Abstract
Staphylococcus aureus MRSA strains belonging to the clonal complex 398 (CC398) are highly prevalent in livestock and companion animals but may also cause serious infections in humans. CC398 strains in livestock usually do not possess well-known virulence factors that can be frequently found in other MRSA sequence types (ST). Since many staphylococcal virulence genes are residing on the genomes of temperate phages, the question arises why livestock-associated (LA-) CC398 strains are only rarely infected by those phages. We isolated and characterized four temperate phages (P240, P282, P630 and P1105) containing genes of the immune evasion cluster (IEC) and/or for the Panton-Valentine leucocidin (PVL). Sequence analysis of the phage genomes showed that they are closely related to known phages and that the DNA region encoding lysis proteins, virulence factors and the integrase exhibits numerous DNA repeats which may facilitate genomic rearrangements. All phages lysed and lysogenized LA-CC398 strains. Integration of IEC phage P282 was detected at ten sites of the hosts’ chromosome. The prophages were stably inherited in LA-CC398 and enterotoxin A, staphylokinase and PVL toxin were produced. The data demonstrate that lysogenic conversion of LA-CC398 strains by virulence-associated phages may occur and that new pathotypes may emerge by this mechanism.
Highlights
Staphylococcus aureus Methicillin-resistant Staphylococcus (S.) aureus (MRSA) strains belonging to the clonal complex 398 (CC398) are highly prevalent in livestock and companion animals but may cause serious infections in humans
To identify virulence gene-containing phages in S. aureus that should be used in subsequent experiments with LA-MRSA CC398, a large number of strains belonging to various MLST types was analysed by microarrays for the presence of virulence genes known to be located on the genomes of temperate phages[10]
For each lysate at least one susceptible indicator strain was found. As it was uncertain how many prophages had been induced in each strain, single plaques were analysed by PCR targeting the respective virulence genes, which resulted in the isolation of four phages containing a different composition of these genes (Table 1)
Summary
Staphylococcus aureus MRSA strains belonging to the clonal complex 398 (CC398) are highly prevalent in livestock and companion animals but may cause serious infections in humans. Since many staphylococcal virulence genes are residing on the genomes of temperate phages, the question arises why livestock-associated (LA-) CC398 strains are only rarely infected by those phages. Sak, scn lukF-PVL, lukS-PVL, sea lukF-PVL, lukS-PVL contribution to the virulence of MRSA is not fully understood, though it destroys polymorphonuclear granulocytes by forming pores, which leads to lysis of the cells[17] It is still unclear why virulence gene containing phages have to date only rarely been detected in LA-MRSA CC398 strains. Van der Mee-Marquet et al.[18] studied a ß-hemolysin-converting ΦSa3 phage in a LA-MRSA CC398 strain and found that the prophage was not stable and needed a helper phage for infection. We determined multiple integration sites of the ΦSa3 phage P282 within the S. aureus chromosome and show that the phage-encoded virulence factors are expressed in LA-CC398, which may increase the virulence of these strains in vivo
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