H. Delencastre et al., METHICILLIN-RESISTANT STAPHYLOCOCCUS-AUREUS DISEASE IN A PORTUGUESE HOSPITAL - CHARACTERIZATION OF CLONAL TYPES BY A COMBINATION OF DNA TYPING METHODS, European journal of clinical microbiology & infectious diseases, 13(1), 1994, pp. 64-73
Fifteen pediatric patients as well as the five nursing staff of the Bu
rn Unit of the Hospital D. Estefania in Lisbon, Portugal, were assayed
at weekly intervals over a fivemonth period in order to identify the
nature and number of methicillin-resistant Staphylococcus aureus (MRSA
) clones associated with colonization and wound infection. Methicillin
resistance was confirmed by a mec-specific DNA probe. MRSA isolates w
ere classified into chromosomal types (clones) on the basis of a varie
ty of techniques: (i) ribotyping; (ii) restriction digestion by the en
donuclease ClaI followed by Southern hybridization with the mech-speci
fic DNA probe and (iii) by hybridization with Tn554; and (iv) pulsed-f
ield electrophoresis (PFE) of SmaI digests followed by (v) Southern hy
bridization with the mecA DNA probe. A sixth, physiological technique
(population analysis) was used to define the mode of phenotypic expres
sion of methicillin resistance in each isolate. All isolates carried a
single, common polymorph (ClaI type III) of the mecA gene. Hybridizat
ion with Tn554 resolved these isolates to two novel patterns (alpha an
d beta), of which one (Tn554 alpha) was predominant (90 %). This patte
rn could be further resolved to four closely related PFE types (A thro
ugh D). In contrast, all isolates with the Tn554 beta pattern belonged
to an additional, grossly different PFE type E. The Tn554 beta class
was also unique in that these bacteria carried the mecA gene in a SmaI
fragment smaller (about 170 kb) than that found in the alpha type str
ains (194 kb). Most isolates (83 %) showed a single heterogeneous (pop
ulation analysis Class 3) mode of resistance expression. The data demo
nstrate the full capacity of the globally rare (ClaI type III) MRSA cl
one for colonization and virulence. The results also document the stab
ility of the complex heterogeneous resistance phenotype as well as the
stability of the chromosomal types under conditions of in vivo carria
ge over a period of several months. In a few isolates the same mecA po
lymorph was present in several, grossly different genetic backgrounds,
suggesting horizontal transfer of the mecA gene.