Phage typing and phage induction in carrier and invasive Staphylococcus epidermidis isolates

Molecular characterization of methicillin-resistant coagulase-negative staphylococci from a neonatal intensive care unit

OBJECTIVE

To evaluate clonal dissemination of methicillin-resistant coagulase-negative staphylococci (CNS).

SETTING

Neonatal intensive care unit of a 180-bed, university-affiliated general hospital.

PATIENTS

Neonates admitted to the neonatal intensive care unit between March 1999 and October 2000, from whom CNS were isolated as a unique pathogen. Patients from other wards from whom epidemiologically unrelated staphylococci strains were obtained served as control-patients.

METHODS

Conventional methods were used for phenotypic characterization of CNS. Methicillin resistance was determined by mecA polymerase chain reaction (PCR) amplification. Genotypic characterization was done by random amplification of DNA with degenerated primers (RAPD) and repetitive element sequence-based PCR (rep-PCR).

RESULTS

Forty methicillin-resistant CNS isolates obtained from neonates were characterized as Staphylococcus epidermidis (33), S. hominis (5), S. warneri (1), and S. auricularis (1). Both RAPD and rep-PCR indicated the presence of 4 different clones among the 33 S. epidermidis isolates. In turn, the 4 randomly selected, epidemiologically unrelated methicillin-resistant CNS strains obtained from control-patients showed 3 new profiles by RAPD and 2 by rep-PCR, which differed from the corresponding patterns mentioned earlier. Persistence of S. hominis in a neonate could be assessed by both genotypic techniques.

CONCLUSIONS

The molecular characterization of the methicillin-resistant CNS studied indicated dissemination of one particular methicillin-resistant CNS clone among the neonates in the ward studied. Although RAPD showed a superior power to discriminate among methicillin-resistant CNS isolates, both RAPD and rep-PCR detected intraspecific and interspecific genomic diversity.

Virioplankton: viruses in aquatic ecosystems

The discovery that viruses may be the most abundant organisms in natural waters, surpassing the number of bacteria by an order of magnitude, has inspired a resurgence of interest in viruses in the aquatic environment. Surprisingly little was known of the interaction of viruses and their hosts in nature. In the decade since the reports of extraordinarily large virus populations were published, enumeration of viruses in aquatic environments has demonstrated that the virioplankton are dynamic components of the plankton, changing dramatically in number with geographical location and season. The evidence to date suggests that virioplankton communities are composed principally of bacteriophages and, to a lesser extent, eukaryotic algal viruses. The influence of viral infection and lysis on bacterial and phytoplankton host communities was measurable after new methods were developed and prior knowledge of bacteriophage biology was incorporated into concepts of parasite and host community interactions. The new methods have yielded data showing that viral infection can have a significant impact on bacteria and unicellular algae populations and supporting the hypothesis that viruses play a significant role in microbial food webs. Besides predation limiting bacteria and phytoplankton populations, the specific nature of virus-host interaction raises the intriguing possibility that viral infection influences the structure and diversity of aquatic microbial communities. Novel applications of molecular genetic techniques have provided good evidence that viral infection can significantly influence the composition and diversity of aquatic microbial communities.

The polyvalent staphylococcal phage phi 812: its host-range mutants and related phages

Ninety-five percent of 782 culture collection strains, as well as hospital strains of Staphylococcus aureus subsp. aureus of different provenance and 43% of 89 culture collection strains of different coagulase-negative species of the genus Staphylococcus, were found to be sensitive to the polyvalent phage phi 812 or to at least one of its host-range mutants or to the polyvalent phages SK311, phi 131, and U16. Thus sensitivity to the polyvalent staphylococcal phages seems to be one of the common features of S. aureus subsp. aureus strains. The adsorption kinetics and one-step growth characteristics of the phages phi 812 and SK311 were estimated. Restriction genomic maps of the phages phi 812 (146.5 kb) and SK311 (141.1 kb) were constructed by use of the restriction endonucleases AvaII, PstI, KpnI, SacI, SmaI, and XhoI. The host-range mutations of the phage phi 812 were localized on this map. Comparison of restriction patterns of the phages phi 812 and SK311 with those of the polyvalent phages U16 and phi 131 suggests that all these phages are closely related. Their genomes differ from each other mostly by some deletions, insertions (1-3 kb), or inversions. Evidence was given that the phage phi 812 together with SK311, phi 131, and U16 belongs in the phage species Twort, the description of which is substantially supplemented with the data on the phage phi 812 reported in this paper.

Molecular typing of Staphylococcus epidermidis and other CNS with repetitive element sequence-based PCR

The chromosomal distribution of the repetitive DNA sequence found in Mycoplasma pneumoniae (REP-MP2) provides an ideal target for detecting DNA fragment patterns specific to individual Staphylococcus epidermidis and S. haemolyticus strains. A REP-MP2 sequence-based PCR (rep-PCR) was developed and applied to CNS isolates. We identified a 450 bp genomic DNA fragment which was common and specific to S. epidermidis isolates and not found in other CNS. In addition, S. epidermidis isolates showed several bands that could be grouped into 14 different fragment patterns. Similarly, S. haemolyticus isolates were classified into 10 groups. Significant correlations between the typing patterns of S. epidermidis and resistance to oxacillin (P< 0.05), gentamicin (P< 0.01), erythromycin (P< 0.02), and sulfamethoxazole-trimethoprim (P< 0.001) were found. The rep-PCR method is a rapid and reproducible discriminatory means for molecular typing of S. epidermidis and other CNS.

Typing of Staphylococcus epidermidis strains by random amplification of polymorphic DNA

The polymerase chain reaction was used to obtain randomly amplified polymorphic DNA profiles for typing of Staphylococcus epidermidis strains. Epidemiologically unrelated S. epidermidis isolates were screened with randomly amplified polymorphic DNA analysis. The discriminating ability of 45 randomly designed 10-mer primers was assessed. The highest discriminatory power was obtained with the 10-mer oligonucleotide OPAM-12. In typing a total of 13 unrelated S. epidermidis strains with OPAM-12, 11 different banding profiles were obtained reproducibly by agarose gel electrophoresis. The discriminatory power of the method with OPAM-12 was estimated using the D value of Hunter and Gaston (1988) to be 0.961. A reproducibility index of 1 was obtained after typing a total of 40 cultures including 12 triplicates and one quadruplicate of the 13 unrelated strains. Following the described procedure, the randomly amplified polymorphic DNA method provided a rapid, simple and reproducible alternative to other S. epidermidis typing systems.

Update on clinical significance of coagulase-negative staphylococci

The clinical significance of coagulase-negative Staphylococcus species (CNS) continues to increase as strategies in medical practice lead to more invasive procedures. Hospitalized patients that are immunocompromised and/or suffering from chronic diseases are the most vulnerable to infection. Since CNS are widespread on the human body and are capable of producing very large populations, distinguishing the etiologic agent(s) from contaminating flora is a serious challenge. For this reason, culture identification should proceed to the species and strain levels. A much stronger case can be made for the identification of a CNS etiologic agent if the same strain is repeatedly isolated from a series of specimens as opposed to the isolation of different strains of one or more species. Strain identity initially can be based on colony morphology, and then one or more molecular approaches can be used to gain information on the genotype. Many of the CNS species are commonly resistant to antibiotics that are being indicated for staphylococcal infections, with the exception of vancomycin. The widespread use of antibiotics in hospitals has provided a reservoir of antibiotic-resistant genes. The main focus on mechanisms of pathogenesis has been with foreign body infections and the role of specific adhesins and slime produced by Staphylococcus epidermidis. Slime can reduce the immune response and opsonophagocytosis, thereby interfering with host defense mechanisms. As we become more aware of the various strategies used by CNS, we will be in a better position to compromise their defense mechanisms and improve treatment.