haemolyticus and methicillin-resistant S aureus (MRSA) [13] and

haemolyticus and methicillin-resistant S. aureus (MRSA) [13] and appears to play a vital role in generating mosaicism in the genetic contexts of mecA. The insertion of IS431 and homologous recombination between different copies of IS431 can result in acquisition, loss and re-arrangements of genetic components [14, 15]. Therefore, IS431 apparently serves as the “adapters” allowing genetic components to be linked and clustered together to form complicated genetic contexts of mecA. In GenBank and literature, e.g. [3], there are many cases in which

mecA is bracketed by two copies of IS431, either at the same or opposite orientations, i.e. the class C1 or C2 mec complex. In these cases, two copies of IS431 have the potential to form a composite transposon mediating the mobilization of mecA but no 8-bp DR could be identified flanking the class C1 or C2 mec complexes. This suggests that the two copies check details of IS431 might have inserted in tandem rather than mobilize together as a unit. Alternatively, IS431 might behave likes IS26[16], an insertion sequence also of the IS6 family, that could lead to adjacent deletions, leaving no DR. No ccr

genes could be identified in this large region containing mecA. In the 1970s and 1980s, it was found that methicillin resistance could be transferred by phages [17–21] in experimental conditions and could be also carried by a transposon, Tn4291, located on a naturally occurring plasmid, 4SC-202 in vivo pI524 [21]. However, these studies were carried out before the identification of mecA and no sequence information was available for the phages carrying methicillin resistance, Tn4291 and pI524. It remains unclear whether methicillin resistance in these experiments was due to the expression of mecA. In particular, Tn4291 mediated resistance

to methicillin Baf-A1 datasheet but not to penicillin, raising the possibility that the methicillin resistance determinant carried by Tn4291 was actually not mecA. mecA is usually transferred by SCCmec, but mecA existed in the absence of any known types of ccr genes have been found in both MRSA and CoNS previously. In particular, no known ccr genes were detected for an half of methicillin-resistant S. haemolyticus isolates from a hospital in Tunisia [22], suggesting that elements carrying mecA but lacking ccr genes might be common in S. haemolyticus. However, the detailed genetic context of mecA were not characterized in these cases and therefore the exact reasons for the absence of ccr genes remain unclear [2]. The present study provides a detailed example that mecA was in a context without ccr genes and might be able to be transferred by a MGE other than SCCmec. A complex SCC-like remnant containing components with various origins This 40-kb region between orfX and orf39 contained five copies of IS431 (designated IS431-1 to −5 from upstream of to downstream of mecA, respectively) and three terminal inverted repeats (IR) of SCC elements (Figure 1).

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