Properties of methicillin-resistant Staphylococcus aureus colonizing patients in a burns unit

MRSA and the environment: implications for comprehensive control measures

Environmental contamination with methicillin-resistant Staphylococcus aureus (MRSA) is established soon after colonized or infected patients become resident. There are many studies that detail the mechanisms of spread and environmental survival of methicillin-susceptible Staphylococcus aureus (MSSA); this knowledge translates directly into the same findings for MRSA. The potential ubiquity of MRSA in a health-care setting poses challenges for decontamination. Whereas patients and medical staff are important sources for MRSA spread, the environmental burden may contribute significantly in various contexts. Effective control measures must therefore include consideration for MRSA in the environment.

Methicillin resistant Staphylococcus aureus in the critically ill

Methicillin resistant Staphylococcus aureus (MRSA) is endemic within many hospitals worldwide. Critically ill patients on intensive care units have increased risk factors making them especially prone to nosocomially acquired infections. This review addresses the current situation regarding the evolution of MRSA and the techniques for identifying and epidemiologically typing it. It discusses specific risk factors, the morbidity and mortality associated with critically ill patients, and possibilities for future antibiotic treatments.

Survival of MRSA on sterile goods packaging

For many years, MRSA (methicillin-resistant Staphylococcus aureus) has been a world-wide problem. Stringent infection control regimens need to be followed to prevent spread. One such measure is the disposal of unused, MRSA-contaminated single-use items, which is quite expensive. An alternative, less costly measure is to store these items temporarily, re-using them once the organism is non-viable. To establish survival times of MRSA on sterile goods packaging, paper and foil samples were contaminated with MRSA (approximately 10(8)-10(9) cfu/sample). The number of pathogens recoverable from the samples was measured at defined times. MRSA was demonstrated to survive on sterile goods packaging for more than 38 weeks. No MRSA was recoverable after 50 weeks. Temporary storage of MRSA-contaminated single-use items for such a long period of time is not an appropriate or reliable means of decontamination, but many be considered for items that would be costly to replace.

Evidence that hospital hygiene is important in the control of methicillin-resistant Staphylococcus aureus

Observational and microbiological data were collected from the patients and environment of a male general surgical ward over a period of 27 months from January 1998. Isolates of methicillin-resistant Staphylococcus aureus (MRSA) from patients and environment were typed by antibiogram, bacteriophage and pulsed field gel electrophoresis of chromosomal DNA. In September 1999, an intervention was put in place which included increasing the domestic cleaning time by 57 hours per week, with emphasis on removal of dust by vacuum cleaning, and allocation of responsibility for the routine cleaning of shared medical equipment. From January 1998 to September 1999, despite standard infection control measures (emphasis on hand hygiene, isolation of affected patients and staggered closure and cleaning of ward bays), 69 patients acquired a strain of E-MRSA16. This strain was also widespread in the ward environment. Typing confirmed that isolates from patients and environment were indistinguishable from one another and that the outbreak was due to a single strain. This strain was responsible for postoperative infection in approximately one third of the patients who acquired it. In the six months following the intervention, only three patients were colonized with the outbreak MRSA and monthly surveys failed to detect this strain in the environment. Thorough and continuous attention to ward hygiene and removal of dust was needed, to terminate a prolonged outbreak of MRSA infection on a general surgical ward, in addition to standard infection control measures. Control of hospital-acquired infection with MRSA requires a combination of measures, none of which are completely effective in isolation.

Antibiotic and biocide resistance in methicillin-resistant Staphylococcus aureus and vancomycin-resistant enterococcus

Concern has been growing regarding the potential of antibiotic and disinfectant co-resistance in clinically important bacteria. In this study, the susceptibilities of methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-sensitive Staphylococcus aureus (MSSA) to chlorhexidine (CHX), the quaternary ammonium compounds cetylpyridinium chloride (CPC) and benzalkonium chloride (BC), triclosan, dibromopropamidine isethionate (DBPI) and triclocarban were compared. MRSA exhibited low-level resistance to CHX and the QACs, with MICs of 1.5 to 3-fold (CHX), and 2 to 4-fold (QACs) higher than MSSA. However, the MIC values for MRSA ranged between 0.025 (the MIC of MSSA) and 1 microg/mL with triclosan, and between <5 (the MIC of MSSA) and 75 microg/mL with DPBI. Nevertheless, these strains remain relatively sensitive to most of these antimicrobial agents. The bactericidal efficacy of CHX, CPC and DBPI (with the exception of one strain) correlated with their MIC value. This was not observed using triclosan; MRSA and MSSA strains were equally susceptible to its killing effect, regardless of MIC. The permeabilizing agent, ethylenediamine tetraacetic acid (EDTA) was unable to potentiate the antibacterial activities of the biocides against any of the strains tested. Attempts to select for staphylococcal strains with increased resistance to triclosan, CPC or CHX, using disc diffusion, step-wise broth, or repeated exposure/recovery technique, were only partially successful, and resistance was found to be unstable. The susceptibilities of vancomycin-resistant enterococcus (VRE) and vancomycin-sensitive enterococcus (VSE) to the biocides were also compared and found to be similar both in terms of MIC testing and time-kill studies.

Methicillin-resistant Staphylococcus aureus versus the burn patient

Methicillin-resistant Staphylococcus aureus (MRSA) has become a frequent cause of nosocomial infection, its increasing prevalence posing serious therapeutic and infection control problems within the hospital environment. MRSA is a major challenge to the burn patient, with potential to cause significant morbidity and mortality. Burn patients have been shown to become colonised and infected more readily than other patient groups. Extensive burn injuries are particularly susceptible to infection as a result of the disruption of the normal skin barrier and accompanying depression of immune responses. Extended hospitalisation and antibiotic therapy have been identified as additional risk factors for MRSA carriage and infection. Microbial surveillance, epidemiological studies and the introduction of strict infection control regimes can reduce the prevalence of MRSA but may be insufficient for eradication or prevention of outbreak situations. Recognition of the clinical importance of MRSA to the burn patient highlights the need to take appropriate measures to minimise transmission and infection in this vulnerable group of patients.

Use of 0.3% triclosan (Bacti-Stat) to eradicate an outbreak of methicillin-resistant Staphylococcus aureus in a neonatal nursery

BACKGROUND

Once established in an institution, methicillin-resistant Staphylococcus aureus (MRSA) outbreaks have proved difficult to eradicate, despite intensive infection control measures. This report describes the nosocomial infection with MRSA of 22 male infants in a neonatal nursery during a 7-month period and the infection control procedures that effectively brought this outbreak under control and eliminated recurrence for more than 3 1/2 years.

METHODS

After a single index case of bullous impetigo caused by MRSA in a neonate discharged from the nursery 2 weeks previously, an additional 18 cases of MRSA skin infections were clustered in a 7-week period. Aggressive infection control measures were instituted, including changes in umbilical cord care, circumcision procedures, diapers, handwashing, gloves, gowns, linens, disinfection, placement in cohorts of neonates and staff, surveillance, and monitoring.

RESULTS

These measures were not effective in slowing the outbreak. The single additional measure of changing handwashing and bathing soap to a preparation containing 0.3% triclosan (Bacti-Stat) was associated with the immediate termination of the acute phase of the MRSA outbreak.

CONCLUSION

The nursery has remained free of MRSA for more than 3 1/2 years, attesting to the success of our program.

Cell surface hydrophobicity and adherence to extra-cellular matrix proteins in two collections of methicillin-resistant Staphylococcus aureus

Non-specific and specific mechanisms of adherence have been examined in two collections of methicillin-resistant Staphylococcus aureus (MRSA). Determination of hydrophobicity by salt aggregation, hydrophobicity indices and of adherence to the extra-cellular matrix proteins fibronectin, vitronectin, laminin and collagen type 1 have failed to reveal any correlation with phage-type, plasmid profile or antibiogram. Further, the strain collections, made over a period of years in two countries, differ markedly in their adherence characteristics; MRSA are heterogeneous in this respect. Such heterogeneity may explain the polarization of views on the epidemicity or 'virulence' of MRSA. With the exception of adherence to collagen a small group of methicillin sensitive S. aureus had characteristics intermediate between the two groups of MRSA.

Partial characterization of an endemic strain of a methicillin- and aminoglycoside-resistant Staphylococcus aureus (MARSA) homogeneously resistant to β-lactam antibiotics

Selected strains of methicillin- and aminoglycoside-resistant Staphylococcus aureus (MARSA) were subjected to a preliminary examination. They were representative of a larger group collected in a routine clinical microbiology laboratory over a period of 2 years. MARSA was endemic in the associated hospital. The characteristics investigated were antimicrobial resistance, the production of beta-lactamase, free and bound coagulase, protein A, DNA-ase, urease, lipase and pigment. The MARSA strains were generally indistinguishable, other than in their antimicrobial resistances. The resistance to methicillin was completely homogeneous. Except with imipenem, growth extended to the edge of discs containing methicillin and the other beta-lactam antibiotics tested when the strains were cultured at 37 degrees C on media without added salt. Homogeneous resistance may confer an epidemiological advantage on strains of this phenotype.

Survival of bacteria under dry conditions; from a viewpoint of nosocomial infection

The viability of non-sporing bacteria in a dry state was examined. The number of viable cells was determined at various time intervals, after inoculation onto cotton lint and a glass plate. Viable cells of Mycobacterium bovis were detected more than 2 months after inoculation, and this was the most resistant species to dry conditions among the bacteria tested. In the case of the Gram-positive cocci tested and one species of Gram-negative rods, Acinetobacter calcoaceticus var. anitratus, the viable cell numbers decreased by only 2 log10 by 25 days after inoculation, indicating that they are highly resistant to dry conditions. In contrast, Gram-negative rods other than A. calcoaceticus lost their viability very rapidly and no viable cells were detected 7 h after inoculation. In the presence of proteins such as bovine serum albumin and horse serum, the viability of bacteria in the dry state increased. This protein effect on bacterial survival was marked for Escherichia coli and Salmonella typhimurium but not for Pseudomonas cepacia and Xanthomonas maltophilia. These results indicate why Gram-positive cocci and A. calocoaceticus are capable of causing airborne infections. It is also suggested that enterobacteria such as E. coli could cause infections when they survive in a dry environment along with proteins derived from body fluids.

A survey of methicillin-resistant Staphylococcus aureus affecting patients in England and Wales

For a six-month period between October 1987 and March 1988, 660 isolates of methicillin-resistant Staphylococcus aureus (MRSA) from 570 patients were sent to the Staphylococcus Reference Laboratory at Colindale to supplement the National reporting survey of MRSA in England and Wales. The isolates were characterized by phage typing, antibiotic susceptibility and by selected biochemical tests. Patient details were also surveyed. Fourteen strains affected more than one hospital and were called multi-hospital epidemic strains. One strain, EMRSA-1, accounted for more than 40% of isolates and of patients. Other epidemic strains were defined. Ten additional strains were restricted to single hospitals. Only 25 primary isolates were non-typable but 67 sporadic typable strains occurred. The patients affected were approximately equally either infected or colonized. The sexes were represented equally. Orthopaedic and geriatric wards were over-represented. Epidemic strains were clumping factor positive while some sporadic strains were weak producers. Urea alkalinization and protein A production could supplement phage typing and antibiotic resistance in strain recognition.

Analysis of infection in a burn ward

One hundred and seventy-two burn wound swabs obtained from 90 patients admitted to the Aljila Hospital Burn Unit, Benghazi over a 3-month period were processed to determine the microbial flora colonizing burns and their resistance patterns to selected locally available topical and systemic agents. Approximately 84.9 per cent of the swab specimens yielded growth of 11 bacterial species and Candida spp.; of these, Ps. aeruginosa, Staph. aureus and Klebsiella spp. predominated in order of prevalence, followed by Enterobacteria. Polymyxin (100 per cent sensitive), amicacin (90.9 per cent sensitive) and carbenicillin (66.7 per cent sensitive) were the most effective of 10 selected antibiotics tested against the 60 pseudomonas isolates. In studies in vitro using six topical agents, Ps. aeruginosa strains were most sensitive, in decreasing order, to mafenide acetate, silver sulphadiazine, acetic acid, silver nitrate and Eusol. Seventy-eight per cent of Staph. aureus isolates were resistant to methicillin and erythromycin and 93 per cent to tetracycline.

Resistance to dicloxacillin, methicillin and oxacillin in methicillin-susceptible and methicillin-resistant Staphylococcus aureus detected by dilution and diffusion methods

A total of 54 Staphylococcus aureus strains of varying methicillin resistance were investigated for their resistance to methicillin, oxacillin and dicloxacillin by different diffusion tests. Inhibition zones were measured around locally prepared paperdiscs with 10 micrograms methicillin, 5 and 10 micrograms oxacillin, 5 and 10 micrograms dicloxacillin, PDM paperdiscs with 10 micrograms methicillin or oxacillin and Neo-sensitabs tablets with methicillin or oxacillin. All diffusion tests were performed both with Mueller-Hinton agar and Danish Blood agar as well as at 37 degrees C and 30 degrees C and read after overnight incubation. Differences in zone diameter under different conditions were found to be independent of the susceptibility level of the strains. Seventeen of the strains were detected as methicillin-resistant (MRSA) by two methods including high inoculum and prolonged incubation at 30 degrees C. The minimum inhibitory concentration (MIC) of the 54 strains was determined by a plate dilution method at 30 degrees C and 37 degrees C. A 10 micrograms locally prepared methicillin disc detected all MRSA strains with no false reactions either at 37 degrees C or 30 degrees C on Mueller-Hinton agar. Investigations with oxacillin discs had to be performed at 30 degrees C or with a 5 microgram disc in order to detect correctly. PDM paperdiscs gave reactions identical to the corresponding locally prepared discs. Methicillin Neo-sensitabs detected all MRSA strains but also included a few susceptible strains among the resistant ones. Addition of blood increased the number of not-detected MRSA strains. All 17 MRSA strains were susceptible to dicloxacillin by the dilution method, and the disc diffusion test showed similar results. Dicloxacillin discs therefore did not detect the presence of MRSA strains. The implications of replacement of the methicillin/oxacillin disc by a dicloxacillin disc are discussed.

Plasmid analysis of 219 methicillin-resistant Staphylococcus aureus strains with uncommon profiles isolated in Lisbon

During the years 1986 and 1987, 219 methicillin-resistant Staphylococcus aureus (MRSA) strains were isolated in a Lisbon hospital. Antimicrobial susceptibilities and genetic analysis showed that resistance to penicillin, methicillin, erythromycin (inducible phenotype), tetracycline, gentamicin, tobramycin, kanamycin, streptomycin, neomycin and trimethoprim were chromosomally encoded. Plasmid DNA was absent from 38.8% of the isolates. Constitutive erythromycin resistance was coded by three plasmids of c. 2.3 Md, c. 2.0 Md and c. 1.6 Md. Chloramphenicol resistance was mediated by two plasmids of c. 2.9 Md and c. 1.8 Md. Small cryptic plasmids of c. 1.65 Md, c. 1.2 Md and c. 1.0 Md were also detected. The majority of the strains revealed antigen 17, were lysed by phages 75, 89 and/or 85, and were either devoid of plasmid DNA, or possessed a c. 1.6 Md plasmid coding for constitutive erythromycin resistance or a c. 1.0 Md cryptic plasmid. These observations cannot rule out that the MRSA Lisbon isolates are a homogeneous group of strains that might have a common origin, and seem to be different from MRSA recently isolated in other countries.

Colonization priority among Staphylococcus aureus strains--correlation with phage-type

We have studied the distribution of phage-type patterns among strains of Staphylococcus aureus isolated from patients in a burns unit. From 51 patients the same phage-type was isolated from succeeding swabs during the observation period. In 20 patients new types were introduced, but the original strain remained. In 23 patients the first strain was replaced by one other strain, in eight patients two or more. Strains of type 95 seemed to have a high colonization priority, whereas strains of group III had a low one. In 1986 phage-typing was performed on two or more S. aureus strains from the same patient, in 4561 instances. Recurrence of strains of the same phage-type pattern was demonstrated in 70% of the patients when the first and the fourth sample were compared. The "newer epidemic" strains of phage-type 95 and of the 94,96 complex had the highest percentage of recurrence (more than 80%) when adjacent samples were compared, and 68-69% when the first and the fourth sample were compared. The good colonization capacity of these strains might be one of the explanations why they occur frequently today although they are resistant only to penicillin.

Methicillin-resistant Staphylococcus aureus: associated morbidity and effectiveness of control measures

The strain of methicillin-resistant Staphylococcus aureus (MRSA) prevalent in south-east England produced in one acute hospital in a year 40 infections (bacteraemia, pneumonia and surgical wound, skin and urinary tract infections) with three attributable deaths. Rigorous measures succeeded in controlling the outbreak despite its extent, but our results suggest that less stringent measures could fail to control outbreaks of this scale. Several subsequent localized outbreaks within the hospital, probably caused by separate re-introductions of MRSA from other hospitals, were controlled by re-instigation of control measures on individual wards. The overall success of the intervention was shown by the decline in the incidence of MRSA infections from 27 in the 6 months beforehand to 2 in the most recent 6 months, and by the decline in the prevalence of colonization among patients 10 or more days in hospital from 52% immediately before the intervention to 3% 7 months after it. The incidence of attributable morbidity and death without control measures warrants a concerted effort to tackle the epidemic in all affected hospitals in Britain.

Protein A and coagulase expression in epidemic and non-epidemic Staphylococcus aureus

Strains of Staphylococcus aureus were divided into groups on the basis of antimicrobial sensitivity and epidemiology and tested for protein A expression in a simple microtitre test, which detected the non-immunological binding of immunoglobulin to protein A on whole cells of S aureus. Isolates of the methicillin resistant strain prevalent in south east England (EMRSA) showed a low expression of protein A compared with the other strains of methicillin resistant S aureus (MRSA), other multiple resistant strains, and sensitive strains. Protein A and coagulase expression in 27 strains of MRSA from 15 countries associated with hospital outbreaks were compared with 27 strains of MRSA from 11 countries reported to be sporadic isolates. Twenty four of the 27 outbreak associated MRSA showed low expression of protein A and high expression of coagulase. Conversely, sporadic strains generally gave higher levels of protein A and a wide variety of coagulase reactions. The results suggest that many epidemic strains of MRSA may have phenotypic characteristics that distinguish them from sporadic strains.