Genetic heterogeneity in methicillin-resistant strains of Staphylococcus aureus revealed by esterase electrophoretic polymorphism

Genetic relationship between methicillin-sensitive and methicillin-resistant Staphylococcus aureus strains from France and from international sources: delineation of genomic groups

Cluster analysis of the SmaI patterns, generated by pulsed-field gel electrophoresis, of 44 methicillin-resistant (MRSA) and 118 methicillin-sensitive (MSSA) Staphylococcus aureus strains isolated in various French hospitals and 61 MRSA and 48 MSSA strains from 20 other countries revealed 20 genomic groups distributed into four distantly related phylogenic branches. Eighty-three of the 105 MRSA strains (79%) were clustered in the six genomic groups of phylogenic branch I; and 154 of the 166 MSSA strains (92.8%) were clustered in the 14 genomic groups of phylogenic branches II, III, and IV. Agreement between genomic group and two other markers, esterase type and phage group, was obtained, emphasizing the clonal structure of the population. The genomic groups were delineated by esterase type. The distribution of the strains within the genomic groups was independent of their geographical origin; French strains were clustered with strains from other countries. The three types of the staphylococcal cassette chromosome mec (SCCmec) complex were distributed according to genomic groups. Most of the time, type I and type II SCCmec complexes were found in the MRSA strains belonging to the same genomic groups. In contrast, the type III SCCmec complex was specific to the MRSA strains belonging to the three genomic groups characterized by a common esterase type.

Methicillin-resistant Staphylococcus aureus typing methods: which should be the international standard?

Methicillin-resistant Staphylococcus aureus (MRSA) has spread to all parts of the world. Effective control measures are dependent on a thorough knowledge of the organism's epidemiology which requires a typing technique that can be universally applied. Many typing methods have been developed for MRSA but none has been adopted as the internationally recognized standard. This review summarizes the information available on each in order to assess their suitability as a reference procedure. The majority of phenotypic and genotypic techniques are not sufficiently discriminatory, reproducible, stable or useful in an outbreak to be acceptable. The methods which do fulfil these requirements and have a potential for standardization, such as pulsed-field gel electrophoresis, binary typing or a combination of more rapid techniques, require further systematic evaluation.

Methylesters of L-arginine and N-nitro-L-arginine induce nitric oxide synthase in Staphylococcus aureus

The presence of L-arginine methylester (AME), L-arginine ethylester (AEE), or N-nitro-L-arginine methylester (NAME) in the growth media of Staphylococcus aureus increased the nitric oxide synthase (NOS) activity approximately 5- to 14-fold. The increase of NOS activity was confirmed by two assay methods, namely assaying the formation of L-[3H] citrulline from L-[3H] arginine and NO formation. The increase of NOS activity was most likely due to increased de novo synthesis, demonstrated by Western immunoblot analysis. The addition of methanol to the culture medium also increased the NOS activity as much as that found with the above three compounds. Evidence is presented to show that AME, AEE, or NAME gave rise to the formation of methanol in vivo by the action of intracellular esterase(s) and that methanol is subsequently involved in the induction of NOS in this bacterial system.

Epidemiology of methicillin-resistant Staphylococcus aureus in a Warsaw hospital

Methicillin-resistant Staphylococcus aureus (MRSA) isolates were collected during two eight-month periods in 1991 and 1994, respectively. In order to study the epidemiology, all 74 strains were characterized by phage-typing, antibiotic resistance patterns and DNA-restriction map after cleavage with SmaI enzyme, and pulsed-filed gel electrophoresis (PFGE). These investigations confirmed that MRSA in the hospital, 1991 and 1994, was not due to the spread of one or two clones, but by the simultaneous occurrence of a few well characterized strains and sporadic, occurring strains of different phage-types. Some of these might have developed from the more commonly occurring strains. Isolates from 1994 were more resistant to antibiotics in vitro, than the 1991 isolates. The typing results also indicated that whilst most of the MRSA strains in 1994 were different compared with those of 1991, some of the strains might have been present in both years. The PFGE-typing was more discriminatory and gave a higher typability than the phage-typing, especially among the multiply resistant isolates of MRSA from 1994. Among the less resistant strains the phage-typability was high and with only few exceptions, there was a good correlation between PFGE-type and phage-type.

Methicillin-resistant Staphylococcus aureus subtyping: interest of combined antibiotyping and esterase electrophoretic typing

Ninety-four methicillin-resistant Staphylococcus aureus isolates (MRSA) were characterized by means of two typing methods, antibiotyping and esterase electrophoretic typing. Antibiotyping, recorded on the basis of susceptibility testing of 13 antimicrobial agents, allowed the description of 18 antibiotypes, four of which comprised 30, 14, 14 and 12 strains respectively. Esterase electrophoretic typing, based on esterase activity against seven synthetic substrates after polyacrylamide-agarose gel electrophoresis, led to the description of 12 electrophoretic types, two of which were predominant with 60 and 20 strains respectively. The combined use of both typing methods yielded 32 combinations, three of which were predominant with 21, 12 and 11 strains respectively. A good differentiation of strains was achieved, particularly when the antibiotype was correlated to the electrophoretic type. Thus, the combination of antibiotyping with esterase electrophoretic typing may be proposed as a well-suited method for the characterization of MRSA strains.

Epidemiology of Staphylococcus aureus in patients with cystic fibrosis

Seven hundred and thirty-four isolates of Staphylococcus aureus, recovered from the sputum of 238 cystic fibrosis patients in six French hospitals, were characterized by esterase electrophoretic typing, capsular polysaccharide serotyping and phage typing and tested against 14 antibiotics for sensitivity. Thirty-four esterase electrophoretic types were found with a genotypic diversity coefficient of 0.91. Five hundred and forty-eight (78.7%) isolates produced capsular polysaccharide and 350 (50.3%) were type 8. Four hundred and sixty isolates (66.6%) were phage typable and 202 (28.2%) were lysed by group III bacteriophages. No esterase electrophoretic type, capsular type or phage type was specific to cystic fibrosis. Isolates belonged to a wide range of types, similar to strains acquired outside hospitals. Eighty-five patients had three or more consecutive isolates over at least 6 months. The ability of S. aureus to persist for long periods of time has been demonstrated in 73% of them. Methicillin-resistance was encountered among 73 strains (9.8%) which were also multiresistant. Two hundred and eighty-nine (39.9%) strains were sensitive to all antibiotics tested except to penicillin. Pristinamycin and co-trimoxazole were the most effective antibiotics. These results could contribute to the elaboration of a rational approach to the prophylaxis and therapy of respiratory staphylococcal infections in cystic fibrosis patients.

Molecular epidemiology for local outbreaks of methicillin resistant Staphylococcus aureus (MRSA). The need for several methods

Subtyping isolates may be useful for epidemiological studies of methicillin-resistant-Staphylococcus aureus (MRSA) outbreaks. Among subtyping methods, DNA-based techniques have been applied very effectively for this purpose. An outbreak of MRSA infections took place in one hospital in Barcelona early during 1991. From the beginning of the outbreak to December 92, 70 MRSA isolates from different patients and sources were collected. All strains were evaluated by restriction endonuclease analysis of plasmid DNA (REAP) and macrorestriction endonuclease analysis of genomic DNA using Sma I and pulsed-field-gel-electrophoresis (PFGE). Plasmid screening and REAP using Hind III demonstrated two plasmid subtypes: subtype A showing a large plasmid, and subtype B showing the same large plasmid plus a smaller one. Subtypes A and B corresponded to the more recent and older isolates, respectively, suggesting the loss of the small plasmid during the epidemic. PFGE using Sma I displayed two closely related profiles (PFGE subtype A and A'; CS = 0.90). These subtypes were different from those subtypes exhibited from 4 methicillin-susceptible-Staphylococcus aureus (MSSA) isolates from the same hospital and from 2 epidemiologically unrelated MRSA isolates. Almost all isolates showing PFGE subtype A preceded those isolates showing PFGE subtype A'. This fact and the similarity between both subtypes suggested minor chromosomal DNA rearrangement during the outbreak from a unique strain. While PFGE using Sma I is a useful tool in evaluation of clonal dissemination, our data suggest epidemic or local outbreaks may need several methods to best delineate the source and spread of MRSA strains. The reproducibility and discriminatory power of REAP makes it a useful adjunct in this context.

Typing of Staphylococcus aureus by pulsed-field gel electrophoresis, zymotyping, capsular typing, and phage typing: resolution of clonal relationships

Sixty-nine Staphylococcus aureus isolates from two epidemiologically unrelated sources were typed by pulsed-field gel electrophoresis after SmaI digestion of chromosomal DNA (genome typing), and the results were compared with those obtained by other typing methods: phage typing with the international set of phages, capsular serotyping with monoclonal antibodies against capsular polysaccharides type 5 and 8, and zymotyping by polyacrylamide agarose electrophoresis for esterase polymorphism. A good correlation of S. aureus types was found by these four typing methods. Differentiation increased in the order capsular typing < zymotyping < phage typing < genome typing, yielding 2, 10, 20, and 26 different S. aureus types, respectively. Five of the 26 genome types were further divided into several subtypes revealing clonal relationships. When 36 French S. aureus isolates were compared with 33 German S. aureus isolates, 3 strains representing clonal populations were identical in both groups. S. aureus isolates from patients with cystic fibrosis were also typed at the beginning and the end of a 4-week summer camp for these patients. The results suggested a possible strain transmission during the summer camp. We conclude that genome typing by pulsed-field gel electrophoresis is a powerful tool not only for strain identification but also for the resolution of the clonal relationships of S. aureus strains.

Esterase electrophoretic polymorphism of human and animal strains of Clostridium perfringens

Esterase electrophoretic polymorphism in human and animal strains of Clostridium perfringens was studied by using polyacrylamide-agarose gel electrophoresis. Five types of esterases, designated E-I to E-V and defined by their hydrolytic specificities toward five synthetic substrates, were found in protein extracts of bacteria grown without glucose (glucose-containing media allowed only the expression of esterase E-I). Mobility variants of esterase E-I, which hydrolyzes alpha- and beta-naphthyl acetates and butyrates, were used as a basis for the distribution of strains into 11 zymogroups. When all five types of esterases and their electrophoretic variants were considered, 77 electrophoretic types (ETs) could be described for the 89 strains tested. Animal strains did not constitute a distinctive subpopulation, as revealed by their distribution in the zymogroups and by clustering analysis. Statistical analysis also emphasized the importance of esterase E-IV (which hydrolyzes only naphthyl acetates) and esterase E-V (which hydrolyzes only alpha-naphthyl acetate) in clustering by the relatedness of the ETs. ETs allowed the epidemiological characterization of stool isolates recovered from elderly inpatient residents and from adolescent chronic-care psychiatric patients. These results indicate that esterase electrophoretic typing may be a marker for epidemiological and ecological analyses.

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

Genetic relationships among 254 isolates of Staphylococcus aureus resistant to methicillin recovered between 1961 and 1992 from nine countries on four continents were determined by analyzing electrophoretically demonstrable allelic variation at 15 chromosomal enzyme loci. Fifteen distinctive electrophoretic types, marking clones, were identified. The mec gene is harbored by many divergent phylogenetic lineages representing a large portion of the breadth of chromosomal diversity in the species, a result that is interpreted as evidence that multiple episodes of horizontal transfer and recombination have contributed to the spread of this resistance determinant in natural populations. Isolates recovered in the United Kingdom, Denmark, Switzerland, Egypt, and Uganda in the 1960s are of a single multilocus enzyme genotype and probably are progeny of an ancestral methicillin-resistant clone. There is geographic variation in the frequency of recovery of the common methicillin-resistant clones, an observation that may in part explain reported regional differences in natural history correlates of resistant organisms.

Typing of methicillin-resistant Staphylococcus aureus with an M13 repeat probe

A bacteriophage M13 tandem repeat has been used to probe EcoRI digested genomic DNA of methicillin-resistant Staphylococcus aureus (MRSA). The patterns generated were found to be useful in typing MRSA and generally confirmed the relationships that had previously been recognized in other studies based on antimicrobial resistance and plasmid profiles. The epidemic MRSA of London hospitals (EMRSA) and the majority of the epidemic MRSA of eastern Australian hospitals (EA MRSA) gave the same pattern. However, two isolates previously classified as EA MRSA gave a different pattern and a third another pattern. One isolate from Dublin, two isolates from Nuneaton and two isolates from Singapore gave the same pattern as the two EA MRSA. With the exception of the early or classic MRSA all the other isolates examined gave their own distinctive patterns. With one exception the classic MRSA belonged to a separate group. The exception was of particular interest because it gave the same pattern as the majority of the EA MRSA. This suggests that there may be an evolutionary relationship between some of the classic MRSA and the EMRSA of London and the EA MRSA of Australia.

Epidemiologic typing of Staphylococcus aureus by DNA restriction fragment length polymorphisms of rRNA genes: elucidation of the clonal nature of a group of bacteriophage-nontypeable, ciprofloxacin-resistant, methicillin-susceptible S. aureus isolates

Analysis of DNA restriction fragment length polymorphisms of rRNA genes (ribotyping) was employed to assist in the epidemiologic investigation of the emergence and spread of ciprofloxacin-resistant Staphylococcus aureus at the Atlanta VA Medical Center because many isolates of interest were nontypeable by phages and harbored few plasmids useful as strain markers. Chromosomal DNAs of selected S. aureus isolates were digested initially with 20 different restriction enzymes. EcoRI appeared to give the best discrimination of hybridization banding patterns (ribotypes) and was used with all study isolates. Overall, 15 different ribotypes were seen among the 50 S. aureus isolates studied (7 ribotypes among 13 methicillin-susceptible S. aureus [MSSA] isolates and 9 ribotypes among 37 methicillin-resistant S. aureus [MRSA] isolates). Seven of eight ciprofloxacin-resistant MSSA (CR-MSSA) patient isolates had identical antibiograms, were nontypeable by phages, and had a single 22-MDa plasmid. Six of these seven CR-MSSA isolates had an identical ribotype pattern. Ribotyping distinguished this CR-MSSA strain or clone from MRSA and other MSSA isolates, including nontypeable isolates that contained a 22-MDa plasmid. Five ciprofloxacin-susceptible MSSA isolates studied had five ribotypes; one pattern was identical to the CR-MSSA clone. Twenty-three CR-MRSA isolates recovered from the Atlanta VA Medical Center had four different ribotypes. Ribotyping proved to be a useful molecular epidemiologic tool in the study of S. aureus because it differentiated isolates which were indistinguishable by more traditional methods. In addition, this technique demonstrated that at our institution, ciprofloxacin resistance emerged in multiple strains of MRSA, as opposed to primarily a single strain or clone of MSSA.

The use of molecular techniques for epidemiologic typing of Candida species

The availability of an epidemiologic typing system for Candida species that is sensitive, rapid, inexpensive, and easy to perform would clearly be an advantage to the mycologist, microbiologist, and epidemiologist in the ongoing struggle to understand the epidemiology and pathogenesis of candidiasis. This is particularly true given the increasing prominence of organisms such as C. albicans and C. tropicalis which are ubiquitous members of the normal flora yet are also important causes of nosocomial bloodstream infection. Unfortunately, the ideal epidemiologic typing system does not yet exist. Current data suggest that the molecular typing methods of restriction endonuclease digestion of genomic DNA with ethidium bromide staining (DEtBr typing) and electrophoretic karyotyping using pulsed-field electrophoresis offer rapid, simple, and sensitive means of discriminating strains of Candida species. These methods appear at present to be the most practical typing methods for both large- and small-scale epidemiologic studies. Other typing methods using specific DNA probes provide a powerful means of identifying strains and will undoubtedly be applied more broadly in the future. Thus far, studies employing molecular typing methods have documented that (1) most patients are colonized by one strain of Candida species, (2) isolates of Candida species recovered from blood or deep tissue sites are generally identical to those obtained from colonization sites before infection developed, and (3) nosocomial transmission of a single strain of C. albicans may occur, particularly in an intensive care unit setting. Given the limitations of the available typing methods and the complex nature of the patients at risk for candidiasis, both the epidemiologist and laboratory scientist must use these methods with clear epidemiologic objectives in mind. Whenever possible, all organisms to be typed should be typed by the same person on the same day, and typing should always include unrelated as well as epidemiologically related isolates. Additional studies, based upon sound epidemiologic principles, will be necessary to clarify the role of the various molecular typing methods as epidemiologic markers of Candida species and to further our understanding of the epidemiology and pathogenesis of candidiasis.

Epidemiological markers for epidemic strain and carrier isolates in an outbreak of nosocomial oxacillin-resistant Staphylococcus aureus

An outbreak of nosocomial infections occurring in a postoperative intensive care unit was caused by a single strain of oxacillin-resistant Staphylococcus aureus. Six patients were infected, or colonized, by this strain, which was traced by using the following four epidemiological markers: antibiogram, bacteriophage type, capsular polysaccharide type, and esterase electrophoretic type. This strain was compared with S. aureus isolates obtained from the noses of 13 carriers from a group of 42 staff members. A good correlation in terms of phenotypic markers was found between the epidemic strain and a strain isolated from one carrier. Both exhibited the same pattern of multiple resistance as well as the same phage type, 77, capsular polysaccharide type, 5, and esterase electrophoretic type, 6. In contrast, an oxacillin-resistant strain, isolated from another carrier, differed from the epidemic strain by susceptibility to rifampin and by susceptibility to four additional bacteriophages. The other 11 strains isolated from carriers were susceptible to oxacillin and exhibited widely different phenotypes. These results confirm the interest of using several epidemiological markers to trace the spread of epidemic S. aureus strains and to delineate the carrier strains.

Correlation between esterase electrophoretic types and capsular polysaccharide types 5 and 8 among methicillin-susceptible and methicillin-resistant strains of Staphylococcus aureus

The relationship between capsular polysaccharide types 5 and 8 and esterase electrophoretic types (zymotypes) in 160 French clinical isolates of Staphylococcus aureus was studied. Methicillin-susceptible strains of capsular types 5 and 8 were represented by 11 zymotypes, indicating a high polymorphism. Methicillin-resistant strains were mainly distributed in only two distinct populations. The predominant population was represented by strains of zymotype 6 and capsular type 5, and the second population was represented by strains of zymotype 14 and capsular type 8.