. Plasmid-mediated gentamicin resistance in up to 50% of enterococcal isolates, along with enhanced penicillin resistance in some strains, leaves few therapeutic options. The emergence of Enterobacteriaceae with chromosomal or plasmid-encoded extended spectrum beta-lactamases presents a world-wide problem of resistance to third generation cephalosporins. Control of these infections rests on (i) monitoring infections with such resistant organisms in an ongoing fashion, (ii) prompt institution of barrier precautions when infected or colonized patients are identified, and (iii) appropriate use of antimicrobials through implementation of antibiotic control programs.

An overview of nosocomial infections, including the role of the microbiology laboratory

An estimated 2 million patients develop nosocomial infections in the United States annually. The increasing number of antimicrobial agent-resistant pathogens and high-risk patients in hospitals are challenges to progress in preventing and controlling these infections. While Escherichia coli and Staphylococcus aureus remain the most common pathogens isolated overall from nosocomial infections, coagulase-negative staphylococci (CoNS), organisms previously considered contaminants in most cultures, are now the predominant pathogens in bloodstream infections. The growing number of antimicrobial agent-resistant organisms is troublesome, particularly vancomycin-resistant CoNS and Enterococcus spp. and Pseudomonas aeruginosa resistant to imipenem. The active involvement and cooperation of the microbiology laboratory are important to the infection control program, particularly in surveillance and the use of laboratory services for epidemiologic purposes. Surveillance is used to identify possible infection problems, monitor infection trends, and assess the quality of care in the hospital. It requires high-quality laboratory data that are timely and easily accessible.

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

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

A nursing challenge: methicillin-resistant Staphylococcus aureus in long-term care

1. Methicillin-resistant Staphylococcus aureus (MRSA) are gram-positive cocci, differing from methicillin-resistant species of S epidermidis because they produce coagulase. MRSA is distinguished from methicillin-sensitive S aureus by its resistance to methicillin and other antibiotics. MRSA has not been found to be an organism of greater virulence than methicillin-sensitive strains of S aureus, but infections caused by MRSA are sometimes of greater concern because of the limited antibiotic treatment options. 2. In the past 25 years there has been an increase in the number of reported infections in patients caused by MRSA. As infections with MRSA have become more prevalent in community hospitals throughout the country, the difficulty in controlling the spread within the community hospitals has mirrored the experiences of the tertiary care centers. 3. Education regarding the transmission of microorganisms and the caregiver's role in prevention is essential for every nursing facility. Interfacility communication is essential for hospital and nursing home staff to plan and manage the care of infected or colonized residents. 4. A facility-specific MRSA policy should optimize the opportunity for infected persons to receive nursing home care--without jeopardizing other residents.