Epidemiología molecular de Staphylococcus aureus resistente a meticilina del linaje CC398 de distintos orígenesresistencia, virulencia y contenido plasmídico stars

Abstract

Staphylococcus aureus is a bacteria frequently found on the skin, on the nose and on the mouth without causing disease. However, it is also an important pathogen that can be involved in infectious processes or food poisoning. Recently, there is a high concern about the increase of methicillin resistant S. aureus (MRSA) strains. These strains are resistant to all ?-lactams, and this fact leads to a serious therapeutic problem. Since 2005, a livestock-associated MRSA belonging to CC398 has been spreading. This clonal lineage has been detected as infectious and colonizer agent in animals, mainly farm animals, but also in humans. In this study, a total of 7 clinical cases detected in a Spanish hospital and caused by MRSA CC398 were characterized. Four of the patients had a working relationship with animals, two patients lived with some farm worker and one patient did not have contact with animals. Moreover, in six cases, some relatives were nasal carriage of MRSA CC398. In order to establish the possible animal-to-human transmission of CC398 strains, pig farms related to 6 of the cases were analyzed. In four of them, this transmission was confirmed due to the fact that similar strains were obtained from animal and human samples. MRSA CC398 isolated strains did not present any virulence tested genes and usually showed a multiresistance phenotype. The dynamic of MRSA nasal colonization was also studied. For this, one of the cases was chosen and several samples of patient and relatives were taken on different occasions after mupirocin treatment. The efficacy of nasal MRSA decolonization seemed to be higher in people with sporadic contact with animals or contact with MRSA colonized humans than people with close contact with colonized animals. Alternatively, with the purpose to know more about the epidemiology of this microorganism and to detect the possible presence of MRSA CC398 strains, the prevalence of MRSA in food and nasal samples of healthy people was studied as well as clinical MRSA strains were characterized. A low prevalence of MRSA (1.6%) was identified in meat samples and molecular typing results suggested that these strains could be from both human (CC5 y CC217) and animal origin (CC398). A moderate frequency of MSSA (19%) and a low prevalence of MRSA (0.4%) were detected in healthy people in this study. A high genetic diversity was found among MSSA strains and most of them were susceptible to all tested antimicrobials. Nevertheless, tst- and eta-positive strains were isolated. Moreover, MSSA CC398 and CC97 were detected in people without contact with animals. A comparative study among clinical MRSA strains of a Spanish hospital isolated in two periods distant in time was carried out. CC5-t067 lineage was still predominant in our country and it was associated with resistance to quinolones, aminoglycosides and in a lesser extent to macrolides. A possible emergence of CC8 is identified in the second period with two PVL positive MRSA strains. The level of resistance among MRSA remains more or less constant in both periods although it seems that there could be a slight downward trend in the second period in which appeared mupirocin resistance. No MRSA CC398 was identified in these strains. Since MRSA CC398 normally presents resistance to tetracycline, 52 tetracycline resistance clinical MRSA isolates were studied. These strains were isolated in a Spanish hospital in 2009 and 2010. The aim was to determine whether tetracycline resistance could be a good marker to detect MRSA CC398. Most of 52 strains presented spa-types associated with CC398 (67.3%) although strains belonging to other CCs were identified: CC1 (11.5%), CC5 (11.5%) y CC8 (9.6%). Resistance mechanisms involved in the unusual phenotype lincosamides resistance and macrolides susceptibility was analyzed in MRSA CC398 strains and other Staphylococcus spp. strains. Different genes located on genetic mobile elements were detected: lnu(A) (in small plasmids), lnu(B) (in large plasmids), vga(A) (in mobilizable plasmids), cfr (in conjugative plasmids) and vga(A)-variant (in transposon). In some cases, novel plasmids (pUR2355, pUR4128, pUR3036, pUR3937, pUR5425) or genetic environments were identified and it seemed to be a relation between these genes and some animal clonal lineages as CC9 and CC398. In order to know more about genetic mobile elements a study about plasmid content of S. aureus strains was performed. An explanding of a plasmid classification system for Gram-positive bacteria was carried out and 15 rep-families and 10 unique sequences were established. Most of the strains presented a high number of plasmids from different sizes. In this study, it was confirmed that plasmids are dynamic structures and that plasmids are able to be transmitted between different genera and species.