genomewide characterization of host-pathogen interactions by ...

Maren Depke
Introduction
linking. In the presence of methicillin, MRSA induce the expression of PBP2a from their mecA
gene, and survive antibiotic treatment because PBP2a resumes peptide cross-linking instead of
the inhibited standard PBPs. Other resistance mechanisms including a methicillin-specific
lactamase (methicillinase) and PBP2 mutations with reduced methicillin binding are discussed
(Chambers 1997, Stapleton/Taylor 2002).
First, hospitals were the primary setting of transmission of and infection with such strains,
certainly also favored by the gathering of people who might be regarded as pre-disposed victims
for infection because of pre-existing illnesses or poor health state which additionally have
frequent injections and insertions of medical devices (Lindsay/Holden 2004).
But nowadays, MRSA has left the hospital as its transmission site and so-called “communityassociated”
MRSA arise. Existence of this new group has been published beginning from 1990 for
different countries. Interestingly, hospital-acquired / healthcare-associated (HA) and communityassociated
(CA) MRSA consist of distinct strains, although of course CA-MRSA can cause
nosocomial infections when transmitted by a carrier into hospital.
CA-MRSA strains have characteristics which distinguish them from HA-MRSA. They often
colonize non-classical niches in the human body and not the nares, and the distribution of the
Staphylococcal Cassette Chromosome mec (SCCmec) types is different between HA- and CA-
MRSA. Additionally, in CA-MRSA a higher frequency of Panton-Valentine leukocidin (PVL)-positive
strains is found, although the PVL virulence factor is not a universal marker for CA-MRSA.
Although the prevalence of CA-MRSA in Europe seems to be low, the number is already
increasing and additionally might be underestimated because of difficulties in monitoring
(symptom-free non-nasal carriage). Such unrecognized and hidden reservoir of MRSA is
challenging with respect to attempts in controlling staphylococcal infectious disease
(Otter/French 2010).
A B C
Fig. I.5: Proportion of MRSA isolates in EARSS-participating counties in 1999 (A), 2001 (B), and 2008 (C).
Graphics from European Antimicrobial Resistance Surveillance System (EARSS), a european wide network of national surveillance
systems, providing reference data on antimicrobial resistance for public health purposes (http://www.rivm.nl/earss/).
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